Reducing Cracking in Solder Joint Interfacial Cu₆Sn₅ with Modified Reflow Profile

抄録

The polymorphic transformation that occurs in the Cu₆Sn₅ intermetallic compound (IMC) at 186°C has the potential to generate stresses that could lead to cracking of that phase in soldered joints during the multiple reflow cycles of a typical printed board assembly process and the thermal cycles to which electronic assemblies are exposed during service. In this paper the authors report on the effect of variations in the cooling stage of a reflow soldering thermal profile on the incidence and extent of cracking in the Cu₆Sn₅ at the interface between solder alloys and copper substrates. The solder alloy/substrate combinations studied were Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu-0.05Ni-1.5Bi/Cu. The cooling conditions were (i) the direct-cooling of a conventional reflow profile, and (ii) an alternative reflow profile with one of three extended isothermal holding periods of 30, 60, and 180 seconds at 140°C during the cooling stage. It was found that the alternative reflow profiles reduced cracking in the interfacial Cu₆Sn₅ IMC layer and this resulted in improved resistance of the reflowed solder ball to failure in high speed impact shear when the distribution of stress tends to favor crack propagation though the interfacial IMC rather than through the bulk solder.