Alloying effect of Ni, Co, and Sb in SAC solder for improved drop performance of chip scale packages with Cu OSP pad finish

While SAC305 and SAC405 have been shown to yield similar or better reliability than SnPb solder in temperature cycle test, it is becoming evident that the same Pb free solders perform poorly under drop/impact conditions. The primary reasons for this reduced performance are the lower ductility of solder and brittle IMC formation at CSP pad and solder interface. Although some improvements are possible by changing the pad finish from NiAu to Cu OSP, the performance is still not as good as NiAu-SnPb combination. In addition, Cu-Solder interface results in the formation of Cu₃Sn intermetallic compound resulting in brittle interface as well as voiding within this IMC with aging. This study focuses on the alloying effect of Ni, Co, and Sb in SAC solder on the IMC formation and the drop performance of packages. The solder alloys considered had small % of either Ni, Co, or Sb added with varying basic composition of Sn, Ag, and Cu. A total of 6 alloys were evaluated against Sn_3.0Ag_0.5Cu solder alloy. The solder balls of each alloy were attached to packages with Cu OSP surface finish using standard package assembly process. The evaluation matrix included package and board level tests as well as interfacial IMC studies. The package level tests (ball shear, ball pull, and zone shear) and IMC studies were conducted on as-soldered and thermally aged samples. Board level drop tests were performed as per JESD22-B111 test method. The results show that changes in SnAgCu composition and the addition of some elements in SnAgCu based solder can significantly improve the drop performance, primarily because of differences in IMC formation and the strength of solder alloys. While lower Ag improves the ductility of solder itself, addition of minor % of Ni or Co retards the formation and growth of Cu₃Sn intermetallic at pad-solder interface, thus minimizing the potential of void formation and failure at this IMC. It is shown that the drop performance of packages with Cu OSP (pad finish)-SnAgCuNi (solder) combination can be as good as or better than those with NiAu-SnPb combination