IMPACT OF CRACKING BENEATH SOLDER PADS IN PRINTED CIRCUIT BOARDS ON RELIABILITY OF BALL GRID ARRAY PACKAGES

摘要

Board laminate cracking starting from the edge of the Cu pads at pad-laminate interface and propagating under Cu pads is referred to as pad craters. Pad craters are primarily caused by mechanical stress or overload condition which may be induced by impact loads and are typically observed near the package corners. However, failures due to loss of adhesion between Cu pad and laminate are not considered pad craters. The reason this failure mode needs attention is because these cracks may propagate under Cu pads and cause a trace crack at the location where the trace meets that particular pad or they could provide an opportunity for an electrically conducting path or short within the PCB, among others. This paper discusses the typical pad crater crack signature observed along with the associated failures modes and reliability risks. Micro-hardness testing (MHT) was performed for boards subjected to eutectic Sn/Pb and lead-free reflow to determine if there was a significant difference in the laminate material properties that could affect the pad crater crack initiation and propagation. The impact of pad crater growth was studied under accelerated stress test condition to comprehend the reliability risk. A Design Of Experiment (DOE) was conducted to determine the extent of pad crater crack growth during thermal cycling. Results indicate an average 20% pad crater crack growth during temp cycling over 900 cycles.