Preceramic
and
postceramic
solder
joints
E. H. Stade, D.D.S.,* M. H. Reisbick, DAD., KS.,** and J. D. Preston, D.D.S.*** Wadsworth Veterans Administration Hospital Center and University California at Los Angeles, Los Angeles, Calif.
of
individual units of a fixed prosthesis may be cast together or joined by soldering. Generally, one-piece castings larger than three units are not recommended because of distortion produced by nonuniform investment expansion within the investment plug. The over-all mesiodistal length of a soldered multiple-unit fixed prosthesis will They are of lesser magnitude than those also have dimensional discrepancies.‘-” shown by large one-piece castings. Regardless, soldered connectors are frequently used and, many times, are the preferred or only choice. When ceramic-metal units are connected, the complexity of the joining procedure increases. Soldering these units prior to ceramic fusion predisposes the castings to warpage or melting because of the required high temperatures. Postceramic soldering induces the hazard of ceramic color change due to investment contamination; also, turther metal brittleness may occur due to the precipitation of iron platinum with additional heat treatment.” Because stress concentrations are found in connectors, their potential for failure is high; these stresses are amplified by the presence of porosity. Another factor influencing the strength of soldered connectors is gap size. Existing literature reveals that the gap between units should be greater than 0.002 to 0.004 inch to allow for thermal expansion of the assembly to be soldered .I However, the upper limit is apparently determined solely by total expected shrinkage.‘9 4 The thickness of a calling card is often suggested as a guideline for gap space. The authors consider this distance, about 0.012 inch, to be the minimum acceptable gap. The purpose of this study was to evaluate the quality of solder joints made under simulated preceramic and postceramic conditions. These joints were evaluated at gap distances of 0.31 mm. (0.012 inch), 0.51 mm. (0.020 inch), and 0.76 mm. (0.030 inch). This investigation was conducted of training in fixed prosthodontics. *Department **Associate ***Director,
in partial
fulfillment
of the requirements
for certification
of Fixed Prosthodontics. Professor and Chairman, Section of Biomaterials. Fixed
Prosthodontic
Training
Program.
527
528
Stade,
Reisbick,
J. Prosthet. Dent. November, 1975
and he&n
Fig. 1. Assembly apparatus
used to orient
units to be soldered.
MATERIALS AND METHODS Wax specimens, 0.050 by 0.150 inch, were invested* and cast with a ceramicmetal alloy? melted with a gas-oxygen flame. Each pair was positioned in a holding device, the gap size was adjusted with a leaf gauge, and each pair was embedded in soldering investmentf (Fig. 1) Sixty-five metal pairs were invested. Five pairs were joined with a contact joint having no gap space? while groups of 20 pairs each were joined with gap spaces of 0.3 1, 0.51, and 0.76 mm. Five contact joints and 10 each of the 0.31, 0.51, and 0.76 mm. gapped pairs were used for the preceramic soldering procedure. Following a drying period of at least 24 hours at room temperature, the invested specimens were placed in a cold electric oven and heated to 1,800° F. After removal from the oven, the joints were immediately soldered using a gas-oxygen torch and the manufacturer’s recommended solder.5 The soldered units were allowed to cool to room temperature before being removed from the investment. The joint areas were trued to remove excess solder and then measured to determine cross-sectional area. The remaining paired specimens; with gap spaces of 0.31, 0.51, and 0.76 mm., were then invested, dried, and fluxed.11 Eighteen karat No. 615 fine solder strips were placed over the gaps. The units were placed in a vacuum oven at 28 inches of mercury and 1,000’ F., and the temperature was raised to 1,525’ F. at a rate of 75’ F. per minute. The units were held for two minutes at 1,525O F., removed from the furnace, and allowed to cool to room temperature. These joints were also trued and measured. “Ceramigold
investment,
Whip-Mix
VZeramcn “0,” J. F. Jelenko $Hi Heat soldering investment, $Ceramco
No. 1 solder, J. F. Jelenko
ANTFlux, Unitek
Corporation,
Corporation,
Louisville,
Ky.
& Company, Inc., New Rochelle, Whip-Mix Corporation, Louisville, & Company,
Monrovia,
Calif.
