Fracture behaviour of flip chip solder joints

Abstract

The paper presents the results of isothermal cyclic shear experiments on flip chip solder joints. Two micro shear testers have been designed and built for this task. One tester is optimised to achieve high precision. Since it is actively compensated for its finite stiffness, this tester is able to record force displacement hysteresis with a resolution of better than 1 mN and 20 nm force and displacement measurements, respectively. The second tester works very similar but fits in a UHV chamber. This way, it enables in-situ SEM observations during the test. The experiments were done exactly the same way with both machines. The experimental program consisted on two types of isothermal experiments. The first type used a cyclic triangular strain wave with constant frequency but different amplitudes as the load function. The second type used a cyclic triangular strain wave with constant amplitude but different frequencies as the load function. The strain wave amplitudes ranged from /spl Delta//spl epsi/=0.3%...4%, the strain wave frequencies ranged from f=0,0004 Hz ... 10 Hz. The test temperature was 300 K. The investigated solder materials have been Sn63Pb37 and Sn95.5Ag4Cu0.5. In order to model crack propagation in FEM simulations, two different criteria have been tested, to develop a simple crack propagation model. The first criterion was plastic strain energy /spl Delta/W/sub pl/ and the second criterion was accumulated inelastic strain /spl epsi//sub acc/. The paper gives crack propagation relations for both criteria.