materials Article

Interfacial Reaction and Mechanical Properties of Sn-Bi Solder joints Fengjiang Wang 1, * 1 2

*

ID

, Ying Huang 1 , Zhijie Zhang 1 and Chao Yan 2, *

ID

Key Laboratory of Advanced Welding Technology of Jiangsu Province, Jiangsu University of Science and Technology, Zhenjiang 212003, China; [email protected] (Y.H.); [email protected] (Z.Z.) School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China Correspondence: [email protected] (F.W.); [email protected] (C.Y.); Tel.: +86-511-8441-5618 (C.Y.)

Received: 6 July 2017; Accepted: 7 August 2017; Published: 9 August 2017

Abstract: Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6 Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6 Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6 Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6 Sn5 in solder matrix and the transition from Cu6 Sn5 to Cu3 Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder. Keywords: Sn-Bi; solder; intermetallic compound; mechanical property; microstructure; aging

1. Introduction Sn-Ag-Cu Pb-free solder alloy is commonly employed as the interconnected material to replace traditional Sn-Pb solder due to its superior wettability, reliability and mechanical properties [1]. The melting temperature (MP) for Sn-Ag-Cu Pb-free solder ranges from 217 ◦ C to 221 ◦ C and is far higher than 183 ◦ C for Sn-37Pb eutectic solder, which means that a higher temperature will be required during assembling the components with boards. However, with continued shrinkage in size and thickness of electronic packaging, higher soldering temperature from Sn-Ag-Cu solder inevitably results in the thermal warpage on the thin or ultra-thin components [2]. To solve this reliability issue, development of Pb-free solder with MP close to or lower than Sn-37Pb solder is recently pursued in electronic industry [3].

Materials 2017, 10, 920; doi:10.3390/ma10080920

www.mdpi.com/journal/materials

Materials 2017, 10, 920 Materials 2017, 10, 920

2 of 16 2 of 16

Solder Solder in in Sn-Bi Sn-Bi binary binary system system provides provides aa low low temperature temperature solution. solution. Based Based on on Sn-Bi Sn-Bi binary binary phase phase diagram shown in Figure 1, with Bi alloyed into Sn, the liquidus temperature of Sn-Bi solder diagram shown in Figure 1, with Bi alloyed into Sn, the liquidus temperature of Sn-Bi solder can can be be ◦ ◦ continuously decreased from 232 °C at the melting point of Sn to 138 °C at eutectic composition of continuously decreased from 232 C at the melting point of Sn to 138 C at eutectic composition of Sn-58Bi. Sn-58Bi. ItItcan canbe beclearly clearlyseen seenthat thatthe thesolid solidsolution solutionSn-Bi Sn-Bisolder solderwith withBi Bicontent contentunder underthe themaximum maximum solubility solubility of of Bi Bi in in Sn, Sn, namely namely 21 21 wt wt %, %, provides provides the the potential potential alternative alternative for for Sn-37Pb Sn-37Pb solder, solder, while while hypoeutectic-to-eutectic Sn-Bi solder with higher Bi content is helpful to realize lower temperature hypoeutectic-to-eutectic Sn-Bi solder with higher Bi content is helpful to realize lower temperature soldering. soldering. Nowadays, Nowadays,many manyresearchers researchers have have studied studied the the microstructure microstructure and and mechanical mechanical properties properties of Sn-Bi solder with different Bi content. Shen et al. [4] investigated the creep performance of Sn-Bi solder with different Bi content. Shen et al. [4] investigated the creep performance of of Sn-Bi Sn-Bi with Bi content of 3%, 10%, 50% and 57% using the nanoindentaiton method and found that with Bi content of 3%, 10%, 50% and 57% using the nanoindentaiton method and found that the the transition transition stress stress decreased decreased with with Bi Bi content. content. Lai Laietetal. al.[5] [5]and andYe Ye et et al. al. [6] [6] studied studied the the hardness hardness and and bend content of of 3.6%, 7.25%, 14.5%, 29% andand 58%, andand found that that Snbendfracture fractureenergy energyofofSn-Bi Sn-Biwith withBiBi content 3.6%, 7.25%, 14.5%, 29% 58%, found 14.5Bi solder had the highest among them. Osório et al. [7] and Silva et al. [8,9] used a directionally Sn-14.5Bi solder had the highest among them. Osório et al. [7] and Silva et al. [8,9] used a directionally solidified solidified method method to to manufacture manufacture Sn-40Bi Sn-40Bi and and Sn-52Bi Sn-52Bi hypoeutectic hypoeutectic solders, solders, and and then then investigated investigated their microstructure and tensile properties. In summary, Bi addition would increase the their microstructure and tensile properties. In summary, Bi addition would increase the mechanical mechanical properties properties of of Sn-Bi Sn-Bi solder. solder.

Figure1.1.Sn-Bi Sn-BiPhase Phasediagram diagram[10] [10]with withdash dashlines linesillustrating illustratingthe theconcentrations concentrationsused usedin in this this study. study. Figure

Moreover, soldering with withCu Cusubstrate, substrate,an anintermetallic intermetalliccompound compound(IMC) (IMC)layer layer formed Moreover, during soldering formed at at the solder/Cuinterface interfaceisisnecessary necessaryfor forthe the metallurgical metallurgical joining. joining. Currently, the non-eutectic the solder/Cu non-eutectic Sn-Bi Sn-Bi solder where Cu Cu66Sn55 and Cu33Sn solder for for interfacial interfacial reaction reaction only includes Sn-5Bi [11] and Sn-10Bi [12,13], where Sn IMCs at the theinterface interfaceand andthe thegrowth growthkinetics kinetics Cu-Sn IMC layers were studied under IMCs were formed at ofof Cu-Sn IMC layers were studied under the the thermal aging, most research was focused on eutectic Sn-58Bisolder. eutectic Leeinvestigated et al. [14] thermal aging, whilewhile most research was focused on Sn-58Bi Leesolder. et al. [14] investigated the interfacial reaction products of Sn-58Bi solder on Cu and electroless nickelthe interfacial reaction products of Sn-58Bi solder on Cu and electroless nickel-immersion gold (ENIG), immersion gold (ENIG),IMC andwere the corresponding Cu6Sn5 and , respectively. Hu et al. and the corresponding Cu6 Sn5 and NiIMC , respectively. HuNi et3Sn al.4[15] studied the growth 3 Sn4were [15] studied the growth kinetics molten of Cu-Sn IMC between Sn-58Bi Cu, and foundfollowed that the kinetics of Cu-Sn IMC between Sn-58Bi and Cu, molten and found that and the IMC thickness IMC thickness followed the linear growth on soldering temperature. Li etstudied al. [16] the systemically studied the linear growth on soldering temperature. Li et al. [16] systemically effect of the third the effect element of the third alloying element on IMC growth molten Sn-58Bithat on Cu, that only alloying on IMC growth of molten Sn-58Bi onofCu, and found onlyand Zn found was effective to Zn was effective to depress the IMC growth. depress the IMC growth. ItIt seems seems that that Bi Bi addition addition into into Sn Sn did did not not enroll enroll into into the the interfacial interfacial Cu-Sn Cu-Sn reaction reaction during during the the formation formation of of Cu-Sn Cu-Sn IMCs. IMCs. However, However, in Sn-58Bi/Cu Sn-58Bi/Cujoint, joint,our ourprevious previouspaper paper[17] [17]and andZou Zou et et al. al. [18] [18] have formation of of Bi-rich Bi-richlayer layerand andthe thesegregation segregationofofBiBiatoms atomswere wereeasily easily developed have found found that the formation developed at at interface, which would deteriorate joint strength. Therefore, Bi existence in Sn-Bi solder thethe interface, which would deteriorate thethe joint strength. Therefore, the the Bi existence in Sn-Bi solder will will the interfacial behaviors occurring in Sn It is necessary to study the effect of affectaffect the interfacial behaviors occurring in Sn and Cu.and It is Cu. necessary to study the effect of different Bi different Bi addition into Sn on the interfacial reaction on Cu. In this study, the relationship between addition into Sn on the interfacial reaction on Cu. In this study, the relationship between Bi distribution Bi distribution from different Bi addition in Sn-Bi solder, the interfacial characteristics at the interface and the resulting joint strength were systematically studied. This paper selects Sn-Bi solders with

Materials 2017, 10, 920

3 of 16

Materials 2017, 10, Bi 920addition in Sn-Bi solder, the interfacial characteristics at the interface and the resulting 3 of 16 from different

joint strength were systematically studied. This paper selects Sn-Bi solders with different Bi content different Bi content ranging from 5–58 wt %, and the effect of Bi content on the microstructure of ranging from 5–58 wt %, and the effect of Bi content on the microstructure of solders, the interfacial solders, the interfacial behaviors between solder and Cu during soldering and the following aging behaviors between solder and Cu during soldering and the following aging conditions, and the shear conditions, and the shear strength on ball joints are studied. strength on ball joints are studied.