N. Y. Ky.
Inc., New Rochelle,
N. Y.
::;r:r“5”
and postceramic
Preceramic
Table I. Tensile strength values and standard deviations
solder joints
529
for torch- and oven-soldered
joints Gap size (mm.1
Soldered
Mean tensile I strenpth la.s.i. J
Parent metal fractures
S.D.
No. soecimens
Torch
0.31 0.51 0.76
39,686 43,315 46,612
0 0 0
12,035 14,159 6,211
10 10 IO
Oven
0.31 0.51 0.76
57,165 60,642 73,710
4* 6* 9*
13,809 9,306 8,259
10 10 10
were included
in the mean tensile strength
*These
specimens
Table II. Factorial Source of variation
analysis of variance Degrees ojjireedom 59 54 5 1 2 2
Total Within cells Between cells Torch to oven Gap Torch/oven to gap *NS =
value for their
for solder technique Mean square 1.215 X IOn 6.387 X 10’ 7.243 X lOa 1.567 X 10’
group.
and gap space data F
P
-
-
52.54 5.958 I.289
p < 0.001 0.01 >p< 0.05 NS*
not significant.
The error
for the study was a standard
deviation
of 1.1 x 104.
All soldered specimens were subjected to a tensile force in an Instron Universal test machine*; the crosshead speed was 0.05 inch per minute. The raw data were subjected to tde analysis of variance, and where differences existed, they were defined using the method of Scheffe.G Scanning electron micrographs were made of selected joint fractures and studied for possible explanation of high or low strength values.
RESULTS Sixty-three specimens were tested. Two of the specimens with no gap space fractured while being trued. The values obtained for the other joints without a gap are not reported, since the sample size is not statistically significant (values obtained were exceedingly low). Nineteen specimens did not rupture at the joint but exhibited parent metal fractures (Table I). These specimens were included in the statistical analysis, since they did give an indication of joint strength. Such units gave strength values close to those cited by the manufacturer, such as 72,500 p.s.i. ultimate tensile strength for Ceramco “0” Micro Fine gold. Gap size-strength relationship. Table I indicates a consistent rise in strength values as the gap size increases. The joints with the highest strength also have the smallest standard deviation. The oven-soldered samples show a rise in parent fracture *Instron
Corp.,
Canton,
Mass.
530
J. Prosthet. Dent. November, 1975
Stade, Reisbick, and Preston
GAP SPACE
Fig. 2, Strength values obtained
(mm)
for oven and torch soldering
Fig. 3. Scanning electron micrograph of low-strength Fig. 4. Scanning electron micrograph of high-strength
fracture. fracture.
at three separate gap spaces.
(Original (Original
magnification magnification
x65.) x160.)
as the gap size increases. Table II, an analysis of variance on pooled gap sizes, shows gap size to be a statistically significant pararneter (0.01 < p < 0.05). Fig. 2 illustrates that ultimate tensile strength increases with gap size. Oven vs. torch soldering. Oven-soldered joints were found to be consistently superior to torch-soldered joints; the results were found to be highly significant (Table II). The F test was also used to compare the variance of gap size with the soldering method. There were no significant differences (F = 1.289). That is, the method of soldering is more important than gap differences. Fig. 2 visually demonstrates the relationship between the soldering methods. Most notably, the 0.76 mm. ovensoldered gap is disproportionately superior. Figs. 3 and 4 are scanning electron micrographs of selected ruptured joints. The low strength value for the specimen in Fig. 3 can be attributed to the large internal
Volume 34 Number 5
Preceramic
and postceramic
solder joints
531
void. Although the joint appeared sound following the soldering procedure, it was structurally weak. Fig. 4 is a micrograph of a very strong sample. Porosity is demonstrable even in high-strength joints when subjected to sufficient magnification. These micrographs are representative of joints produced by both methods of soldering. The 0.76 mm. joints produced by the torch method presented technical difficulties. These gap spaces were too wide to allow complete filling with solder. This phenomenon was not observed with smaller gap spaces nor in any of the oven-soldered joints. DISCUSSION The most striking finding in this investigation was the significant superiority