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Epoxy Flux - A Low Cost High Reliability Approach for PoP Assembly
Keywords: Epoxy, Flux, PoP
Package on package (PoP) is a packaging rapidly prevails in mobile devices of electronic industry, due to its flexibility in combining memory and processor into one component with reasonably low profile. However, similar to BGA, the solder joints of assembled PoP are prone to crack upon dropping onto floor, thus need reinforcement by underfilling. Underfilling process needs underfill materials 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 is compatible with solder paste. Applied by 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, then placed onto the footprint pad on 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 stacked PoP is subsequently reflowed in oven together with other surface mount components placed on printed solder paste. Epoxy flux combines soldering and reinforcement with one single process. With a controlled pick up flux volume, venting channel is formed, allowing outgassing at reflow. The low stress characteristics of epoxy flux prevents formation of solder extrusion. Overall, epoxy flux provides a low cost and high reliability solution for PoP assembly.
Ning-Cheng Lee, Vice President, Technology
Indium Corporation
Clinton, NY

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