Epoxy Flux – A Low Cost High Reliability Approach for PoP Assembly

Abstract

Package on package (PoP) is a packaging that rapidly prevails in mobile devices of the electronic industry, due to its flexibility in combining memory and processor into one component with a reasonably low profile. However, similar to BGA, the solder joints of assembled PoP are prone to cracking upon dropping onto the floor, thus needing reinforcement by underfilling. The underfilling process needs an underfill material plus additional dispensing equipment, dispensing and flowing steps, and subsequent time-consuming curing. Furthermore, underfilled PoP suffers solder extrusion upon rework, particularly when reworking at the opposite side of the board right underneath the PoP. Epoxy flux is a new material developed to address the issues described above. It is a liquid epoxy with fluxing power, and it is compatible with solder paste. Applied by a dipping process, epoxy flux can be used at the mounting of bottom package and top package. First, the bottom package is dipped in a film of epoxy flux, and then placed onto the footprint pad on a PCB with or without solder paste. Then, the top package is dipped in epoxy flux and placed on top of the bottom package. The PCB with a stacked PoP is subsequently reflowed in the oven together with other surface mount components placed on printed solder paste. Epoxy flux combines soldering and reinforcement into one single process. With a controlled pick up flux volume, a venting channel is formed, allowing outgassing at reflow. The low stress characteristics of epoxy flux prevents the formation of solder extrusion. Overall, epoxy flux provides a low cost and high reliability solution for PoP assembly.