Testing of Backward Soldered BGAs on a Military Product

The move to lead free (Pb-free) electronics by the commercial industry has resulted in an increasing number of ball grid array components (BGAs) which are only available with Pb-free solder balls. The reliability of these devices is not well established when assembled using a standard tin-lead (SnPb) solder paste and reflow profile, known as a backward compatible process. Previous studies in processing mixed alloy solder joints have demonstrated the importance of using a reflow temperature high enough to achieve complete mixing of the SnPb solder paste with the Pb-free solder ball. Research has indicated that complete mixing can occur below the melting point of the Pb-free alloy and is dependent on a number of factors including solder ball composition, solder ball to solder paste ratio, and peak reflow times and temperatures. Increasing the lead content in the system enables full mixing of the solder joint with a reduced peak reflow temperature, however, previous research is conflicting regarding the effect that lead percentage has on solder joint reliability in this mixed alloy solder joint. Previous work by the authors established a protocol for soldering Pb-free BGAs with SnPb solder paste based on solder ball size and target lead content in the final solder joint. The units from that testing were subjected to thermal cycling between -55°C and 125°C and compared to a SnPb baseline assembly. Results showed that mixed alloy joints performed as well as or better than standard SnPb joints under these conditions. This study continues the previous work with evaluation of reliability in a production design. Functional assemblies were built using Pb-free BGAs in a SnPb solder process and subjected to life testing including accelerated aging and highly accelerated life testing (HALT). Results from this testing are compared to SnPb baseline units and previous product development test results. Introduction Europe’s Restriction of Hazardous Substances (RoHS) and similar lead-free directives from nations outside the European Union have caused the electronics industry to move towards lead-free soldering. Some manufacturers building high-reliability electronics and/or products with a long service life which are outside of the scope of these directives continue to use conventional tin-lead (SnPb) solder due to the uncertain reliability of lead-free solder joints. Because component manufacturers are moving to lead-free production, many components are no longer available in SnPb. As availability of SnPb BGAs and chip scale packages (CSPs) continues to diminish, fabricators who continue to use a SnPb process will be compelled to accommodate lead-free versions of these packages by either reballing the package, or using them as-is in a backward compatible process. Reballing is currently an acceptable process for many, but carries with it the disadvantages of potential damage to the package, voiding of the component manufacturer’s warranty, and/or significant cost impacts. Soldering of lead-free components with SnPb solder, called backward compatibility, has therefore become an area of particular interest. The mixing of metallurgies can induce new reliability concerns because solder joint reliability depends on loading conditions, material properties and microstructure of the solder joint. Furthermore, the microstructure of the solder joint continues to evolve over time depending on temperature and mechanical loading conditions. One of the microstructural features known to influence the reliability of conventional SnPb solder is the lead (Pb) phase, which coarsens with time, temperature and mechanical loading, causing reliability concerns [1][2]. It is therefore important to understand any impact to reliability resulting from the mixing of solder alloys, particularly in the use of components which have a volume of lead-free solder that is significant to the final solder volume in the joint, such as BGAs and CSPs. Numerous studies have been conducted to investigate the reliability of the mixed solder joints subjected to various loading conditions and metallurgical combinations. In general, results from these studies indicate that for lead-free As originally published in the IPC APEX EXPO Conference Proceedings.