of the oven-soldered joints, These joints were often stronger than the parent alloys being joined. This is not too surprising, since the compositions of the solders differ. The low-fusing solder contains copper in sufficient quantity to allow age-hardening to occur. Since the specimens were bench cooled, as necessary for ceramic units, agehardening was a probable occurrence. Controversy exists as to whether the increased strength of heat treatment is preferable, since it is accompanied by a decrease in ductility with concomitant brittleness. Porosity is an important consideration when studying solder joint strengths. Voids can be attributed to flux entrapment due to overfluxing and underheating. However, this does not account for the voids seen in the low-strength joints formed by torch soldering without flux. Rather, this phenomenon may be associated with the degree of wetting of solid metal by the molten solder. This is related to the relative interfacial energies between dental gold solders and a gold alloy.’ The surface tension of solder is greatly influenced by its temperature. This is of critical importance when using high-fusing solders, since the melting differential between the castings and the solder is 90’ F., or less. (The melting range of Ceramco “0” metal is 2,100’ to 2,150° F., and for Ceramco No. 1 solder, it is 2,010’ to 2,090’ F.) This may partially explain the low strength values obtained with torch-soldered joints. Joint porosity may also occur due to overheating; this is the result of volatilization of the base metals with lower melting temperatures. The probability of this accounting for the porosity in the torch-soldered joints is low, since the high-fusing solder was 95 per cent precious metals. Gap space has also been shown to have an effect on joint porosity.” That is, contact or very small gaps are responsible for large amounts of porosity. The present study shows that gap space has a significant effect on joint strength. An increase in strength occurs as the gap space enlarges. Fig. 2 contrasts the two methods of heating the solder joints. The increases produced by the oven method can be explained on the basis of differences in surface tension of the solders and uniformity of heat application. The no-gap joints, utilized on a limited basis in this study, were definitely inferior. Their weakness is due to poor capillary flow between the units. SUMMARY This investigation evaluated preceramic and postceramic soldering procedures. Three gap spaces and two soldering methods were evaluated. Wider gaps produced stronger joints. The strongest joints were achieved by using wider gaps and the oven
532
Stade,
Reisbick,
J. Yrostbet. Derrt. November, 1975
and Preston
soldering technique. Very wide gaps (0.76 mm.) are not recommended because of possible distortion through excessive solder shrinkage. However, in practice, many solder joints exceed the 0.31 mm. recommended minimum; these joints will not show decreased strength. The authors express their appreciation versity, Northridge, for statistical support
to Donald C. Butler. in this study.
Ph.D.,
California
State Uni-
References 1. Ryge, G.: 2. Fusayama, by Various 1964. 3. Stackhouse, 1967. 4. Steinman,
Dental Soldering Procedures, Dent. Clin. North Am., Nov., 1958, pp. 747-757. T., Wakumoto, S., and Hosoda, H.: Accuracy of Fixed Partial Dentures Made Soldering Techniques and One-Piece Casting, J. PROSTHET. DENT. 14: 334-342, J. A.: Assembly
6. 7. 8.
Units by Soldering,
J. PROSTHET.
R. R.:
DENT.
18: 131-139,
Warpage Produced by Soldering With Dental Solders and Gold Alloys, J. 4: 384-395, 1954. Smith, D. L., Burnett, A. P., Brooks, M. S., and .4nthony, D. H.: Iron-Platinum Hardening in Casting Golds for Use With Porcelain, J. Dent. Res. 49: 283-288, 1970. Guenther, W. C.: Analysis of Variance, Englewood Cliffs, N. J., 1964, Prentice-Hall, Inc. O’Brien, W. J., Hirthe, W. M., and Ryge, G.: Wetting Characteristics of Dental Gold Solders, J. Dent. Res. 42: 675-680, 1963. Ryge, G., and Fairhurst, C. W.: Effects of Certain Variables on the Strength of Soldered Joints, Int. Assoc. Dent. Res. Abst. No. M-10, March, 19.57, p. 109. PROSTHET.
5.
of Dental
DENT.
DR. STAI)E VETERANS DENTAL MIAMI,
ADMINISTKATIOW
DR. PRESTOX WAIISWORTH
VETERASS
Los ANGELES,
CALIF.
DR.