2. 2. Experimental Experimental Sn-Bi solderswith with different Bi content were manufactured in this study. The detailed Sn-Bi solders different Bi content were manufactured in this study. The detailed compositions compositions include two solid solution ofcompositions of Sn-5Bi and Sn-15Bi, two hypoeutectic include two solid solution compositions Sn-5Bi and Sn-15Bi, two hypoeutectic compositions of compositions of Sn-30Bi and Sn-45Bi, and an eutectic composition of Sn-58Bi. These solder Sn-30Bi and Sn-45Bi, and an eutectic composition of Sn-58Bi. These solder alloys were preparedalloys from were prepared and the Bi. For each solder, Sn and Bi melted raw materials were pure Sn and Bi.from For pure each Sn solder, weighted Sn andthe Bi weighted raw materials were in a ceramic ◦ C for melted ceramic crucible 600protection °C for 30 min with the molten salt of 1.3KCl:1LiCl. crucibleinata 600 30 minat with from the protection molten saltfrom of 1.3KCl:1LiCl. Mechanical stirring ◦ Mechanical stirring was required to homogenize the solder alloy. The molten solder was cooled to was required to homogenize the solder alloy. The molten solder was cooled to 300 C, and then chilled 300 and with then graphite chilled and casted with graphite mold. and°C, casted mold. To prepare the joints for To prepare the joints for interfacial interfacial observation, observation, the the wetted wetted samples samples were were prepared prepared with with solder solder globules on Cu substrate. The solder globule with a weight about 0.2 g was manufactured globules on Cu substrate. The solder globule with a weight about 0.2 g was manufactured in in the the molten rosin. Oxygen-free Cu (OFC) with dimensions of 15 mm × 15 mm × 1 mm was used as the molten rosin. Oxygen-free Cu (OFC) with dimensions of 15 mm × 15 mm × 1 mm was used as the substrate. substrate. Before Before wetting, wetting, the the wetted wetted surface surface on on copper copper plate plate was was carefully carefully polished polished to to aasurface surfacefinish finish with 1 μm using diamond solution, and then ultrasonically cleaned with acetone and distilled with 1 µm using diamond solution, and then ultrasonically cleaned with acetone and distilledwater. water. During During wetting, wetting, the the Sn-Bi Sn-Bi solder solder globule globule was was coated coated with with aa commercial commercial rosin-based rosin-based soldering soldering flux flux ◦ and placed on the Cu plate. The soldering temperature and time was 250 °C and 60 s, and placed on the Cu plate. The soldering temperature and time was 250 C and 60 s,respectively. respectively. To To study study the the interfacial interfacial evolution evolution in in solder solder joints joints under under aging aging conditions, conditions, the the wetted wetted samples samples were were ◦ then aged in silicone oil with an aging temperature of 135 °C and aging time from 10 to 40 days. The then aged in silicone oil with an aging temperature of 135 C and aging time from 10 to 40 days. wetted and aged joints were cross-sectioned for interfacial analysis. To prepare the samples The wetted and aged joints were cross-sectioned for interfacial analysis. To prepare the samples for for microstructural observation, the as-casted solder alloys, the as-wetted and aged samples were then microstructural observation, the as-casted solder alloys, the as-wetted and aged samples were then epoxy-mounted epoxy-mounted and and polished polished to to aa 0.05 0.05 μm µm surface surface finish finish with with colloidal colloidal silica. silica. The The microstructure microstructure in in the solder system and the interfacial structure in the joint system were observed by scanning the solder system and the interfacial structure in the joint system were observed by scanning electron electron microscopy (SEM, ZEISS ZEISSMERLIN, MERLIN, Germany), elemental composition was detected by microscopy (SEM, Germany), andand the the elemental composition was detected by energy energy dispersive spectrum (EDS, OXFORD INCA, Germany). The thickness of IMC layer at the dispersive spectrum (EDS, OXFORD INCA, Germany). The thickness of IMC layer at the interface was interface wasbydetermined byarea dividing area onby theitsIMC layer by its measured and determined dividing the on thethe IMC layer measured length, and was length, averaged bywas five averaged by five SEM images. SEM images. To To evaluate evaluate the the effect effect of of Bi Bi content content on on the the mechanical mechanical properties properties of of Sn-Bi Sn-Bi as-soldered as-soldered joints, joints, aa ball ball shear shear test test was was used used with with the the method method and and the the various various parameters parameterson onprinted printedcircuit circuitboard board(PCB) (PCB) illustrated in Figure 2. Sn-Bi solder ball with the diameter of 0.76 mm was reflowed on Cu illustrated in Figure 2. Sn-Bi solder ball with the diameter of 0.76 mm was reflowed on Cu pad pad with with 0.6 ◦ C for 0.6 mm mm diameter diameter at at the the temperature temperature of of 250 250 °C for 60 60 s. s. The The shear shear test test was was finished finished with with the the shear shear height of 20 20 µm μmand andshear shearspeed speedofof mm/s. After shear tests, fracture surface observed height of 0.10.1 mm/s. After shear tests, thethe fracture surface was was observed with with SEM, and the element mapping at the fracture morphology was detected by EDS. SEM, and the element mapping at the fracture morphology was detected by EDS.

Figure 2. Schematic diagram on the ball shear test. Figure 2. Schematic diagram on the ball shear test.

Materials 2017, 10, 920 Materials 2017, 10, 920

4 of 16 4 of 16

3. Results and Discussion 3. Results and Discussion 3.1. Microstructure of Sn-Bi Solder Alloys 3.1. Microstructure Sn-Bi Solder From the Sn-Biofbinary phaseAlloys diagram shown in Figure 1, there exists an eutectic reaction at 138 ◦C °C with the composition of Sn-58Bi. β-Sn phase is in theFigure solid 1, solution rich in with the solubility limit From the Sn-Bi binary phase diagram shown there exists anBieutectic reaction at 138 of 21the wtcomposition % occurringof at eutectic temperature, there is rich almost solution for Snlimit in Bi. with Sn-58Bi. β-Sn phase is thewhile solid solution in Bino with the solubility of Accordingly, the microstructural evolutionwhile for Sn-Bi system is illustrated 3. Sn-5Bi 21 wt % occurring at eutectic temperature, theresolder is almost no solution for Snin inFigure Bi. Accordingly, or belongs toevolution the solid solution the is temperature line, β-Sn theSn-15Bi microstructural for Sn-Biregion. solderWith system illustrated reaching in Figurethe 3. liquidus Sn-5Bi or Sn-15Bi solid solution will gradually the liquidreaching phase until line. belongs to thephases solid solution region.crystallize With thefrom temperature the reaching liquidus the line,solidus β-Sn solid After the solvus line, Bi will precipitate from the solid solution phases, and the final microstructure solution phases will gradually crystallize from the liquid phase until reaching the solidus line. After for solder of β-Sn solid matrixphases, with finely distributed Bi precipitates, as the the solvus line,isBicomposed will precipitate from thesolution solid solution and the final microstructure for the illustrated in Figure of 3a.β-Sn For Sn-30Bi and Sn-45Bi solder alloys, the Bi composition in Snsolder is composed solid solution matrixhypoeutectic with finely distributed Bi precipitates, as illustrated Bi solder is over the solubility limit in the β-Sn, but less than the eutectic composition. After reaching in Figure 3a. For Sn-30Bi and Sn-45Bi hypoeutectic solder alloys, the Bi composition in Sn-Bi solder is the line during phases will be formed in the liquid solder. overliquidus the solubility limit insolidification, the β-Sn, butthe lessprimary than theβ-Sn eutectic composition. After reaching the liquidus When the solder temperature continually to 138 °C, eutectic reaction occurs When in the left line during solidification, the primary β-Sndecreases phases will be formed in the liquid solder. the liquid solder ◦ phase in order to produce a striped eutectic microstructure with alternating Sn and Bi phases. temperature continually decreases to 138 C, eutectic reaction occurs in the left liquid phase in order Therefore, final microstructure for Sn-30Bi and Sn-45Bi isSn then the primary to producethe a striped eutectic microstructure with alternating andcomposed Bi phases.ofTherefore, the β-Sn final phases surrounded by stripes of the eutectic phases, as illustrated in Figure 3b. Sn-58Bi is the solder microstructure for Sn-30Bi and Sn-45Bi is then composed of the primary β-Sn phases surrounded with an eutectic and therefore an eutectic reaction directly is occurs to produce by stripes of thecomposition, eutectic phases, as illustrated in Figure 3b. Sn-58Bi the solder with the an eutectic eutectic structure when thetherefore temperature of liquid solderdirectly reachesoccurs 138 °C. The finalthe microstructure of Sn-58Bi composition, and an eutectic reaction to produce eutectic structure when ◦ solder is fully composed of the striped eutectic structure with alternating β-Sn and Bi phases, as the temperature of liquid solder reaches 138 C. The final microstructure of Sn-58Bi solder is seen fully in Figure 3c. composed of the striped eutectic structure with alternating β-Sn and Bi phases, as seen in Figure 3c.

