Degradation of thermal performance of ball grid arrays after thermal cycling

The thermal performance of ball grid array packages depends upon many parameters including die size, use of thermal balls, number of perimeter balls, use of underfill, and printed circuit board heat spreader and thermal via and spreader design. Thermal cycling can affect the integrity of thermal paths in and around the BGA as a result of the cracking of solder balls and delamination of the package, including at underfill interfaces. In this study, the impact of thermal cycling on the thermal performance of BGA's was investigated and quantified. A number of test boards which included a range of the parameters cited above were experimentally examined. A baseline thermal resistance was measured for each case, which was verified with numerical thermal modeling. The boards were then subjected to thermal cycling from -40degC to 125degC. Every 250 cycles the thermal performance was measured. Packages expected to be least reliable (with large die and no underfill), showed an increase in thermal resistance after 750 thermal cycles. Further increases in thermal resistance were observed with continuous thermal cycling until solder joint failure occurred at 1250 cycles, preventing additional measurements. The correlation between thermal cycling and thermal resistance was then analyzed using a numerical structural simulation model that predicted crack initiation in the solder joints. The first observed thermal resistance increase occurred in close proximity to the number of cycles where the finite element model predicted initiation of cracking in the thermal solder balls