While already in widespread use in the consumer market for handheld, portable electronics, package on package (PoP) continues to gain in popularity, and seems likely to be implemented in other types of electronic assemblies. In order to define internal assembly processes for PoP devices, an assembly and reliability study was undertaken. The first goal of this study was to assess the performance of different dippable fluxes and solder pastes in terms of assembly yield and quality, and the second goal was to assess the reliability of different variations of the test vehicle under accelerated thermal cycling. Thermal cycling was selected as the test method in this case to help define appropriate assembly processes for PoP devices used in higher reliability assemblies, where performance under thermal cycling would be a significant consideration.
As with other small area array packages, both the top and bottom devices in the PoP stack are offered with a range of different alloys. This study includes three different alloy variations – a SAC305 stack, a SAC405 stack, and a stack with a SAC105 upper package, and a SAC125 lower package. In addition to the variation in ball alloy, different dippable materials were included in the study. Three different manufacturer’s dip fluxes were used, as were three different dippable solder pastes, one from each of three manufacturers. The dippable pastes were all SAC305 – two with Type V powder, and the third with Type VI powder. Rework and underfill were also included as variables in some cells.
Assemblies were subjected to thermal cycling from -40°C to 125°C to assess the reliability of the different assembly variations. Results of the thermal cycling are presented, and preferred assembly methods based on quality and thermal cycling performance are discussed.
Key words: Package on Package, Dip Flux, Dip Paste, Reliability