Figure Figure 3. 3. Microstructural Microstructural evolution evolution in in Sn-Bi Sn-Bi solder solder system: system: (a) (a) solid solid solution solution Sn-Bi; Sn-Bi; (b) (b) hypoeutectic hypoeutectic Sn-Bi Sn-Bi and and (c) (c) eutectic eutectic Sn-Bi. Sn-Bi.

Figure 4 shows the back scattered-electron (BSE) images on the microstructure for Sn-5Bi, SnFigure 4 shows the back scattered-electron (BSE) images on the microstructure for Sn-5Bi, Sn-15Bi, 15Bi, Sn-30Bi, Sn-45Bi and Sn-58Bi solder alloys. There are two kinds of phases in them: grey β-Sn Sn-30Bi, Sn-45Bi and Sn-58Bi solder alloys. There are two kinds of phases in them: grey β-Sn phases and phases and white Bi-rich phase. The distribution of Bi phases was obviously changed by Bi content. white Bi-rich phase. The distribution of Bi phases was obviously changed by Bi content. Meanwhile, Meanwhile, the compositions of phases for the regions marked in Figure 4 were also detected by EDS the compositions of phases for the regions marked in Figure 4 were also detected by EDS analysis. analysis.

Materials 2017, 10, 920

5 of 16

Materials 2017, 10, 920

5 of 16

Figure 4. SEM images on the microstructure of Sn-Bi solder alloy: (a) Sn-5Bi; (b) Sn-15 Bi; (c) Sn-30Bi; Figure 4. SEM images on the microstructure of Sn-Bi solder alloy: (a) Sn-5Bi; (b) Sn-15 Bi; (c) Sn-30Bi; (d) Sn-45Bi; (e) Sn-58Bi; (f,g) higher magnification on Sn-5Bi and Sn-15Bi solder; and (h) Sn-15Bi with (d) Sn-45Bi; (e) Sn-58Bi; (f,g) higher magnification on Sn-5Bi and Sn-15Bi solder; and (h) Sn-15Bi with furnace cooling. furnace cooling.

As shown in Figure 4a for Sn-5Bi solder, the microstructure was mainly composed of β-Sn grains As shown Figure 4a for Sn-5Bi solder, the microstructure was mainly composedwith of β-Sn grains surrounded by in rarely distributed small Bi particles. Bi distribution was magnified the image surrounded by rarely distributed particles. distribution was magnified thephases. image shown in Figure 4f. It can be clearlysmall seen Bi that Bi phasesBiwere mainly precipitated fromwith the Sn From EDS results on β-Sn phase (region A), there still exists about 3.17 wt % Bi with solid solution in β-Sn after solidification. With Bi content in Sn-Bi increased from 5 wt % to 15 wt %, large amounts of

Materials 2017, 10, 920

6 of 16

shown in Figure 4f. It can be clearly seen that Bi phases were mainly precipitated from the Sn phases. From EDS results on β-Sn phase (region A), there still exists about 3.17 wt % Bi with solid solution in β-Sn after solidification. With Bi content in Sn-Bi increased from 5 wt % to 15 wt %, large amounts of Bi particles were obviously observed both along and within the β-Sn grains. The more greatly magnified BSE image on Sn-15Bi solder is shown in Figure 4g. In region B, the solid solubility of Bi atoms in β-Sn phase was about 5.49 wt % from EDS analysis. Therefore, other Bi atoms were precipitated both along β-Sn phase grains and within β-Sn phase grains with very small sizes such as the region C in Figure 4g, due to the limited solubility of Bi in Sn at room temperature. Therefore, just as shown in Figure 3, the microstructure of Sn-5Bi and Sn-15Bi solid solution solders is composed of the β-Sn matrix embedded with fine Bi particles along or within the β-Sn phase grains. The microstructure in Figure 3 was predicted from the equilibrium crystallization. However, it should be mentioned that the cooling rate within the graphite mold should be much higher than the equilibrium solidification in the present study. There should be some differences between experimental and predicted structures. Therefore, we especially observed the microstructure of Sn-15Bi solder under furnace cooling, as seen in Figure 4h, which was completely different from microstructure in Figure 4b under rapid cooling conditions. Due to the adequate precipitation of Bi atoms from β-Sn, large amounts of bulk Bi phases were observed along β-Sn grains. Because the substrate and the component also provide the cooling channels for Sn-Bi in the solder joints, most Bi atoms will also be kept in β-Sn phases due to the non-equilibrium crystallization. Figure 4c,d show the BSE microstructure of Sn-30Bi and Sn-45Bi hypoeutectic solders. The dark and bright regions are primary β-Sn phases and Sn-Bi eutectic structure, respectively. As mentioned above, the primary β-Sn phases were firstly solidified from the liquid solder, and then the eutectic reaction occurred on the remaining liquid to form the eutectic structure among them. The increase in Bi content in Sn-Bi hypoeutectic solder promotes the increase of the amount of eutectic structure and a decrease in the primary β-Sn phases. Therefore, the microstructure of Sn-30Bi and Sn-45Bi hypoeutectic solders is composed of the primary β-Sn phases surrounded by the networked Sn-Bi eutectic phases. Finally, Figure 4e shows the microstructure of Sn-58Bi solder. It is a typical striped eutectic structure with alternated β-Sn phases and Bi phases due to the instantaneous phase separation during solidification. Similarly, the primary β-Sn phases still formed in the right corner of solder microstructure due to the non-equilibrium solidification. 3.2. Microstructure of Sn-Bi Solder Joints at the Interface Figure 5a–e show the BSE images on the interfacial structure in Sn-Bi solder joints after wetting. It is clearly seen that an IMC layer was produced between Sn-Bi solder and Cu substrate. Its composition was confirmed as Cu6 Sn5 by EDS analysis and was independent of the Bi content in the solder. At the interfaces of Sn-5Bi and Sn-15Bi solder shown in Figure 5a,b, the morphology of IMC layer shows a typical scallop type, and the thickness of IMC layer is about 1 µm. Besides the interfacial Cu6 Sn5 , it is interesting to find that a huge quantity of Cu6 Sn5 IMC flakes was also produced in the solder matrix, but the amount of these IMC flakes in Sn-15Bi solder was less than that in Sn-5Bi. The precipitated Bi particles from soldering were uniformly distributed within the solder matrix. In Sn-30Bi solder joint shown in Figure 5c, the IMC thickness grew a little more than that in Sn-5Bi or Sn-15Bi solder joints. The scallop-type morphology was also observed for the IMC layer. Different from the interfaces in Sn-5Bi or Sn-15Bi, Bi-rich phase segregation was clearly observed in the grooves between the scallop-type grains. In the solder matrix, Bi atoms existed with both small particles and large bulk phases, while Cu6 Sn5 IMC flakes were seldom observed in the bulk solder compared with Figure 5a or Figure 5b. In Sn-45Bi and Sn-58Bi solder joints, as shown in Figure 5d,e, more Bi-rich phases accumulated at the interface between Sn-Bi solder and the Cu6 Sn5 IMC layer, while no Cu6 Sn5 flakes were observed in the solder matrix. The morphology of the IMC layer also became slightly flattened but thicker. The IMC thickness is about 2 µm.

