Acoustic Microscopy of Interfacial Crack in Sn-Ag-Cu Alloy Solder Joints under Shear Fatigue Testing

Abstract

Copper plates of 1 mm in thickness were joined with a solder layer (60 μm in thickness) of Sn-3.8 mass% Ag-1.2 mass% Cu alloy, and then subjected to the shear fatigue testing. At required numbers of fatigue cycles, the specimen was subjected to the acoustic microscopy to measure the shape and the size of the bonded region. After fatigue testing, the fracture surface was observed through scanning electron microscope to verify the distribution of fine penny-shaped cracks in the bonded region. The area of the bonded region estimated from the acoustic image monotonically decreased with increasing number of fatigue cycles. The larger the initial bonded region, the longer was the fatigue life. The density of the penny-shaped crack in the bonded region decreased with the distance from the edge of the bonded region, and increased with the number of fatigue cycles. The average size of the penny-shaped crack slightly increased with the distance from the edge of the bonded region, but showed no dependence on the number of fatigue cycles. The numerical calculation was also conducted with a simple model to obtain the bonded area as a function of the number of fatigue cycles. The calculated change in the bonded area with the number of fatigue cycles agreed well with the estimated one from the acoustic image. Calculated distributions of the density and the average size of the penny-shaped crack explained the change in the measured ones with the distance from the edge of the bonded region.