Solders for Thick Gold Plating Consideration of solder characteristics, joint shear strength, and soldering conditions determines the choice of a solder-flux system BY DAVI D H. BREWER ABSTRACT. A study was made to find a produce joints that appear "cold" and tie, acicular crystals of AuSn4 in the solder more suitable than 63Sn-37Pb for are extremely weak. Certain other solder. These large crystals give the use with thick gold plating. Conclusions high-temperature solders that are bet­ solidified solder a crystalline appear­ were based upon the results of wettabil- ter adapted for use with gold plating ance that can be confused with a 'cold ity and shear-strength tests. The test often cannot be used because of the solder joint.' methods used provided results that are required use of rosin fluxes or the "3. Except for very thin deposits of reproducible with equipment that is un­ approximately 0.01 mil and less, alloy complicated. presence of organic insulating materi­ Eutectic 63Sn-37Pb solder is satisfac­ als. golds containing small quantities of tory for use with thick gold plating only This investigation was made to find nickel, cobalt, or silver cause consid­ if soldering time and temperature are a solder more suitable for use with erably weaker solder joints than do closely controlled to prevent excessive thick gold plating than 63Sn-37Pb. pure golds of comparable thicknesses. alloying of tin and gold. Used under Thick gold plating was considered to The weak solder joints are due pri­ conditions that produce excessive alloy­ be a deposit of 0.3 mil or greater. The marily to a dewetting condition. The ing, 90In-10Ag has a higher shear solder initially wets the gold surfaces strength than that of 63Sn-37Pb. Al­ investigation was made in two phases: 1. A study of the wettability of gold and then draws back to leave ridges of though high in shear strength, 80Au- solder connecting the joint. 20Sn solder is restricted in use because plating by the various solders. of its high soldering temperature. 2. A determination of the shear "4. Pure golds and alloy golds con­ A study of the wettability and shear- strength of the solders after they were taining nickel, cobalt, or silver are strength test results presented in this alloyed with gold. significantly, although not markedly, paper will aid in the choice of a solder- All of the solders used in the inves­ more readily wetted by tin-lead sol­ flux system for almost any intended use tigation are commercially available. ders than is bare copper. involving thick gold plating. "5. Pure golds are only slightly Background more readily wetted than alloy golds. Introduction An earlier study of gold-plating for "6. Thinner golds of both pure and Technological advancement in the solderability by Harding and Pressly* alloy types are slightly more readily electronics industry has led to increas­ presents the following six conclusions: wetted than thicker golds." ing use of gold-plated components. "1. Thin, pure gold plating of ap­ In the same report, Harding and Gold plating may be used to minimize proximately 0.05 mil and less and very Pressly make the following recom­ the resistance of a conductor, de­ thin, alloy gold plating of approx­ mendations: crease contact resistance, prevent cor­ imately 0.01 mil and less dissolve "Thin gold plating (approximately rosion, or, in thin deposits, to increase readily in tin-lead solder and have no 10 millionths) is definitely a soldering solderability. Difficulty is often en­ measurable detrimental effects on the aid and has no detrimental effects on countered in soldering, however, when structure or strength of the solder the structure or strength of solder design considerations dictate that a joints. joints. thick gold plating be used. "2. Thick gold plating of approx­ "Thick gold plating (approximately The pins of a miniature connector imately 0.05 mil and greater dissolves 50 millionths and greater) should be block are a typical example of a thick readily in tin-lead solder at normal avoided when strength is important. gold-plating application. While it soldering temperatures. The gold-tin- "In those applications where thick would be desirable to have a thin lead alloy formed is significantly gold plating is required for other rea­ plating on the end of the pin that is weaker than eutectic tin-lead solder, sons, as thin a gold as can be tolerated soldered to the conductor, repeated its strength varying inversely with the should be used; pure gold deposits abrasion of the opposite end by the gold content. Thus, any factor that should be chosen; and the soldering mating connector demands that a will increase the amount of gold in the operation should be precisely con­ thick plating be used. The small size solder will produce weaker solder trolled to minimize soldering tempera­ of the pins makes it impractical to use joints; that is, increased gold thick­ ture and soldering time." two different plating thicknesses. ness, increased soldering temperature, Emphasis is placed on the fact these Excessive alloying of tin-lead solder and increased soldering time. The conclusions and recommendations are with gold has long been known to gold-tin-lead alloy formed is weaker for tin-lead solders, and the use of than the eutectic tin-lead solder be­ gold plating, to enhance solderability. DAVID H. BREWER is with the Kansas cause of the formation of large, brit- City Division, Bendix Aerospace-Electron­ Solder Selection ics Co., Kansas City, Mo. "Harding, W. B., and Pressly, H. B., The choice of solders to be included Paper presented at the AWS National Fall "Soldering to Gold Plating," Technical in this investigation was based upon Meeting held in Baltimore, Md., during Proceedings, 1963, American Electroplaters' Oct. 5-8, 1970. Society, pp. 90-106. several factors. Table 1 lists the solder WELDING RESEARCH SUPPLEMENT | 465-s Wettability Study Table 1—Properties of Solders Considered for Investigation Method Eutectic (E) or liquidus (L) Wettability of the solders chosen melt for study was determined by melting a Solder temperature, specific quantity of each solder on a composition % °F Comments circular specimen punched from a 63Sn-37Pb 361 E Common, general-purpose solder for electrical gold-plated panel. After cooling, the connections. outline of the wetted area was traced on translucent paper cross-ruled in 80ln-15Pb-5Ag 315 E Indium-base solders are weaker without tin, but 0.1 in. squares (each unit having an 90ln-10Ag 446 L gold-indium intermetallics may not be as brittle. area of 0.01 sq in.). The included squares were counted, and fractional 96.5ln-3.5Bi» 304 L Recommended by manufacturer "for soldering parts were estimated to the nearest thin films of gold without appreciable gold tenth of a unit. scavenging." A supplementary method of ex­ 97.5Pb-1.5Ag-1.0Sn 588 E pressing solder wettability was used— Fig. 1. This method consists of deter­ 68Sn-32Cd 349 E Alloys containing cadmium are used for solder­ mining the angle between the surface 83Pb-17Cd 478 E ing in the jewelry industry. of the solder (at its leading edge) and the surface of the base metal on which 80Au-20Sn 536 E Gold-base alloys recommended by manufac­ turer for soldering "because of their excellent the gold is plated. For this purpose, a 88Au-12Ge 673 E wettability, good thermal conductivity, high representative cross section of the strength, resistance to many etchants, and specimen is photomicrographed at 94Au-6Si 698 E low melting point." Alloys with melting points 50X. A V2 in. chord is drawn on the higher than that of 80Au-20Sn were found in­ photomicrograph from the leading 27AU-73T1 268 E capable of being fluxed by rosin fluxes. Most edge of the solder to its top surface. of them are quite brittle. The 27Au-73Tl alloy The included angle is termed the was so brittle that it could not be rolled into "contact angle." preforms needed for the shear specimens. Results from the two methods of 56.5Bi-43.5Pb 255 E expressing solder wettability were cor­ related, and acceptable limits were a Composition determined by laboratory analysis. defined for each method. compositions that were initially con­ and gold, despite their frosty appear­ Specimen Preparation sidered, together with the preliminary ance. Test panels for the wettability study data obtained in preparation for the High melting temperatures excluded consisted of two 10V2 by 12 in. study. Except for 63Sn-37Pb, solders 88Au-12Ge and 94Au-6Si solders sheets of 0.020 in., 70-30 half-hard containing a high percentage of tin from the investigation because of the brass. Preparation before plating, pri­ were avoided because of the brit- presence of organic insulating and marily done to remove tarnish, con­ potting materials in the components to tleness of the gold-tin intermetallic sisted of vapor degreasing, cleaning compounds that would be formed. be soldered, and the required use of anodically in an alkaline cleaner The 63Sn-32Cd composition was in­ rosin-alcohol and rosin-terpene fluxes. (Oakite 191), and rinsing. For econo­ cluded because of its reported use in Of the original twelve solders con­ my, and to prevent edge build-up of the jewelry industry. sidered, the following nine were cho­ plating, the back and edges of each sen for study: In general, gold- and indium-base panel were masked with platers' tape solders were considered to be the 1. 63Sn-37Pb. to leave an exposed area of 8V2 by most promising. It was reasoned that 2. 80In-15Pb-5Ag. 10 in. The panels were then dipped in the gold-base solders would pick up 3. 96.5In-3.5Bi a 2 lb/gal solution of ammonium per- additional gold from the plating, 4.