Microstructure, joint strength and failure mechanism of Sn-Ag,Sn-Ag-Cu versus Sn-Pb-Ag solders in BGA packages

摘要

The microstructure, joint strength and failure mechanisms of Sn-Agand Sn-Ag-Cu versus Sn-Pb-Ag systems on Cu/Ni/Au BGA pad metallizationhave been investigated after multiple reflow and high temperaturestorage. Sn-Pb-Ag system gave rise to a two-layer structure, i.e.Ni₃Sn₄ and (Au,Ni)Sn₄, at the interfaceafter aging at 150° C. However, such a structure was not detected inboth lead-free systems. Only a layer of Ni₃Sn₄phase in the Sn-Ag system and a layer of Cu-Sn-Ni-Au intermetalliccompound in Sn-Ag-Cu system were found at the interfaces, even after1000 hours at the afore-mentioned temperature. The formation of the(Au,Ni)Sn₄ ternary compound was due to re-settlement of Au atthe interface which led to severe brittle failure in the Sn-Pb-Agsystem. In contrast, Sn-Ag and Sn-Ag-Cu systems failed exclusivelyinside the solder after aging at 150° C up to 1000 hours. The solderball joint strength of the three systems and failure modes were alsoevaluated. Both lead-free systems showed good resistance to thermalaging with a solder ball joint strength maintained at about 1.60 to 1.70kgf. The Sn-Pb-Ag system, on the other hand, degraded in mechanicalperformance over aging time, reaching a strength as low as 1.20 kgf. Thegrowth rates of intermetallic layers at 125, 150, and 175° C, and theactivation energy were also determined in this study