Materials 2017, 10, 920 Materials 2017, 10, 920

7 of 16 7 of 16

Figure Interfacialstructure structureofofSn-Bi Sn-Bisolder solder joints joints after Sn-15 Bi;Bi; (c)(c) Sn-30Bi; Figure 5. 5. Interfacial after soldering: soldering:(a) (a)Sn-5Bi; Sn-5Bi;(b) (b) Sn-15 Sn-30Bi; Sn-45Bi; and Sn-58Bi. (d)(d) Sn-45Bi; and (e)(e) Sn-58Bi.

The effect of aging on the interfacial behaviors of solder joints was then studied. Figure 6 The effect of aging on the interfacial behaviors of solder joints was then studied. Figure 6 illustrates illustrates the representative backscattered electron (BSE) images on Sn-Bi/Cu samples aged at 135 ◦ C with the°Crepresentative backscattered electron images on Sn-Bi/Cu samplesobviously aged at 135 with aging duration of 10, 20, 30 and 40 (BSE) days, respectively. The IMC thickness increased aging oftime. 10, 20, and 40 respectively. Thewith IMCincreasing thickness obviously withduration the aging In30 Sn-5Bi anddays, Sn-15Bi solder joints, of the IMCincreased thickness with at thethe aging time. the In Sn-5Bi solder joints,between with increasing of the thickness at the interface, interface, Cu3Sn and IMCSn-15Bi layer was produced Cu6Sn5 and Cu,IMC which was sketched by the thered Cuarea Sn IMC layer was produced between Cu Sn and Cu, which was sketched by the red area in 3 6 with 5 in Figure 6, and its thickness also increased the aging time. In the solder matrix, because Figure 6, and its thickness also increased with the aging time. In the solder matrix, because some some Bi atoms were dissolved into the β-Sn grains during the soldering condition, they were Bi atoms were dissolved into Bi theparticles β-Sn grains during the soldering condition, they were segregated segregated to form small during isothermal aging, which were uniformly distributed in to form particles isothermal uniformly distributed in the solder thesmall solderBimatrix as during observed in Figureaging, 6. Thewhich large were amounts of Cu6Sn 5 IMC flakes formed inmatrix the seen large in Figure 5a,b, were combined thesoldering aging time. In Snas soldering observed condition, in Figure as 6. The amounts of Cucoarsened flakes formedwith in the condition, 6 Sn5 IMCand Sn-45Bi5a,b, hypoeutectic solder joints, the most important observation at Sn-30Bi the interface the as 30Bi seenand in Figure were coarsened and combined with the aging time. In and is Sn-45Bi obvious growth of joints, Cu6Sn5 the layer andimportant the prohibition on the formation of Cu3Sn betweengrowth Cu6Sn5 of hypoeutectic solder most observation at the interface is layer the obvious Cu. Bi-rich phases were obviously coarsened in the solder matrix and were also accumulated at Cuand 6 Sn5 layer and the prohibition on the formation of Cu3 Sn layer between Cu6 Sn5 and Cu. Bi-rich

Materials 2017, 10, 920

8 of 16

Materials 10, 920 of 16 phases were2017, obviously coarsened in the solder matrix and were also accumulated at the8 interface between solder and IMC layer due to the Sn consumption during the reaction to form Cu6 Sn5 . In the the interface between solder and IMC layer due to the Sn consumption during the reaction to form Sn-58Bi solder joint, the rapid growth on the Cu6 Sn5 layer and the inhibition on the Cu3 Sn layer were Cu6Sn5. In the Sn-58Bi solder joint, the rapid growth on the Cu6Sn5 layer and the inhibition on the similarly observed at the interface, but it is interesting to find that the IMC thickness in the Sn-58Bi Cu3Sn layer were similarly observed at the interface, but it is interesting to find that the IMC thickness solder was thinner thanwas that in Sn-30Bi and solder joints under the same in joint the Sn-58Bi solder joint thinner than that in Sn-45Bi Sn-30Bi and Sn-45Bi solder joints under theaging same time. At the interface between Sn-58Bi solderSn-58Bi and IMC layer, the continuous layerBi-rich always existed aging time. At the interface between solder and IMC layer, the Bi-rich continuous layer during the aging duration. always existed during the aging duration.

Figure 6. Interfacial evolution of Sn-Bi solder with different Bi content the aging Figure 6. Interfacial evolution of Sn-Bi solder jointsjoints with different Bi content underunder the aging condition. condition.

It is Itwell known that thetheIMC interfaceininsolder solder joints follows empirical is well known that IMCgrowth growth at at the the interface joints follows the the empirical diffusion formula: diffusion formula: √ X = Dt + X0 , (1) =√

+

,

(1)

where X is the IMC thickness, XX IMCthickness, thickness, time isdiffusivity the diffusivity 0 0isisthe the initial initial IMC t ist is thethe time andand D isD the where X is total the total IMC thickness, of theofIMC layer. Therefore, the thickness(X(X plotted with the square the IMC layer. Therefore, thegrowth growthof of IMC IMC thickness − X−0) X was plotted with the square root of root 0 ) was 1/2)) with of aging days withthe theresults results shown in Figure It is clearly seenSn-5Bi that Sn-5Bi and Sn-15Bi aging days(t(t1/2 shown in Figure 7. It 7. is clearly seen that and Sn-15Bi had the had 1/2 1.2 1/2 , respectively. 1/2 and 1/2, respectively. lowest rate rate of μm/day With Bi content in Bi Sn-Bi solderin increasing the lowest of1.0 1.0μm/day µm/day and 1.2 µm/day With content Sn-Bi solder to 30 wt % and 45 wt %, the rapid growth on the IMC layer occurred at the interface with the growing with increasing to 30 wt % and 45 wt %, the rapid growth on the IMC layer occurred at the interface 1/2 and 5.4 μm/day 1/2, respectively.1/2 rate of 4.6rate μm/day with Bi content in the solder reachingin the 1/2 and the growing of 4.6 µm/day 5.4 µm/day However, , respectively. However, with Bi content the eutectic composition of Sn-58Bi, the growth rate of the IMC layer at the interface decreased to 3.0 solder reaching the eutectic composition of Sn-58Bi, the growth rate of the IMC layer at the interface μm/day1/2. Therefore, in all of these Sn-Bi solders, Sn-5Bi and Sn-45Bi solder had the lowest and decreased to 3.0 µm/day1/2 . Therefore, in all of these Sn-Bi solders, Sn-5Bi and Sn-45Bi solder had the highest growth rate on IMC thickness, respectively. lowest and highest growth rate on IMC thickness, respectively.

Materials 2017, 10, 920

9 of 16

Materials 2017, 10, 920

9 of 16

Figure 7. Linear relationship betweenthe thethickness thickness of the square rootroot of aging daysdays in in Figure 7. Linear relationship between of total totalIMC IMCand and the square of aging Sn-Bi solder joints. Sn-Bi solder joints.

In the case of the formation of Cu6Sn5 IMC layer at the interface, it is commonly obtained from

In case of the formation Cu6 Sn layer at the interface, it is commonly obtained from thethe reaction between Sn and Cuofatoms with the following reaction: 5 IMC the reaction between Sn and Cu atoms with the following reaction: 6Cu + 5Sn → Cu6Sn5,

(2)

where Cu and Sn atoms are from the solder respectively. In the case of the (2) 6Cu +matrix 5Sn →and CuCu 6 Snsubstrate, 5, formation of the Cu3Sn IMC layer at the interface during isothermal aging, it is obtained from the wherefollowing Cu and reactions: Sn atoms are from the solder matrix and Cu substrate, respectively. In the case of the

