While military and aerospace electronics are currently exempt from Restriction of Hazardous Substances Directive (RoHS) prohibitions on Pb in electronics, the lead-free transition in the consumer electronics industry has influenced exempt industries. Lead-free solders, particularly Sn-Cu and Sn-Ag solder alloy, are becoming more and more popular in the electronic packaging industry, due to the environmental concerns associated with lead. Among the various Sn-Cu and Sn-Ag solder alloys, meanwhile, electromigration of the flip chip lead-free solder bump has recently been one of the most important reliability issues in flip chip packages. The chip package mounted on the PCB is considerably affected by the surface finish materials or technologies of copper pad. Currently, electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) processing as environment-friendly surface finishes process are widely used. Accordingly, the interfacial reaction between the surface finish and the lead-free solder of a solder joint has become an important package reliability issue. Many studies have been conducted to evaluate the reliability of Pb-free solder joints with respect to solder alloy types and surface finishes. Nevertheless, there is little understanding of correlation between interfacial reaction characteristic and reliabilities with respect to various surface finishes. Therefore, we systematically investigated the effects of PCB surface finishes on the interfacial reaction characteristic and reliabilities of Sn-0.7Cu and novel Sn-3.5Ag solder joints.
The ENIG finish shows slow growth kinetics for the (Cu,Ni)6Sn5 and Ni3Sn4 intermetallic compound (IMC) phases at both Sn-0.7Cu and Sn-3.5Ag solder joints, while the OSP finish show extensive growth of Cu-Sn total IMC (Cu6Sn5+Cu3Sn) under annealing and electrical current stressing. moreover, the ENIG finish shows a strong diffusion barrier effect as compared to the OSP finish, which leads to better electrical reliability at electromigration lifetime evaluation. Cu consumption was found to be closely related to the electrical resistance behavior, which indicates that Cu consumption leads to void propagation and consequently to abrupt electric open failure of the solder bump. Both Sn-0.7Cu and Sn-3.5Ag solder bump showed very similar electrical failure behavior.
The shear strength of both Sn-0.7Cu and Sn-3.5Ag solder joints increased and the shear energy decreased with increasing shear speed, which is related to time-independent plastic hardening and time-dependent strain-rate sensitivity. At interface between each surface finish and solders, brittle fracture mode was dominant with increasing shear speed.
We revealed that ENIG surface finish have better interfacial reaction characteristic and electrical and mechanical reliability than OSP surface finish regardless of solder alloy compositions.