Failure Mechanism Study for Low-k Device Bond Pad Crack Post Temperature Cycle

Copper (Cu) wires are increasingly used in semiconductor devices to provide cost-effective packaging. Many of the challenges for Cu wire have been evaluated to meet the stringent quality requirements, especially for automotive applications. The reliability requirement for automotive devices are more stringent than others. The wafer technology used in this study was CMOS40, with an Al thickness is 28KA and a 53um pad opening and the package studied was a MAPBGA. The failure mode during ATE test is OPEN fail post temperature cycling and was associated with delamination at the bond pad interface. Contamination was not observed at the delaminated interface. Closer visual inspection showed the die attach fillet was high on the side wall of the 7-mil die in the same area as the failed pins. Mechanical simulation was done to investigate the failure mechanism. The result showed that high die attach fillet can cause higher stress and delamination that can lead to Cu ball failure during temperature cycling. The simulation showed an exponential drop in the energy release rate as the fillet height decreased. Lowering of the filet height on the die edge and making it more uniform was done by optimizing the die attach epoxy dispense pattern. Electrical test was performed on the assembled parts at T0, post MSL3/260C, and post 700 temperature cycles (-55 degrees C to 150 degrees C). After filet height optimization, all units pass electrical test with no failures.