Formation of AgSn plates in Sn-Ag-Cu alloys and optimization of their alloy composition

Abstract

Owing to activc R & D efforts, substantial progrcss toward a full transition to Pb-free soldering technolog? has been made reccntly. At present, the leading cwndidatc solders arc ncar-ternary eutectic Sn-Ag-Cu alloys for various soldering applications. The near-eutectic ternary Sn-Ag-Cu alloys yield three phases upon solidification, p-Sn Ag3Sn and CueSni. During solidification, the cquilibriuin eutectic tnnsfonnation is kinetically inhibited. Wlulc the Ag3Sn phase micleatcs with minimal undercooling. tlie p-Sn phase requires a hpical undercooling of I5 to 80 C for nucleation. As a consequence of tlus dispariy in tlie required undercooling. large. plate-like ABSn stmcturcs can grow rapidly within tlie liquid phasc. dining cooling, before the final solidification of solder joints. When large Ag3Sn plates are present in solder joints, they may adversely affect the mechanical behavior and possibly reduce tlic fatigue life of solder joints by providing a preferential crack prop7gation path along the interface betneen a large Ag3Sn plate and tlie p-Sn phase. In tlic present paper, an in-dcptli study to understand the formation of largc Ag3Sn plates in Sn-Ag-Cu solder balls and joints is reported. It is also demonstrated how large Ag& plate fonnation can be minimized in solder joints through optimizing their alloy compositions or controlling cooling rates.