WLCSP Mechanical Reliability - High Speed Pull Testing (Lead-Free Solder Alloys and Electroless Nickel UBM)

摘要

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 [1-2]. This mechanical strength of the solder interconnect is especially important in portable electronic devices, where WLCSP solder bumps are used in high numbers. Mechanical testing of WLCSP solder joints has shown that the intermetallic layer that is formed between the lead-free solder bump and the UBM is often brittle and can lead to premature failure [3-4]. Because electroless nickel UBMs form different intermetallics with the solder compared to either sputtered or electroplated UBMs, the mechanical strength of the solder joint is also expected to be different.rnSmall variations in the composition of SnAgCu based solder alloys have been shown to result in significant differences in mechanical reliability [5]. The rate of intermetallic formation and distribution are highly dependent on the solder alloy composition, the UBM composition, and reflow profile. In this study, three solder alloys (SAC-105, SAC-305, SAC-405) were investigated in combination with e-Ni/Au and Ni/Pd UBMs. The strength of the solder interconnect was evaluated using high speed solder pull testing [6].