The mechanical strength of microvias in reflow cycling and environmental aging

Microvias are an integral part of electronics miniaturization as they enable high density interconnections in printed circuit boards (PCBs). Consequently, also the importance of their reliability and quality has increased. In this paper, the mechanical strength of microvias was studied using shear testing. The main aim was to examine if shear testing could be used to analyze the quality of the microvias and their failure modes. The test was performed by applying lead-free solder paste on to test pads with microvias. A reflow process was then used to form solder bumps. Test parameters included number of performed reflow cycles, number of microvias in a test pad and used surface finish. In addition, the effect of thermal cycling and constant temperature/humidity aging on the shear test results was examined. All the failures observed in the shear test occurred within the test PCB. That is, no bulk solder or intermetallic failures were observed. Increasing the number of reflow cycles from one to five caused a more significant drop in the measured shear energy values of the microvias than in the maximum shear force values. No significant difference could be observed in the shear test results between the test pads with different number of microvias. The aging of the samples did not change the failure mode. The thermal cycling test had a more significant effect on the shear test results than the temperature/humidity test. Based on failure analysis, the samples failed within the epoxy layer between the copper pad and the glass fibers of the microvia layer. The microvias were observed to fail directly beneath the copper pads, which was caused by deliberately using an unoptimized manufacturing process. The performed shear test showed promising results as a test method for the reliability and quality testing of microvias.