Impact of Topside Cooling on the Solder Joint Lifetime of eWLB Packages

The impact of topside cooling (TSC) on the solder joint lifetime of an embedded wafer-level ball grid array (BGA) package (eWLB) is analyzed using both simulation and experiments. The investigations focus on TSC setups using thermal gap pad (TGP) materials as thermal interface material (TIM). Material characterization of the TGP shows that the material exhibits excessive relaxation behavior, which is not reported in the supplier datasheet. For the experimental testing, a housing is developed which avoids any negative impact of the PCB mounting on the solder joint lifetime. In a TSC setup close to the application where the TGP is compressed by deflecting the heatsink towards the component, the relaxation of TGP reduces the topside loading to a level that no impact can be observed, neither by simulation nor experimentally for the investigated TGP material for 30% and 50% compression. In order to intentionally provoke over-stress during testing, mechanical springs are added to the housing. Both simulation and experiments show that the eWLB package can withstand lower force level without significant impact on its solder joint lifetime, but that the solder joint lifetime significantly reduces for higher spring forces.