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Through an aggressive product development program which includes experiment and simulation, Amkor has developed the next level of WLCSP (CSpn1TM), a product which exhibits superior board level reliability when subjected to drop impact, a strong requirement for portable electronics. Failure mechanism of WLCSP under drop test has been established. Depending on type of WLCSP and test board design, 3 primary failure modes can be observed, i.e. copper (Cu) board trace crack, Cu RDL (Redistribution Layer) vertical crack and CulUBM (Under Bump Metallization) delamination. CSpnl can exhibit distinct failure modes under different test board and/or CSpnl designs, resulting in a vast difference in drop test lifetimes. The primary failure mode is shifted whenever the weakest link is removed through design improvement. This paper will focus on detailed analysis of copper board trace crack under drop test, using an integrated approach of testing, failure analysis, material characterization and modeling. Board design guidelines are formulated to understand the effects of 1/0 position, board trace routing direction, board trace width, tear drop design, PCB pad size, stack-up thickness, and alloy materials on board trace reliability. Comparison is also made on possible impact on Cu RDL reliability.

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The effects of solder joint geometry on wafer-level chip-scale package reliability have been studied both through simulations and board level reliability testing. In reliability tests on a 3.9×3.9mm2 die, an enhancement of nearly 2× in thermal cycling reliability was achieved by optimizing the solder joint and under-bump pad stack. In particular, undersizing the printed circuit board pad to produce a more spherical solder joint and reducing the polymer via size under the bump appear to be very important for improving thermal cycling results. Data collected here shows that joint geometry changes can be implemented without compromising drop performance. Methods learned were applied to the qualification of a 6.0×6.0mm2 die, a large platform for WLCSP applications.

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