Investigation on PAD Crater Failure of BGA Solder Joints

ABSTRACT In this investigation, several pad crater failures from different vendors have been analyzed, and the root cause and mechanism have been discussed detailedly. The thermal stability and cure level of base materials of PCB against thermal impact from high temperature leadfree reflow process do play the key role in resulting in the pad crater failure. In these cases of this study the parameter ΔTg (cure factor) after reflow is found to be much more susceptible to the crater failure. Therefore an appropriate cure reagent and a good cure process are significant for strong binding of cohesive dielectric base enough to withstand pad crater failure. A horizontal cross section method was first applied to detect the open circuit crack site in the crater failure cases. The crack of open circuit should occur at the neck of pad with trace sooner or later if the crater occurred. In order to mitigate or eliminate the problem of pad crater, the ΔTg and pad strength after reflow shall be evaluated before selecting the PCB. Meanwhile the leadfree reflow process and design of pad, as well as PCB materials shall be taken into account seriously.