Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging

Abstract

A synergistic approach is applied to address the major concerns about the use of Zn-containing lead-free solders for electronic packaging.Using computational thermodynamics as a predictive tool, the phase stability of the Ag-Al-Cu-In-Sn-Zn system is examined to design a Zn-containing lead-free solder with melting characteristics similar to near-eutectic Pb-Sn solder. Theoretically, it is found that a Sn-0.3mass%Al-4.2mass%In-7.8mass%Znsolderhas a meltingpoint(liquidustemperature) of 185 C and a solidificationrange of 10 C.It is demonstrated thatenvironmentallybenignfluxescontainingtin-organometallicssignificantlyimprovethewettingbehaviorcomparedtorosinfluxesusedforlead-tinsolders.FortheSn-ZneutecticsolderonaCusubstrateat260 C,itisfoundthatthecontactangleisreducedfrom150 toabout25 whentin-organometallic fluxes are used instead of rosin flux. Severe accelerated tests (85% relative humidity at 85 C) for up to six weeks show that themechanical properties of Sn-Zn eutectic solder interconnects are not affected adversely by the environment.(Received July 25, 2003; Accepted September 26, 2003)Keywords: computational thermodynamics, fluxes, lead-free solders, mechanical properties, wetting angle, zinc-containing solders