The Effects of Localized Rework on Microstructural Evolution of Interfacial IMC in Mirrored BGA Electronic Packaging

The hot air rework soldering process is a wellestablished practice in PCB assembly for rectifying faulty electronic components and minimizing defects. However, there remains a limited understanding of the microstructural evolution occurring in reworked, adjacent, and mirror-side components of a mirrored BGA electronic packaging. This paper aims to address this gap by analyzing the microstructural evolution of interfacial intermetallic compounds (IMCs) in these components during the hot air rework soldering, employing scanning electron microscopy (SEM). The findings reveal that the localized rework process caused grain coarsening in the reworked component and its adjacent and mirrored components. This phenomenon is attributed to an increase in Cu concentration in the solder melt, promoting larger Cu6Sn5 IMC grain growth. Interestingly, the assessment of Cu6Sn5 grain size in the mirrored components of the PCB reveals larger grains compared to the reworked side. This discrepancy can be attributed to the slower cooling rate experienced by the mirrored components during localized reflow, leading to grain boundary diffusion of Cu6Sn5 grains, as evidenced by observed necking and grain coalescence.