formation of the Cu3 Sn IMC layer at the 3Cu interface during aging, it is obtained from (3) the + Sn → Cu3Snisothermal , following reactions: (4) 9Cu + Cu6Sn5 → 5Cu3Sn, 3Cu + Sn → Cu3 Sn, (3) where the latter reaction plays the dominant role by consuming the Cu6Sn5 IMC layer. In the case of + Cu 6 Snsolder 5 → 5Cu 3 Sn, it is also obtained from the reaction (4) the formation of Cu6Sn5 particles or9Cu flakes in the matrix, Cu and Sn, which means that Cu atoms must diffuse through the Cu-Sn IMC layer into the wherebetween the latter reaction plays the dominant role by consuming the Cu6 Sn5 IMC layer. In the case of solder matrix with a longer distance. the formation of Cu6 Sn5 particles or flakes in the solder matrix, it is also obtained from the reaction From Figure 7, it can be noted that Bi addition into Sn had the obvious effect on the growth rate between Cu and Sn, which means that Cu atoms must diffuse through the Cu-Sn IMC layer into the of IMC at the interface. In the Sn-Bi/Cu system, Zou et al. [18] verified that Bi atoms were also an solderimportant matrix with a longer distance. diffusing component to be partly dissolved into Cu6Sn5 IMC. From Figures 5 and 6, the From Figure 7, itSn can be noted that addition into Sn of had theIMC obvious the but growth addition of Bi into solder changed notBi only the distribution Cu-Sn in theeffect solder on matrix rate of IMC the interface. In the Sn-Bi/Cu system,Therefore, Zou et al. that atoms were also the at morphology of Cu-Sn IMC at the interface. we[18] try verified to explain theBi interfacial behaviors from diffusing the changecomponent on the diffusing fluxes of atoms caused from Bi addition. The also an important to be partly dissolved into Cudifferent From Figures 5 6 Sn5 IMC. schematic diagrams on the effect of different Bi content in the Sn-Bi solder on the interfacial and 6, the addition of Bi into Sn solder changed not only the distribution of Cu-Sn IMC in the solder morphology and morphology IMC growth behavior are IMC illustrated in Figure 8. It should be noted the the matrix but also the of Cu-Sn at the interface. Therefore, wethat try Cu to was explain dominant species in Cu-Sn interdiffusion [19]. In Sn-5Bi or Sn-15Bi solder joints illustrated in Figure interfacial behaviors from the change on the diffusing fluxes of atoms caused from different Bi addition. 8a, a Cu6Sn5 IMC layer with the scallop-type morphology was produced during soldering. Cu atoms The schematic diagrams on the effect of different Bi content in the Sn-Bi solder on the interfacial were also diffused from the substrate into solder through the groove between two scallops, and then morphology and IMC growth behavior are illustrated in Figure 8. It should be noted that Cu was the reacted with Sn atoms to form huge Cu6Sn5 precipitates within the solder matrix. During isothermal dominant in Cu-Sn interdiffusion [19]. not In Sn-5Bi or Sn-15Bi solder joints illustrated Figure 8a, aging,species Cu atoms diffused from the substrate only reacted with Sn atoms diffused from theinsolder a Cu6to Snpromote with the scallop-type morphology was produced during soldering. Cu atoms 5 IMC layer the continuous growth of Cu6Sn5 IMC layer, but also reacted with the existing Cu6Sn5 IMC were layer also diffused from the substrate into solder through the groove between two scallops, and to form a Cu3Sn layer between Cu6Sn5 and Cu. For the Cu6Sn5 IMC layer at the interface, besides then thewith transition to Cuto 3Sn, it was alsoCu broken produce Cuwithin 6Sn5 flakes the solder matrix closeisothermal to the reacted Sn atoms form huge precipitates the in solder matrix. During 6 Sn5 to Thediffused growth on IMCthe thickness wasnot attributed to them. In Sn-30Bi or Sn-45Bi solder joint, aging,interface. Cu atoms from substrate only reacted with Sn atoms diffused from theas solder shown in Figure 8b, Bi particles were precipitated in the grooves of the scallop-type Cu 6Sn5 grains to promote the continuous growth of Cu6 Sn5 IMC layer, but also reacted with the existing Cu6 Sn5 during soldering, which was not observed in Figure 8a because Bi atoms were mainly dissolved in βIMC layer to form a Cu3 Sn layer between Cu6 Sn5 and Cu. For the Cu6 Sn5 IMC layer at the interface, Sn phases to produce the solid solution phases for Sn-5Bi or Sn-15Bi. Therefore, the diffusion of Cu besides the transition to Cu3 Sn, it was also broken to produce Cu6 Sn5 flakes in the solder matrix close atoms into the solder matrix through the grooves of scallops was prohibited. On the other hand, with to thethe interface. The growth on IMC thickness was attributed to them. In Sn-30Bi or Sn-45Bi solder growing on interfacial Cu6Sn5 IMC, more Bi atoms were also accumulated at the interface between

joint, as shown in Figure 8b, Bi particles were precipitated in the grooves of the scallop-type Cu6 Sn5 grains during soldering, which was not observed in Figure 8a because Bi atoms were mainly dissolved in β-Sn phases to produce the solid solution phases for Sn-5Bi or Sn-15Bi. Therefore, the diffusion of

Materials 2017, 10, 920

10 of 16

Cu atoms into the Materials Materials 2017, 2017, 10, 10, 920 920

solder matrix through the grooves of scallops was prohibited. On the other10hand, 10 of of 16 16 with the growing on interfacial Cu6 Sn5 IMC, more Bi atoms were also accumulated at the interface between solder IMCdue layer the depletion Sn atoms, but enough produce solder and the IMC layer to the depletion of atoms, but not enough to produce aa solder and the and IMCthe layer due to due the to depletion of Sn Snof atoms, but were werewere not not enough to to produce a continuous Bi-rich layer. These enriched Bi phases would inevitably promote the diffusion of continuous Bi-rich layer. These enriched Bi phases would inevitably promote the diffusion of Bi continuous Bi-rich layer. These enriched Bi phases would inevitably promote the diffusion of Bi atoms from to Cu Sn according according atoms Cu Cu atoms from interface interface into into the the Cu Cu666Sn Sn555 phases. phases. During During the the transition transition from from Cu Cu666Sn Sn555 to Cu333Sn Sn according to Equation Equation (4), (4), the the Bi Bi atoms atoms were were then thenprecipitated precipitated to to produce produce Bi Bi segregation segregation at at the the interface interface between between to to Equation (4), the Bi atoms were then precipitated to produce Bi segregation at the interface between Cu Sn and Cu accompanied with the formation of Sn. Figure 9a,b show the enlarged images on Cu 3 Sn and Cu accompanied with the formation of Cu 3 Sn. Figure 9a,b show the enlarged images Cu33Sn and Cu accompanied with the formation of Cu33Sn. Figure 9a,b show the enlarged images on on the interfaces interfaces and Sn-45Bi/Cu Sn-45Bi/Cuaged agedjoints jointswith with30 30 days days aging, aging, respectively. respectively. The The Bi the Sn-30Bi/Cu the interfaces of of Sn-30Bi/Cu Sn-30Bi/Cu and and Sn-45Bi/Cu aged joints with 30 days aging, respectively. The Bi segregation is Sn and Cu, which may be attributed to the reason that segregation is clearly observed between Cu 6 Sn 5 and Cu, which may be attributed to the reason that 6 5 segregation is clearly observed between Cu6Sn5 and Cu, which may be attributed to the reason that the solubility solubility of Sn was was lower lower than than that that in in Cu Cu666Sn Sn555[20]. [20]. the the solubility of Bi Bi atoms atoms in in Cu Cu333Sn Sn was lower than that in Cu Sn [20].

(a) (a)

(b) (b)

(c) (c)

Figure diagrams on the interfacial behavior in (a) Sn-5Bi/Sn-15Bi; (b) Sn-30Bi/Sn-40Bi; Figure 8. diagrams on on the the interfacial interfacial behavior behaviorin in(a) (a)Sn-5Bi/Sn-15Bi; Sn-5Bi/Sn-15Bi; (b) (b) Sn-30Bi/Sn-40Bi; Sn-30Bi/Sn-40Bi; 8. Schematic Schematic diagrams and and (c) (c) Sn-58Bi Sn-58Bi solder solder joints. joints.

Figure at the interface between Cu Sn Cu (a) Sn-30Bi; (b) and SnFigure 9. 9. BiBisegregation segregation atat thethe interface between Cu66Cu Sn556and and Cu in inCu (a)in Sn-30Bi; (b) Sn-45Bi; Sn-45Bi; and (c) (c)and Sn9.Bi segregation interface between Sn (a) Sn-30Bi; (b) Sn-45Bi; 5 and 58Bi aged solder joint with a time of 30 days. 58Bi aged solder joint with timeaoftime 30 days. (c) Sn-58Bi aged solder jointawith of 30 days.

