Thermal fatigue properties of lead-free solders on Cu and NiP under bump metallurgies

Three Pb-free solders, SnCu0.7, SnAg3.8Cu0.7 and SnAg3.5 were evaluated on both electroless NiP and electroplated Cu under bump metallurgies (UBM) for flip chip applications. Eutectic SnPb37 solder was also evaluated as a baseline comparison with the Pb-free solders. Test dice with a size of 12.6×7.5 mm² were direct flip chip attached to test boards with variety of solder alloy/UBM combinations. In order to accelerate solder bump fatigue, no underfill encapsulation was used on the assembled parts. Due to high CTE mismatch between the Si and PCB and low stand-off height of the flip chip assembly, conditions of 0 to 100°C and -40 to 125°C air-to-air thermal cycling were performed to maximize cycles to failure and to distinguish the fatigue life among the solder alloys/UBMs. The results showed that the SnCu0.7 solder, on both electroless NiP and electroplated Cu UBMs, had the longest thermal fatigue life among all the solder/UBM interconnect structures evaluated. The SnAg3.8Cu0.7 on electroplated Cu had a thermal fatigue life comparable to eutectic SnPb37 while SnAg3.5 on electroless NiP had the worst thermal fatigue life. The failure mechanism varied among the Pb-free solder/UBM combinations. The SnCu on both NiP and Cu UBMs had cohesive failure inside the solder bump due to extensive creep in this alloy during thermal cycling. Both SnAg3.5 on electroless NiP UBM and SnAg3.8Cu0.7 on electroplated Cu UBM showed fatigue cracks initiated and propagated through intermetallics and along the intermetallic/solder interfaces, resulting in a shorter thermal fatigue life. Based on these results, the SnCu0.7 solder alloy appears to be the best choice for Pb-free flip chip interconnect