FRACTURE OF LEAD-FREE SOLDER JOINTS AS A FUNCTION OF STRAIN RATE, LOCAL END GEOMETRY AND THICKNESS

摘要

The fracture of SAC305 solder was investigated as a function of strain rate using Cu-solder-Cu double cantilever beam (DCB) specimens joined with a series of 2 mm long discrete solder joints of 150 (□) m thickness. The joints were then fractured with various strain rates under mode I and mixed-mode loading conditions. The failure of each joint in the DCB was accompanied by a sharp drop in the applied load. These maximum loads were used to calculate the initiation critical strain energy release rate, G_(ci), of the solder joints using a finite element model. The results showed a substantial increase of about 75% in the solder joint fracture toughness at strain rates of 0.05 to 1 s~(-1) compared to that under quasi-static loading conditions. This trend suggests that the solder G_(ci) increases rapidly with strain rate whereas the G_(ci) of the IMC is relatively independent of strain rate. Negligible changes in G_(ci) were measured when the solder joint thickness was increased to 400 (□)m. The dependence of crack initiation on the local shape of the solder joint at its end was investigated by fabricating the 2 mm long joints with either a square end (using Kapton tape) or a rounded end (using an embedded wire). Interestingly, these two local geometries produced almost identical values of G_(ci), suggesting that initiation was not a strong function of the shape of the solder joint.