Investigation on PCB related failures in high-density electronic assemblies

This paper presents an investigation on field returned open and short failures related to printed circuit board (PCB), including via hole crack, prepreg crack and insufficient circuit etching. After an experimental study with cross section, time domain reflectometry (TDR), and finite element (FE) modelling, it was found that weak plating and corrosion induced via hole crack was a major root cause of interconnect open failures. Such a failure was always mistakenly treated as solder joint open. Except for via hole crack, PCB prepreg crack was another contributor for open failures. The prepreg material for some lot code in production was in poor quality in terms of long-term durability. Dye and pry and cross section investigation showed that for some field returned units, the pad-to-prepreg interface was the weakest adhesion part in the entire solder joint interconnects. Crack propagated across the whole support and hence pad cratering occurred upon overstress. For the circuit short problem, insufficient etching of copper traces and pads was found to be a dominant contributor in harsh environment for a long time. Particularly, corrosion induced dendrite growth brought a high risk of circuit short of insufficiently etched conductors for the fine-pitch lead-free application. In summary, it is important to monitor and control the incoming quality of incoming PCBs so that the the risk of high field return rate and high cost for repair can be minimized.