Reliability study of Sn3Ag0.5Cu solder joints in QFN/PCB assemblies

The reliability of thermally cycled Sn3.0Ag0.5Cu (SAC305) Quad Flat No-Lead (QFN) component interconnections was characterized by imaging and detailed microstructural analysis by X-ray microscopy, field emission scanning electron microscopy/electron backscatter diffraction (FESEM/EBSD), and scanning transmission electron microscopy (STEM). The 0–100 °C thermal cycling test (TCT) was run for 6000 cycles. Four different test sets including two printed circuit board (PCB) surface finish materials (either immersion tin (ImmSn) or electroless nickel-immersion gold (ENIG)) and QFN components with two interconnection pitches (either 0.4 mm or 0.5 mm) were tested. Weibull characteristic lifetimes ranged from 3500 to 6000 cycles and shape parameters ranged from 5.1 to 8.7. Although samples with ENIG showed higher characteristic lifetimes, especially with the 0.5 mm cases, it did not have a major effect on reliability in these tests. The recrystallization of the as-soldered tin matrix, the coarsening of the fine Ag 3 Sn particles, and the crack initiation under the wedge-shaped burr at the component corner occurred between 500 and 750 cycles, whereas the intermetallic compound (IMC) layer growth was slow during the thermal cycling test in every test assembly. Thus, it was concluded that the layer growth has a negligible effect on the inelastic strain accumulation and the subsequent recrystallization of the critical corner joints. • The reliability of thermally cycled SAC305 QFN interconnections was characterized. • 0–100 °C thermal cycling test (TCT) was run for 6000 cycles. • Weibull characteristic lifetimes ranged from 3500 to 6000 cycles. • The recrystallization of corner joints started between 500 and 750 cycles. • IMC layer growth has a negligible effect on the inelastic strain accumulation.