Polarity effect of electromigration on intermetallic compound formation in SnPb solder joints

Abstract

The polarity effect of electromigration on the interfacial reactions of micro ball grid array (μBGA) solder joints was studied in terms of microstructural evolution. A dummy μBGA package with the same pair of solder joints was used to obtain reproducible results for a practical application. The μBGA package with Ni(P)/SnPb/Ni(P) structure showed a polarity effect on intermetallic compound (IMC) formation after stress by a current density of 3.0 × 10 3 A cm −2 at 120 °C, compared to the no-current case. Electric current enhanced the growth of Ni 3Sn 4 at the anode and retarded it at the cathode because of the change in Sn diffusion induced by electromigration. The IMC growth at the anode was about one order of magnitude faster than that at cathode. The growth of IMC at the anode had a parabolic dependence on time since the square of thickness of IMC increases linearly with time. The real μBGA package has unique and more complicated geometry compared to ordinary diffusion couples. So not only the polarity growth of IMC but also the fast dissolution of Cu and Ni at specific positions was found with the μBGA package. The unique geometry of μBGA solder joints caused heat generation at the upside interfaces due to Joule heating. Because of the heat generation and the high interfacial energy at the triple point of upside interfaces, the electroless Ni(P) finishes and Cu trace are consumed rapidly in the region where the current is crowded. Thus electromigration induced the rapid dissolution of Ni and Cu into solder and then formed (Cu,Ni) 6Sn 5 at the triple point of the current crowding region at high temperature. This is a rapid failure process because the localized fast dissolution of Ni and Cu accelerated the solid-state reaction between solder and electroless Ni(P) or Cu trace.