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Improved SMT and BLR of 0.35 mm Pitch Wafer Level Packages
Improved SMT and BLR of 0.35 mm Pitch Wafer Level Packages
This study shows optimized print process can be achieved using standard materials, while also considering board level reliability.
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Authored By:
Brian Roggeman and Beth Keser
Qualcomm Technologies, Inc.
San Diego, CA, USA

Summary
The surface mount process to successfully mount 0.35mm pitch Wafer level packages (WLP) was studied. Processes including solder paste printing and flux dipping were examined and optimized. The paste print process was optimized in terms of printer setup to use standard materials. Flux dipping was used as an alternative to paste printing to address situations where print challenges cannot be overcome.

Flux dipping often results in a decrease in board level due to the decrease in solder joint standoff, so a novel dip encapsulant was also included. The dip encapsulant is a polymer adhesive which encapsulates the BGA joints after reflow, providing some gain in reliability over standard flux dipping. For board level reliability, temperature cycling was performed using -40 to 125 C temperature swings.

The lifetime and failure modes of the package were analyzed. This study shows optimized print process can be achieved using standard materials, yet alternative SMT processes can also be adopted to successfully assemble fine pitch WLP packages, while also considering board level reliability.

Conclusions
This study examined improvements in the SMT process for successfully integrating 0.35mm pitch wafer level packages. The solder paste print result was shown to be sensitive to printer parameters, including pressure, speed and squeegee type.

It was shown that an optimized setup could be achieved on a challenging stencil aperture area ratio of 0.55. More importantly, the results were achieved using a standard Type 4 paste, and otherwise standard equipment sets and stencil type. Type 5 paste further improved the print result in terms of maximizing transfer efficiency and minimizing variability, however it was determined that the finer powder size was not required if print setup was optimized.

As an alternative to printing, a novel solder joint encapsulant was investigated. Full solder joint coverage was only achieved after significant adjustments to the dip process. When full coverage could be achieved, such as on edge/corner solder joints, the reliability was improved over a standard paste print process and tacky flux. Ultimately paste printing with well controlled print process can achieve excellent SMT results and high reliability of 0.35mm WLP packages.

Initially Published in the SMTA Proceedings

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