From Figure 6, it is interesting to find Cu From find that that the the formation formation of of Cu Cu33Sn Sn was was very very little. little. The The transition transition From Figure Figure 6, 6, it it is is interesting interesting to to find that the formation of 3 Sn was very little. The transition from Cu 66Sn 55 to Cu 33Sn was almost depressed at the interface because the dissolution of Bi atoms into from Cu Sn to Cu Sn was almost depressed at the interface because the dissolution of Bi atoms into from Cu6 Sn5 to Cu3 Sn was almost depressed at the interface because the dissolution of Bi atoms IMC possibly increased the driving force on the formation of Cu 3 Sn. On the other hand, IMC possibly increased the the driving force onon thetheformation On the other hand, Bi Bi into IMC possibly increased driving force formationofofCu Cu3Sn. 3 Sn. On the other hand, Bi precipitation in the grooves of Cu 6 Sn 5 also inhibited the diffusion of Cu atoms the solder matrix. precipitation in the grooves of Cu 6Sn5 also inhibited the diffusion of Cu atoms into into the solder matrix. precipitation in the grooves of Cu6 Sn5 also inhibited the diffusion of Cu atoms into the solder matrix. Therefore, the diffusing Cu atoms were mainly accumulated at the which promoted the Therefore, the thediffusing diffusingCu Cuatoms atoms were mainly accumulated at interface, the interface, interface, which promoted the Therefore, were mainly accumulated at the which promoted the rapid rapid growth of the thickness of the Cu 6 Sn 5 layer during isothermal aging due to the lack of formation rapid growth the thickness theSn Cu6layer Sn5 layer during isothermal aging to the lack of formation growth of theof thickness of theofCu during isothermal aging duedue to the lack of formation of 6 5 of Cu 33Sn at the interface and Cu 66Sn 55 phases in the solder matrix. Similarly, as illustrated in 8c of Cu Sn at the interface and Cu Sn phases in the solder matrix. Similarly, as illustrated in Figure Figure 8c Cu3 Sn at the interface and Cu6 Sn5 phases in the solder matrix. Similarly, as illustrated in Figure 8c for for Sn-58Bi solder, the from Sn-30Bi or hypoeutectic solder joint is the for Sn-58Bi solder, the difference difference Sn-30Bi or Sn-45Bi Sn-45Bi hypoeutectic solder is formation the formation formation Sn-58Bi solder, the difference fromfrom Sn-30Bi or Sn-45Bi hypoeutectic solder jointjoint is the of a of a continuous Bi-rich layer close to the interface during isothermal aging. The transition from Cu66Sn 5 of a continuous Bi-rich layer close to the interface during isothermal aging. The transition from continuous Bi-rich layer close to the interface during isothermal aging. The transition from Cu6Cu Sn5Sn to5 to Cu33Sn to Sn was was also also depressed depressed by by Bi Bi segregation segregation at at the the interface interface observed observed in in Figure Figure 9c. 9c. The The continuous continuous CuCu 3 Sn was also depressed by Bi segregation at the interface observed in Figure 9c. The continuous Bi-rich layer partly blocked the interdiffusion of the Cu-Sn couple, and accordingly caused a slower Bi-rich Cu-Sn couple, Bi-rich layer layer partly partly blocked blocked the the interdiffusion interdiffusion of of the the Cu-Sn couple, and and accordingly accordingly caused caused aa slower slower growth rate of IMC at the interface compared with Sn-30Bi or Sn-45Bi hypoeutectic solder joint. In growth oror Sn-45Bi hypoeutectic solder joint. In all all growth rate rate of of IMC IMCat atthe theinterface interfacecompared comparedwith withSn-30Bi Sn-30Bi Sn-45Bi hypoeutectic solder joint. In of these solder joints, another effect on the IMC growth rate is the ratio of aging temperature to the of these solder joints, another effect on the IMC growth rate is the ratio of aging temperature to the all of these solder joints, another effect on the IMC growth rate is the ratio of aging temperature to solidus of which is as 0.93, 0.99, 0.99 and 0.99 for Sn-15Bi, Sn-30Bi, Snsolidus of solder, solder, which is calculated calculated as 0.84, 0.84, 0.93,0.93, 0.99,0.99, 0.990.99 and and 0.990.99 for Sn-5Bi, Sn-5Bi, Sn-15Bi, Sn-30Bi, Snthe solidus of solder, which is calculated as 0.84, for Sn-5Bi, Sn-15Bi, Sn-30Bi, 45Bi 45Bi and and Sn-58Bi, Sn-58Bi, respectively. respectively. It It means means that that there there exists exists aa higher higher diffusion diffusion rate rate of of Cu Cu and and Sn Sn atoms atoms in in Sn-Bi Sn-Bi solders solders with with higher higher Bi Bi content. content.

Materials 2017, 10, 920

11 of 16

Sn-45Bi and Sn-58Bi, respectively. It means that there exists a higher diffusion rate of Cu and Sn atoms Materials 10, 920 11 of 16 in Sn-Bi2017, solders with higher Bi content. 3.3.Materials 10, 920 on Sn-Bi Solder Joints Shear 2017, Properties

11 of 16

Figure displacement-load curves after ball shear tests on Sn-Bi as-soldered joints Figure 10 showson the displacement-load curves 3.3. Shear Properties Sn-Bi Solder Joints and the corresponding strength ofof Sn-Bi correspondingaverage averageshear shearstrength strengthwith withthe theeffect effectofofBiBicontent. content.The Theshear shear strength SnFigure 10 shows the displacement-load curves after ball shear tests on Sn-Bi as-soldered joints solder joints decreased withwith the increase in Bi content, and then increased for Sn-58Bi Bi solder joints decreased the increase in Bi content, andslightly then slightly increased for eutectic Sn-58Bi and the corresponding shear strength theon effect of Bi content. The shear strength of Sn-the solder. To obtain the effectaverage of Bi content Sn-Biwith solder the crack and failure mode, eutectic solder. To obtain the effect of Biincontent in Sn-Bi solder on propagation the crack propagation and failure Bi solder joints decreased with the increase in Bi content, and then slightly increased for Sn-58Bi fracture surface morphologies and the corresponding element mapping were detectedwere by EDS, and the mode, the fracture surface morphologies and the corresponding element mapping detected by eutectic solder. To obtain the effect of Bi content in Sn-Bi solder on the crack propagation and failure results arethe shown in Figures 11–15. The shear direction wasdirection always from to down EDS, and results are shown in Figures 11–15. The shear was up always fromfor upall to solder down mode, the fracture surface morphologies and the corresponding element mapping were detected by joints thesejoints images. for allin solder in these images. EDS, and the results are shown in Figures 11–15. The shear direction was always from up to down for all solder joints in these images.

(a) (a)

(b)

(b)

Figure 10. of of Bi content in Sn-Bi as-soldered joints on (a) load-displacement curves and (b) shear Figure 10.Effect Effect Bi content in Sn-Bi as-soldered joints on (a) load-displacement curves and Figure 10. Effect of Bi content in Sn-Bi as-soldered joints on (a) load-displacement curves and (b) shear strength. (b) shear strength. strength.

Figure 11. Fracture morphologies of as-soldered Sn-5Bi solder bump: (a) SEM image and elemental Figure 11. 11. Fracture Fracture morphologies morphologies of of as-soldered as-soldered Sn-5Bi Sn-5Bi solder solder bump: bump: (a) (a) SEM SEM image image and and elemental elemental Figure mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b). mapping; (b) interfacial fracture at at higher higher magnification; magnification; and and (c) (c) EDS EDS analysis analysis on on image image (b). (b).

Materials 2017, 10, 920 Materials 2017, 10, 920 Materials 2017, 10, 920

12 of 16 12 of 16 12 of 16

Figure 12. Fracture morphology as-soldered Sn-15Bi Sn-15Bi solder (a)(a) SEM image andand elemental Figure 12. Fracture morphology ofof solderbump: bump: SEM image elemental Figure 12. Fracture morphology ofas-soldered as-soldered Sn-15Bi solder bump: (a) SEM image and elemental mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b). mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b). mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b).

Figure 13. Fracture morphology of as-soldered Sn-30Bi solder bump: (a) SEM image and elemental

Figure 13. Fracture morphology as-soldered Sn-30Bi Sn-30Bi solder (a)(a) SEM image andand elemental Figure 13. Fracture morphology ofofas-soldered solderbump: bump: SEM image elemental mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b). mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b). mapping; (b) interfacial fracture at higher magnification; and (c) EDS analysis on image (b).