HOSPITAL
546/160 FLA. 33125 SERVICE
ADMIKISTRATIOW
90073
RBISBKK
UNIVERSITY OF CALIFORNIA AT Los SCHOOL OF DENTISTRY Los AKGELES, CALIF. 90024
ANGELES
HOSPITAL
CENTER
and
postceramic
solder
joints
E. H. Stade, D.D.S.,* M. H. Reisbick, DAD., KS.,** and J. D. Preston, D.D.S.*** Wadsworth Veterans Administration Hospital Center and University California at Los Angeles, Los Angeles, Calif.
of
individual units of a fixed prosthesis may be cast together or joined by soldering. Generally, one-piece castings larger than three units are not recommended because of distortion produced by nonuniform investment expansion within the investment plug. The over-all mesiodistal length of a soldered multiple-unit fixed prosthesis will They are of lesser magnitude than those also have dimensional discrepancies.‘-” shown by large one-piece castings. Regardless, soldered connectors are frequently used and, many times, are the preferred or only choice. When ceramic-metal units are connected, the complexity of the joining procedure increases. Soldering these units prior to ceramic fusion predisposes the castings to warpage or melting because of the required high temperatures. Postceramic soldering induces the hazard of ceramic color change due to investment contamination; also, turther metal brittleness may occur due to the precipitation of iron platinum with additional heat treatment.” Because stress concentrations are found in connectors, their potential for failure is high; these stresses are amplified by the presence of porosity. Another factor influencing the strength of soldered connectors is gap size. Existing literature reveals that the gap between units should be greater than 0.002 to 0.004 inch to allow for thermal expansion of the assembly to be soldered .I However, the upper limit is apparently determined solely by total expected shrinkage.‘9 4 The thickness of a calling card is often suggested as a guideline for gap space. The authors consider this distance, about 0.012 inch, to be the minimum acceptable gap. The purpose of this study was to evaluate the quality of solder joints made under simulated preceramic and postceramic conditions. These joints were evaluated at gap distances of 0.31 mm. (0.012 inch), 0.51 mm. (0.020 inch), and 0.76 mm. (0.030 inch). This investigation was conducted of training in fixed prosthodontics. *Department **Associate ***Director,
in partial
fulfillment
of the requirements
for certification
of Fixed Prosthodontics. Professor and Chairman, Section of Biomaterials. Fixed
Prosthodontic
Training
Program.
527
528
Stade,
Reisbick,
J. Prosthet. Dent. November, 1975
and he&n
Fig. 1. Assembly apparatus
used to orient
units to be soldered.
MATERIALS AND METHODS Wax specimens, 0.050 by 0.150 inch, were invested* and cast with a ceramicmetal alloy? melted with a gas-oxygen flame. Each pair was positioned in a holding device, the gap size was adjusted with a leaf gauge, and each pair was embedded in soldering investmentf (Fig. 1) Sixty-five metal pairs were invested. Five pairs were joined with a contact joint having no gap space? while groups of 20 pairs each were joined with gap spaces of 0.3 1, 0.51, and 0.76 mm. Five contact joints and 10 each of the 0.31, 0.51, and 0.76 mm. gapped pairs were used for the preceramic soldering procedure. Following a drying period of at least 24 hours at room temperature, the invested specimens were placed in a cold electric oven and heated to 1,800° F. After removal from the oven, the joints were immediately soldered using a gas-oxygen torch and the manufacturer’s recommended solder.5 The soldered units were allowed to cool to room temperature before being removed from the investment. The joint areas were trued to remove excess solder and then measured to determine cross-sectional area. The remaining paired specimens; with gap spaces of 0.31, 0.51, and 0.76 mm., were then invested, dried, and fluxed.11 Eighteen karat No. 615 fine solder strips were placed over the gaps. The units were placed in a vacuum oven at 28 inches of mercury and 1,000’ F., and the temperature was raised to 1,525’ F. at a rate of 75’ F. per minute. The units were held for two minutes at 1,525O F., removed from the furnace, and allowed to cool to room temperature. These joints were also trued and measured. “Ceramigold
investment,
Whip-Mix
VZeramcn “0,” J. F. Jelenko $Hi Heat soldering investment, $Ceramco
No. 1 solder, J. F. Jelenko
ANTFlux, Unitek
Corporation,
Corporation,
Louisville,
Ky.
& Company, Inc., New Rochelle, Whip-Mix Corporation, Louisville, & Company,
Monrovia,
Calif.
