Soft soldering gold coated surfaces

P. A. Ainsworth
1971 Gold Bulletin  
Gold plated components are frequently required to be assembled by soft soldering, and it is important that the joints should be sound. This article reviews the factors influencing the mechanical properties of tin-lead soldered joints between: gold surfaces and describes practical methods for ensuring their reliability in quantity production. Gold possesses a unique combination of properties of particular value in the construction and operation of electrical and electronic equipment. These
more » ... ipment. These properties include high resistance to tarnish and corrosion, low electrical resistivity and contact resistance, ease of thermal compression and ultrasonic bonding and, particularly in the case of certain gold-based alloys, high resistance to mechanical wear. Most efficient use is made of these properties by employing gold, usually in the electroplated form, either as an intermediate layer in certain microelectronic devices, or as a finish on such components as connectors, terminations, and printed circuits. Components coated in this way are frequently required to be assembled by soft soldering, but although gold is very easily wetted by molten tin-lead alloys using non-corrosive rosin fluxes (1, 2, 3, 4) the joints so formed are often found to be mechanically weak. Published information on the reliability of soldered joints on gold is confusing, apparently contradictory results having been reported by different authors, and the purpose of this article is therefore briefly to review the factors influencing joint properties and the conditions under which reliable joints may be made. Factors Affecting Joint Properties Gold dissolves extremely rapidly in molten tin-lead solders, values quoted in the literature for the rate of solution in 60 per cent tin-40 per cent lead at 250°C, for example, varying from 4 µm/s for pure gold wire (5) to 10 tm/s for an electroplated gold of unspecified composition (6). The solid solubility of gold in both tin and lead is, however, very small, and the addition of even less than 0.5 per cent gold to tin-lead solders results in the appearance of the intermetallic compound AuSn4 in the solid alloy. This compound is hard and brittle and has a strong tendency to grow from the melt in the form of coarse plates or needles. Consequently, when present in sufficient quantities the compound embrittles the solder, the ductility and impact strength of a tin-40 per cent lead-gold alloy, for example, falling rapidly when the gold content exceeds 4 to 5 per cent and reaching a very low level with 8 to 10 per cent gold (6, 7). Clearly, if rapid solution of the gold during soldering causes the gold content of the solder to rise above about 5 per cent the joint itself will be very brittle. Moreover, a continuous layer of AuSn4 forms during soldering at the solder-gold interface and when this increases in thickness beyond a certain very low value the bond -between the compound and the gold substrate is considerably weakened, thus further reducing joint strength. It will be apparent from the foregoing that the properties of soldered joints on gold-plated surfaces are likely to be affected by a number of factors including the time and temperature of soldering and the characteristics of the gold plate. These factors have been examined by a number of workers, one of the most comprehensive investigations being that by Harding and Pressly (3), some of whose results are summarised in the graph on page 49. In general, it has been found that soldered joints on metals which are themselves wetted by molten solder, and which have been freshly plated with either pure or alloyed golds up to about 1 jim thick, have mechanical properties similar to those of joints made directly to the bare substrate. The gold completely dissolves in the solder enabling the solder to wet and bond to the substrate and the amount of AuSn 4 which precipitates in the bulk of the solder is insufficient to impair joint properties. As the plating thickness is increased above about 1 -1 µm the gold is not completely dissolved during most soldering operations and the increasing amount of intermetallic compound that is formed, particularly at the interface, causes a progressive fall in shear strength and an even more rapid fall in resistance to 47
doi:10.1007/bf03215142 fatcat:hmwgyzfxyzfh3arv4gqmh6j3ae