During the ball shear test, because there exists a shear height about 20 μm between the shear During the ball shear test, because there exists a shear height about 20 μm between the shear tool and the ball PCB surface, thebecause shear force is always on the bulk solder ball, and the the joint During test, there exists imposed a shear height about 20 µm between tool and the the PCB shear surface, the shear force is always imposed on the bulk solder ball, and the jointshear strength accordingly reflects the bulk solder strength or the interfacial adhesion strength between tool and the PCB surface, the shear force is always on the bulk solder ball, and the joint strength accordingly reflects the bulk solder strengthimposed or the interfacial adhesion strength between solder and Cu substrate. If the shear strength of the bulk solder ball is higher than the interfacial solder and Cu substrate. the bulk shearsolder strength of the bulk solder ball is higher than strength the interfacial strength accordingly reflectsIfthe strength or the interfacial adhesion between adhesion strength, the fracture propagates along the interface between solder and Cu substrate, adhesion the If fracture propagates along the bulk interface between and Cu the substrate, solder and Custrength, substrate. the shear strength of the solder ball issolder higher than interfacial which represents a brittle failure mode. Otherwise, ductile failure mode will be observed with the which represents a brittle failure mode. Otherwise, ductile failure mode will be observed with the adhesion strength, the fracture propagates along the interface between solder and Cu substrate, which fracture occurring inside the bulk solder ball. Meanwhile, there also exists the quasi-ductile failure fracture occurring inside the bulk solder ball. Meanwhile, there also exists the quasi-ductile failure represents a brittle mode. Otherwise, failure mode be observed with thewith fracture mode with morefailure than 50% fracture occurringductile inside the solder or thewill quasi-brittle failure mode mode with more than 50% fracture occurring inside the solder or the quasi-brittle failure mode with occurring the fracture bulk solder ball. Meanwhile, there also exists the quasi-ductile failure mode with more inside than 50% occurring along the interface, which represents a combination of partial more than 50% fracture occurring along the interface, which represents a combination of partial failure mode and partial brittle mode.or The brittle failure area with fracture propagated moreductile than 50% fracture occurring insidefailure the solder the quasi-brittle failure mode with more than ductile failure mode and partial brittle failure mode. The brittle failure area with fracture propagated

50% fracture occurring along the interface, which represents a combination of partial ductile failure mode and partial brittle failure mode. The brittle failure area with fracture propagated along the interface is distinguished with the prominent Cu element mapping from the ductile failure area, and

Materials 2017, 10,10, 920 Materials 2017, 920

1316 of 16 13 of

along the interface is distinguished with the prominent Cu element mapping from the ductile failure is also in Figures It seems Bi addition into Sn-based solder, solder, even with wt %5 Bi area,marked and is also marked 11–15. in Figures 11–15.that It seems that Bi addition into Sn-based even5with content, inevitably produces brittle failure mode, which is completely different from the results wt % Bi content, inevitably produces brittle failure mode, which is completely different from theon Sn-Ag-Cu Pb-free solder.Pb-free The ballsolder. shear test Sn-Ag-Cu as-soldered joint always produces ductile results on Sn-Ag-Cu Theon ball shear test on Sn-Ag-Cu as-soldered jointthe always failure inside bulkfailure solderinside [21,22]. difference is Their attributed to the reason that solder produces thethe ductile the Their bulk solder [21,22]. difference is attributed to Sn-Bi the reason exhibits a higher strength but a weaker ductility than Sn-Ag-Cu solder [23] and the Bi element is very that Sn-Bi solder exhibits a higher strength but a weaker ductility than Sn-Ag-Cu solder [23] and the Bi element is very prone to embrittlement observed from the fracture morphologies, the prone to embrittlement [6]. As observed from[6]. the As fracture morphologies, the fracture surfaces almost fracture almost containfailure all themodes: above-mentioned failure modes: quasi-ductile mode for Sncontain all surfaces the above-mentioned quasi-ductile mode for Sn-5Bi (Figure 11), quasi-brittle 5Bi for (Figure 11), (Figure quasi-brittle modemode for Sn-15Bi (Figure 12), brittle(Figures mode for mode Sn-15Bi 12), brittle for Sn-30Bi and Sn-45Bi 13Sn-30Bi and 14) and and Sn-45Bi ductile to (Figures 13 and 14) and ductile to quasi-ductile mode for Sn-58Bi (Figure 15). quasi-ductile mode for Sn-58Bi (Figure 15).

Figure Fracturemorphology morphologyof ofas-soldered as-soldered Sn-45Bi Sn-45Bi solder and elemental Figure 14.14. Fracture solderbump: bump:(a) (a)SEM SEMimage image and elemental mapping; interfacialfracture fractureatathigher highermagnification; magnification; and mapping; (b)(b) interfacial and (c) (c) EDS EDSanalysis analysison onimage image(b). (b).

Figure 15. Fracture morphology of as-soldered Sn-58Bi solder bump: (a) SEM image and elemental mapping; (b) higher magnification on fracture surface with the observation of ductile solder and brittle interface; and (c) higher magnification on the brittle area in image (b).

Materials 2017, 10, 920

14 of 16

In Sn-5Bi solder joints, the performance of bulk solder matrix was obviously improved by both Bi solid solution in the β-Sn phases and large amounts of Cu6 Sn5 flakes produced from the soldering reaction, as observed in Figure 5a. Therefore, during the ball shear test, partial brittle fracture occurred at the interface with the circular area marked in Figure 11a for Sn-5Bi solder bump, which represents the quasi-ductile failure mode because the retained solder area is more than 50%. The circular area was then observed with a 10,000× magnification and the fracture morphology is shown in Figure 11b. EDS analysis was used to confirm the composition at this fracture surface with the result shown in Figure 11c. The interfacial fractured area still contains much Sn-Bi solder with large amounts of Sn and a small amount of Bi besides the Cu atoms from Cu6 Sn5 . Figure 12 shows the fracture morphologies, elemental mapping and the EDS result on the interfacial fracture area for Sn-15Bi solder joints. Compared with the Sn-5Bi solder bump, a more brittle result occurred along the interface to reach a quasi-brittle failure mode with the interfacial fracture area more than 50%. Similarly, the brittle fracture was also observed with a higher magnification with the morphology shown in Figure 12b. It is a typical scallop-type IMC exposed fracture morphology with very little retained solder. The composition of scallops was confirmed as the Cu6 Sn5 phase by the EDS spectrum in Figure 12c. Therefore, although both of the solder alloys were solid solution compositions, more Bi content in Sn-15Bi solder further increased the solder strength and resulted in more fractures along the interface instead of inside the bulk solder and less retained solder on the IMC exposed area compared with Sn-5Bi solder. With Bi content in Sn-based solder increased to 30% or 45% in order to constitute the Sn-Bi hypoeutectic solder alloy, the fracture of solder joints in Figure 13 or Figure 14 completely occurred along the interface between solder and Cu substrate with a full brittle failure mode, although there still existed a small retained solder area in Figure 14a for the Sn-45Bi solder joint. From the elemental mapping on fracture morphologies of Sn-30Bi and Sn-45Bi, it is interesting to find that the whole fracture was covered with the uniform distribution of Bi atoms besides the Cu atoms and Sn atoms from Cu6 Sn5 . Figures 13b,c and 14b,c show the magnified observation on the brittle fracture area and the corresponding compositional analysis in Sn-30Bi and Sn-45Bi, respectively. In contrast to Sn-5Bi or Sn-15Bi solder bump, the solder residues and cleavage fracture from coarsened Bi phases were easily observed with the amounts increased with Bi content, which came from the Bi precipitation in the grooves or near the interface in Figure 5c,d. Figure 15 shows the elemental mapping on Sn-58Bi eutectic solder bump and the enlarged fracture morphologies. Most fractures occurred inside the solder. From the fracture morphology on the Sn-58Bi solder joint in Figure 15a, most fractures occurred inside the bulk solder ball, which illustrates a ductile or ductile-to brittle fracture mode. With a highly magnified observation on the interfacial brittle area shown in Figure 15c, small Bi particles were uniformly distributed on the IMC layer. The shear strength of Sn-Bi solder bumps should be decided by the performances of solder and the fracture modes during ball shear tests. Ye et al. [6] studied the effect of Bi content in Sn-Bi solder on the mechanical performance of bulk solder alloys, and found that the hardness of Sn-Bi solder reached the highest with Bi content at 7–15 wt %, and then decreased with continuous increasing on Bi content in Sn-Bi solder. Therefore, Sn-5Bi and Sn-15Bi solder bumps show a higher shear strength in Figure 10, and the fracture mode also transits from quasi-ductile to quasi-brittle with the cracks mainly occurring inside the solder bump to the interface between the solder and IMC layer. However, it is interesting to discover that the fractures for Sn-30Bi and Sn-45Bi solder bumps fully occurred at the interface because they had a lower strength than Sn-5Bi and Sn-15Bi solder bumps. As shown in Figure 8b, large amounts of Bi elements were segregated at the interface between the bulk solder and Cu6 Sn5 IMC layer. In Figures 13b and 14b, we also observed the existence of Bi cleavage at the fracture surfaces. It seems that the degradation on Sn-30Bi and Sn-45Bi solder joints were mainly attributed to Bi segregation at the interface. The brittle fracture mode occurring along the interface produces a lower shear strength on Sn-30Bi and Sn-45Bi solder bumps. Sn-58Bi solder had the lowest strength out of all of the Sn-Bi solder alloys in this study; accordingly, the fracture mainly occurred inside the