N. Y. Ky.
Inc., New Rochelle,
N. Y.
::;r:r“5”
and postceramic
Preceramic
Table I. Tensile strength values and standard deviations
solder joints
529
for torch- and oven-soldered
joints Gap size (mm.1
Soldered
Mean tensile I strenpth la.s.i. J
Parent metal fractures
S.D.
No. soecimens
Torch
0.31 0.51 0.76
39,686 43,315 46,612
0 0 0
12,035 14,159 6,211
10 10 IO
Oven
0.31 0.51 0.76
57,165 60,642 73,710
4* 6* 9*
13,809 9,306 8,259
10 10 10
were included
in the mean tensile strength
*These
specimens
Table II. Factorial Source of variation
analysis of variance Degrees ojjireedom 59 54 5 1 2 2
Total Within cells Between cells Torch to oven Gap Torch/oven to gap *NS =
value for their
for solder technique Mean square 1.215 X IOn 6.387 X 10’ 7.243 X lOa 1.567 X 10’
group.
and gap space data F
P
-
-
52.54 5.958 I.289
p < 0.001 0.01 >p< 0.05 NS*
not significant.
The error
for the study was a standard
deviation
of 1.1 x 104.
All soldered specimens were subjected to a tensile force in an Instron Universal test machine*; the crosshead speed was 0.05 inch per minute. The raw data were subjected to tde analysis of variance, and where differences existed, they were defined using the method of Scheffe.G Scanning electron micrographs were made of selected joint fractures and studied for possible explanation of high or low strength values.
RESULTS Sixty-three specimens were tested. Two of the specimens with no gap space fractured while being trued. The values obtained for the other joints without a gap are not reported, since the sample size is not statistically significant (values obtained were exceedingly low). Nineteen specimens did not rupture at the joint but exhibited parent metal fractures (Table I). These specimens were included in the statistical analysis, since they did give an indication of joint strength. Such units gave strength values close to those cited by the manufacturer, such as 72,500 p.s.i. ultimate tensile strength for Ceramco “0” Micro Fine gold. Gap size-strength relationship. Table I indicates a consistent rise in strength values as the gap size increases. The joints with the highest strength also have the smallest standard deviation. The oven-soldered samples show a rise in parent fracture *Instron
Corp.,
Canton,
Mass.
530
J. Prosthet. Dent. November, 1975
Stade, Reisbick, and Preston
GAP SPACE
Fig. 2, Strength values obtained
(mm)
for oven and torch soldering
Fig. 3. Scanning electron micrograph of low-strength Fig. 4. Scanning electron micrograph of high-strength
fracture. fracture.
at three separate gap spaces.
(Original (Original
magnification magnification
x65.) x160.)
as the gap size increases. Table II, an analysis of variance on pooled gap sizes, shows gap size to be a statistically significant pararneter (0.01 < p < 0.05). Fig. 2 illustrates that ultimate tensile strength increases with gap size. Oven vs. torch soldering. Oven-soldered joints were found to be consistently superior to torch-soldered joints; the results were found to be highly significant (Table II). The F test was also used to compare the variance of gap size with the soldering method. There were no significant differences (F = 1.289). That is, the method of soldering is more important than gap differences. Fig. 2 visually demonstrates the relationship between the soldering methods. Most notably, the 0.76 mm. ovensoldered gap is disproportionately superior. Figs. 3 and 4 are scanning electron micrographs of selected ruptured joints. The low strength value for the specimen in Fig. 3 can be attributed to the large internal
Volume 34 Number 5
Preceramic
and postceramic
solder joints
531
void. Although the joint appeared sound following the soldering procedure, it was structurally weak. Fig. 4 is a micrograph of a very strong sample. Porosity is demonstrable even in high-strength joints when subjected to sufficient magnification. These micrographs are representative of joints produced by both methods of soldering. The 0.76 mm. joints produced by the torch method presented technical difficulties. These gap spaces were too wide to allow complete filling with solder. This phenomenon was not observed with smaller gap spaces nor in any of the oven-soldered joints. DISCUSSION The most striking finding in this investigation was the significant superiority of the oven-soldered joints, These joints were often stronger than the parent alloys