Interfacial reactions with lead (Pb)-free solders

Solder interconnection technology plays a key role in various levels of microelectronics packaging, such as controlled collapse chip connection (or C4), solder-ball connection in ball-grid-array (BGA), or IC package assembly to a printed circuit board (PCB). Soldering technologies employed in these applications are quite different in terms of solder materials and processes. The metallurgical phenomena, however, during the soldering processes are very similar to each other, such as wetting of a molten solder to a metallic surface, dissolution of the base metal into the molten solder, formation of intermetallics, and solder joint formation by solidification. A good understanding and thereby the control of the interfacial reactions, namely, dissolution of the base metal(s) and concurrent formation of intermetallics at the soldering interfaces, are critical in producing sound and reliable solder joints. In the present paper, the interfacial reactions of Cu and Ni base metals with several Pb-free solders at their corresponding reflow temperatures are investigated. The interfacial reactions studied simulate the solid-liquid reaction which occurs during the initial solder joint formation as well as its rework process, if necessary. The Pb-free solders used in this investigation include; 58%Bi-42%Sn, 96.5%Sn-3.5%Ag, 95%Sn-5%Sb, and 100%Sn. The Sn/Pb eutectic solder is also included as a bench mark to compare the reaction kinetics with those of the Pb-free solders. The dissolution kinetics of Cu and Ni into the Pb-free solders as well as the growth kinetics of the intermetallic phases are measured using cross-sectional metallography of reaction couples and are compared with those obtained from the Sn/Pb eutectic solder.