Materials 2017, 10, 920

15 of 16

solder bump because the shear strength of solder bump was lower than the interfacial strength, but we still observed the existence of Bi segregation at the interface from the small brittle area in Figure 15c. Sn-58Bi solder joint also shows a slightly higher shear strength due to its ductile fracture mode. 4. Conclusions Sn-Bi solders with different Bi content can realize a low-to-medium-to-high soldering process with the melting temperature ranging from 138 ◦ C to 232 ◦ C. The addition of Bi into Sn changed not only the melting point of solder but also the microstructure, interfacial behaviors in solder with Cu and joint strength. To obtain the effect of Bi content in Sn-Bi solder on them, this study selected five Sn-Bi solders with Bi content ranging from 5–58 wt % including Sn-5Bi and Sn-15Bi solid solution compositions, Sn-30Bi and Sn-45Bi hypoeutectic compositions and Sn-58Bi eutectic composition. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied, and the relationship between them was discussed. The following results can be concluded: (1)

(2)

(3)

(4)

The Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles; hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases; and eutectic Sn-Bi solder was mainly composed of eutectic structure from short striped Sn and Bi phases. During soldering with Cu, Bi content in Sn-Bi solder had an obvious effect on the interfacial behavior of solder joints. In the solder matrix of joints, large amounts of fine Cu6 Sn5 flakes were produced with Bi particles in solid solution Sn-5Bi or Sn-15Bi, while they were almost depressed in hypoeutectic Sn-30Bi or Sn-45Bi and eutectic Sn-58Bi. At the interface of solder/Cu, the increase in Bi content slightly increased the interfacial Cu6 Sn5 IMC thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6 Sn5 . During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The continuous formation of Cu6 Sn5 in the solder matrix partly counteracted the growth of interfacial IMC in solid solution Sn-Bi solder joints, while the accumulation of Bi atoms at the interface promoted the rapid growth of the interfacial Cu6 Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6 Sn5 in solder matrix and the transition from Cu6 Sn5 to Cu3 Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.

Acknowledgments: This research is supported by projects funded by the Jiangsu Planning Project of Science and Technology (Grant No. BK2012163, BK20150466) and Six Talent Peaks Project in Jiangsu Province (Grant No. 2015-XCL-028). Author Contributions: F.W. and C.Y. conceived and designed the experiments; Y.H. performed the experiments; Z.Z. analyzed the data; F.W. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2.

Zhang, L.; Tu, K.N. Structure and properties of lead-free solders bearing micro and nano particles. Mater. Sci. Eng. R Rep. 2014, 82, 1–32. [CrossRef] Sidhu, R.S.; Aspandiar, R.; Vandervoort, S.; Amir, D.; Murtagian, G. Impact of Processing Conditions and Solder Materials on Surface Mount Assembly Defects. JOM 2011, 63, 47–51. [CrossRef]

Materials 2017, 10, 920

3. 4. 5. 6. 7. 8.

9.

10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21.

22. 23.

16 of 16

Kotadia, H.R.; Howes, P.D.; Mannan, S.H. A review: On the development of low melting temperature Pb-free solders. Microelectron. Reliab. 2014, 54, 1253–1273. [CrossRef] Shen, L.; Septiwerdani, P.; Chen, Z. Elastic modulus, hardness and creep performance of SnBi alloys using nanoindentation. Mater. Sci. Eng. A 2012, 558, 253–258. [CrossRef] Lai, Z.; Ye, D. Microstructure and fracture behavior of non eutectic Sn-Bi solder alloys. J. Mater. Sci. Mater. Electron. 2016, 27, 3182–3192. [CrossRef] Ye, D.; Du, C.; Wu, M.; Lai, Z. Microstructure and mechanical properties of Sn-xBi solder alloy. J. Mater. Sci. Mater. Electron. 2015, 26, 3629–3637. [CrossRef] Osório, W.R.; Peixoto, L.C.; Garcia, L.R.; Mangelinck-Noël, N.; Garcia, A. Microstructure and mechanical properties of Sn–Bi, Sn–Ag and Sn–Zn lead-free solder alloys. J. Alloys Compd. 2013, 572, 97–106. [CrossRef] Silva, B.L.; Reinhart, G.; Nguyen-Thi, H.; Mangelinck-Noël, N.; Garcia, A.; Spinelli, J.E. Microstructural development and mechanical properties of a near-eutectic directionally solidified Sn–Bi solder alloy. Mater. Charact. 2015, 107, 43–53. [CrossRef] Silva, B.L.; Evangelista da Silva, V.C.; Garcia, A.; Spinelli, J.E. Effects of Solidification Thermal Parameters on Microstructure and Mechanical Properties of Sn-Bi Solder Alloys. J. Electron. Mater. 2017, 46, 1754–1769. [CrossRef] Лякишев, H., II. The Manual of Metal Binary Phase Diagram; Chemical Industry Press: Beijing, China, 2008. Yoon, J.-W.; Jung, S.-B. Investigation of interfacial reactions between Sn–5Bi solder and Cu substrate. J. Alloys Compd. 2003, 359, 202–208. [CrossRef] Lai, Z.; Ye, D. Microstructure and Properties of Sn-10Bi-xCu Solder Alloy/Joint. J. Electron. Mater. 2016, 45, 3702–3711. [CrossRef] Lai, Z.; Kong, X.; You, Q.; Cao, X. Microstructure and mechanical properties of Co/Sn-10Bi couple and Co/Sn-10Bi/Co joint. Microelectron. Reliab. 2017, 68, 69–76. [CrossRef] Lee, S.M.; Yoon, J.W.; Jung, S.B. Interfacial reaction and mechanical properties between low melting temperature Sn-58Bi solder and various surface finishes during reflow reactions. J. Mater. Sci. Mater. Electron. 2015, 26, 1649–1660. [CrossRef] Hu, X.; Li, Y.; Min, Z. Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate. J. Mater. Sci. Mater. Electron. 2013, 24, 2027–2034. [CrossRef] Li, J.F.; Mannan, S.H.; Clode, M.P.; Whalley, D.C.; Hutt, D.A. Interfacial reactions between molten Sn–Bi–X solders and Cu substrates for liquid solder interconnects. Acta Mater. 2006, 54, 2907–2922. [CrossRef] Wang, F.; Zhou, L.; Wang, X.; He, P. Microstructural evolution and joint strength of Sn-58Bi/Cu joints through minor Zn alloying substrate during isothermal aging. J. Alloys Compd. 2016, 688, 639–648. [CrossRef] Zou, H.F.; Zhang, Q.K.; Zhang, Z.F. Interfacial microstructure and mechanical properties of SnBi/Cu joints by alloying Cu substrate. Mater. Sci. Eng. A 2012, 532, 167–177. [CrossRef] Tu, K.N.; Thompson, R.D. Kinetics of interfacial reaction in bimetallic Cu–Sn thin films. Acta Metall. 1982, 30, 947–952. [CrossRef] Shang, P.J.; Liu, Z.Q.; Li, D.X.; Shang, J.K. Bi-induced voids at the Cu3Sn/Cu interface in eutectic SnBi/Cu solder joints. Scr. Mater. 2008, 58, 409–412. [CrossRef] Sujan, G.K.; Haseeb, A.S.M.A.; Nishikawa, H.; Amalina, M.A. Interfacial reaction, ball shear strength and fracture surface analysis of lead-free solder joints prepared using cobalt nanoparticle doped flux. J. Alloys Compd. 2017, 695, 981–990. [CrossRef] Wang, J.; Nishikawa, H. Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging. Microelectron. Reliab. 2014, 54, 1583–1591. [CrossRef] Wang, F.J.; Huang, Y.; Du, C.C. Mechanical properties of SnBi-SnAgCu composition mixed solder joints using bending test. Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 2016, 668, 224–233. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).