Optimized Design of Various Ag Decorated Sn-xAg-Cu0.7 Solder Bump on Cycling Fatigue Reliability for Wafer-Level Chip Scale Packaging

Effects of Ag content (0 ∼3 wt%) in Sn-xAgCu0.7 solders on microstructure characteristics and low cycling fatigue at different temperature conditions are overall investigated. To increase Ag content, the solidus points 228.8 °C of Sn-Cu0.7 gradually decrease to 218.5 °C and temperature range of solid-liquid coexistence phase reduces at the same time. The Sn-Cu0.7 matrix consisted of small particles of Cu6Sn5 within β-Sn equiaxial grains and did not significantly influence solder hardness. Moreover, much intermetallic compound of plate-like Ag3Sn and rod-like Cu6Sn5 existed in Sn-xAgCu0.7 solders enables to enhance the hardness due to dense network of Ag3Sn precipitation and near eutectic point. As a result of plastic displacement decreases with higher Ag additions, better fatigue lifetime could be achieved at Ag content to 1.5 wt%. Besides, crack stemmed from thicker intermetallic compounds (IMC) layer in Sn-3.0Ag-Cu0.7 solder interface will decrease fatigue performance especially for 80 °C and 120 °C.