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WLCSP Solder Bumping Mechanical Reliability Testing (Lead-Free Solder Alloys and Electroless Nickel UBMs)
Keywords: WLCSP, Wafer Bumping, Flip Chip
Electroless Nickel/Gold is widely considered the lowest cost under-bump-metallization (UBM) in the electronics packaging industry. Its implementation has been driven by the need to further reduce the costs of manufacturing. Recent results have shown that the use of an electroless nickel UBM can also increase the mechanical reliability of electronic devices when used with lead free solders. The mechanical strength of the solder interconnect is extremely important in portable electronic devices, where WLCSP products are used in high numbers. Accelerated mechanical testing has shown that the intermetallic layer that is formed between the solder and the UBM, is one of the weakest interfaces that can lead to premature failure. Because the electrolessly plated nickel forms different intermetallics compared to either sputtered or electroplated UBMs, the mechanical strength is also expected to be different. In this study, lead free solder spheres where placed onto electroless nickel-plated semiconductor I/O pads. The strength of the solder interconnect was evaluated using high speed pull testing, SEM, and FIB analysis. Three different electroless UBM layers were investigated. e-Ni/Au, e-Ni/Pd/Au, and e-Ni/Pd. The flash of palladium and/or gold had a significant affect on the intermetallic phases formed. The thickness of all three of these metal layers was varied to evaluate the affect this had on the intermetallic formation. Four different lead free solder alloys were evaluated (SnAgCu). Several different reflow profiles and reflow techniques were evaluated for their affect on the interconnect strength.
Andrew Strandjord, Senior Manager Advanced Packaging Technologies
Pac Tech
Santa Clara, CA

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