A Comprehensive Solder Joint Reliability Study of SnPb and Pb Free Plastic Ball Grid Arrays (PBGA) Using Backward and Forward Compatible Assembly Processes

High reliability electronic equipment producers are continuing to manufacture tin-lead (SnPB) electronic products using the European Union Pb-in-solder exemption to enable RoHS (restriction on certain hazardous substances) compliance. These companies face chalanges from a component supply chain that is rapidly converting to Ph free offerings and has minimal fiscal motivation to continue to produce SnPb product. These supply chain constraints have created a growing issue with the lack of availability of certain SnPb ball grid array (BGA) components that may force companies to use Pb-free BGAs with their SnPb assembly processes. Pb-free BGAs with SnPb assembly, often referred to as mixed alloy processing, provides an alternative to immediate, complete product conversion to Pb free manufacturing. Despite a number of industry studies, implementation of mixed alloy or backward compatible processes remains a risk because the long term attachment reliability of these solder joints has not yet been characterized thoroughly. This paper presents the results of attachment reliability testing of backward compatible mixed alloy test boards assembled using parameters that produce both full and partial Pb mixing. The component used in the study was a 376 I/O PBGA with Sn-3.0Ag-0.5Cu (SAC 305) solder balls. To address the dwell time dependence 0 C to 100 C temperature cycling tests were conducted using 10, 30, and 60 minute dwell times. To facilitate a comprehensive reliability comparison, additional test cells were incorporated using SAC- SAC assembly, SnPb-SnPb assembly, and SnPb BGA-SAC paste (forward compatible) All tests cells were evaluated using the multiple dwell times and each was tested to 63% or greater failure rate. Post-cycling failure analysis was conducted on representative test samples from each cell. Failure analysis included metallographic analyses using optical microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) to verify the failure mode and evaluate the microstructure. The ATC test data and failure results are discussed in terms of the relationship to the initial as well as evolving microstructure and mechanical behavior that results from temperature cycling. The reliability of the mixed alloy assemblies is discussed relative to that of the pure SAC and SnPb assemblies and the results indicate that mixed assemblies can provide acceptable reliability, even when mixing is not complete.