Effect of High Temperature Storage on Reliability of Sn-Ag-Cu Flip Chip Solder Bumps

In this study, flip chip (FC) daisy chain test dies with Sn- Ag-Cu solder bumps on Al/Ni(V)/Cu UBM were subjected to a high temperature storage (HTS) test at 175degC for up to 2000 hours. Solder bump shear tests were then performed and the electrical resistance of a daisy-chain pair of solder bumps was measured to investigate the effect of high temperature storage on the reliability of Sn-Ag-Cu solder bumps. After HTS at 175degC for only 24 hours, the solder bump shear strength dropped significantly. But from 100 to 2000 hours, the bump shear strength levels off. Similar to the bump shear strength change, at 175degC for only 24 hours, the electrical resistance of a daisy-chain pair of solder bumps dropped substantially. From 24 to 2000 hours, the electrical resistance increases at first and soon becomes stable. During high temperature storage, Ni atom dissolves into the Cu6Sn5 phase and a complex (Cu,Ni)6Sn5 IMC phase was formed. With prolonged HTS, more Ni atoms dissolve into (Cu,Ni)6Sns IMC. At the same time, the total IMC thickness increased monotonically. During HTS, fine Ag3Sn IMC particles also coarsened noticeably. Fracture surface observation reveals that in the as- received state all solder bumps failed in the bulk solder, while after 24 hours at 175degC brittle failure occurred. For the brittle failure, solder bumps failed not only at the interface of the solder and IMC layer but also at the interface of the IMC layer and UBM. The obtained knowledge of the reliability of Sn-Ag-Cu solder bumps on Al/Ni(V)/Cu UBM will be important for improving the design and processing of the UBM and the reliability of FC packages.