In a study of factors affecting wetting in solder systems, tin, lead, and tin-lead alloys were allowed to spread as sessile drops on copper and brass substrates without fluxes in vacuum. X-ray microprobe analyses and optical micrographs show the alloying reactions, which not only occur at the interface but also throughout the bulk of the solder. For example, with tin on copper substitutes, copper diffuses upward into the molten tin, and two separate bands of cu-sn alloy are formed at the interface. The upper, a 67 cu-33 sn alloy, streams upward through the sessile drop to form columnal sections that end at the drop surface and are encased in a 47 cu-53 sn alloy. Ordinary 50 sn-50 pb solder on copper exhibits up to five regions of definite but different compositions. Tin diffuses out of the solder forming cu-sn alloys that leave behind lead-rich areas. The zinc component in brass produces different results with solders. Various binary and ternary alloys are formed at the interface and throughout the solder. Other details of interfaces between tin, lead, or tin-lead solders spread on copper or brass are presented in the article.
Am. Soc. for Metals--conf. on Copper, Oct. 16-19, 1972, Cleveland, Ohio, PP. 1-12