EXPERIMENTAL-STUDY OF SOLDER JOINT RELIABILITY IN A 1256 PIN, 0.4 MM PITCH PQFP

A no-clean surface mount process for the assembly of Plastic Quad Flat Packs (PQFPs) with 15.7 mil (0.4 mm) pitch and 256 leads was establis hed. The reliability of solder joints was assessed through accelerated life-time testing method. Samples were tested under a temperature cyc ling range of between -55 degrees C and 125 degrees C to study the mic rostructure change and fatigue life of solder joints. Observations of microstructure changes such as the growth of intermetallics, grain coa rsening as well as thermal fatigue cracks were made. The cracks were s een to initiate particularly at heel and toe fillet regions of solder joints. The temperature cycling test results were then modeled by Weib ull distribution, and the reliability and failure rate functions for t he current surface mount solder joint process were determined. Under t he temperature cycling test condition, the mean time to failure of the assembly was 7327 cycles. If operating under a temperature range of b etween 23 degrees C and 99 degrees C with one cycle per day, about 50% of the PQFPs were predicted to survive for about 15,753 cycles, or ap proximately 43 years.