being joined. This is not too surprising, since the compositions of the solders differ. The low-fusing solder contains copper in sufficient quantity to allow age-hardening to occur. Since the specimens were bench cooled, as necessary for ceramic units, agehardening was a probable occurrence. Controversy exists as to whether the increased strength of heat treatment is preferable, since it is accompanied by a decrease in ductility with concomitant brittleness. Porosity is an important consideration when studying solder joint strengths. Voids can be attributed to flux entrapment due to overfluxing and underheating. However, this does not account for the voids seen in the low-strength joints formed by torch soldering without flux. Rather, this phenomenon may be associated with the degree of wetting of solid metal by the molten solder. This is related to the relative interfacial energies between dental gold solders and a gold alloy.’ The surface tension of solder is greatly influenced by its temperature. This is of critical importance when using high-fusing solders, since the melting differential between the castings and the solder is 90’ F., or less. (The melting range of Ceramco “0” metal is 2,100’ to 2,150° F., and for Ceramco No. 1 solder, it is 2,010’ to 2,090’ F.) This may partially explain the low strength values obtained with torch-soldered joints. Joint porosity may also occur due to overheating; this is the result of volatilization of the base metals with lower melting temperatures. The probability of this accounting for the porosity in the torch-soldered joints is low, since the high-fusing solder was 95 per cent precious metals. Gap space has also been shown to have an effect on joint porosity.” That is, contact or very small gaps are responsible for large amounts of porosity. The present study shows that gap space has a significant effect on joint strength. An increase in strength occurs as the gap space enlarges. Fig. 2 contrasts the two methods of heating the solder joints. The increases produced by the oven method can be explained on the basis of differences in surface tension of the solders and uniformity of heat application. The no-gap joints, utilized on a limited basis in this study, were definitely inferior. Their weakness is due to poor capillary flow between the units. SUMMARY This investigation evaluated preceramic and postceramic soldering procedures. Three gap spaces and two soldering methods were evaluated. Wider gaps produced stronger joints. The strongest joints were achieved by using wider gaps and the oven
532
Stade,
Reisbick,
J. Yrostbet. Derrt. November, 1975
and Preston
soldering technique. Very wide gaps (0.76 mm.) are not recommended because of possible distortion through excessive solder shrinkage. However, in practice, many solder joints exceed the 0.31 mm. recommended minimum; these joints will not show decreased strength. The authors express their appreciation versity, Northridge, for statistical support
to Donald C. Butler. in this study.
Ph.D.,
California
State Uni-
References 1. Ryge, G.: 2. Fusayama, by Various 1964. 3. Stackhouse, 1967. 4. Steinman,
Dental Soldering Procedures, Dent. Clin. North Am., Nov., 1958, pp. 747-757. T., Wakumoto, S., and Hosoda, H.: Accuracy of Fixed Partial Dentures Made Soldering Techniques and One-Piece Casting, J. PROSTHET. DENT. 14: 334-342, J. A.: Assembly
6. 7. 8.
Units by Soldering,
J. PROSTHET.
R. R.:
DENT.
18: 131-139,
Warpage Produced by Soldering With Dental Solders and Gold Alloys, J. 4: 384-395, 1954. Smith, D. L., Burnett, A. P., Brooks, M. S., and .4nthony, D. H.: Iron-Platinum Hardening in Casting Golds for Use With Porcelain, J. Dent. Res. 49: 283-288, 1970. Guenther, W. C.: Analysis of Variance, Englewood Cliffs, N. J., 1964, Prentice-Hall, Inc. O’Brien, W. J., Hirthe, W. M., and Ryge, G.: Wetting Characteristics of Dental Gold Solders, J. Dent. Res. 42: 675-680, 1963. Ryge, G., and Fairhurst, C. W.: Effects of Certain Variables on the Strength of Soldered Joints, Int. Assoc. Dent. Res. Abst. No. M-10, March, 19.57, p. 109. PROSTHET.
5.
of Dental
DENT.
DR. STAI)E VETERANS DENTAL MIAMI,
ADMINISTKATIOW
DR. PRESTOX WAIISWORTH
VETERASS
Los ANGELES,
CALIF.
DR.
HOSPITAL
546/160 FLA. 33125 SERVICE
ADMIKISTRATIOW
90073
RBISBKK
UNIVERSITY OF CALIFORNIA AT Los SCHOOL OF DENTISTRY Los AKGELES, CALIF. 90024
ANGELES
HOSPITAL
CENTER
