The influence of strain-rate dependent solder material models on the interconnection stress of BGA components in Jedec drop test

Abstract

The strain-rate dependent yield property of solder has a strong influence on the stress distributions in solder interconnections during JEDEC drop tests. In this paper the effect of this material behavior is shown by FEM drop test simulations of productive BGA components in order to predict realistic failure mechanism.Two lead free solder alloys SnAg1.3Cu0.5 (SAC1305) and SnAg3.5 (SA35) were characterized in a high strain-rate tensile tester in a range of 40s -1 to 800s -1 . The specimens were produced in a casting process with a high stress dimension close to real solder joints, in order to create similar size effects. The stress-strain behavior was recorded with high resolution using strain gauge sensors. The evaluation of the yield stress dependency on strain-rate was done by the measured stress data. Hereby the SAC1305 solder revealed a high sensitivity of yield stress on the applied strain-rate, while the SA35 solder marginally increased its yield stress.The influence of both strain-rate dependent solder models on the interconnection stress distribution was tested against a simple bilinear and an elastic material model. The simplified models cause excessive stress in the copper pads of substrate and PCB due to their high and neglected yield behavior, respectively. The accurate solder models significantly reduce the copper stress by generating locally higher plastic deformations and a wider distribution of plastic strain in the solder balls. The different strain-rate sensitivity affects the distribution of plastic strain between solder and copper. High strain-rate sensitive solders reduce the plastic strain in the solder with higher PCB deformations and increase the stress and strain in the copper compared to materials with a low sensitivity.The application of strain-rate dependent solder material models is necessary for realistic stress interpretations in drop test conditions. The neglect of this material behavior leads to unrealistic stress distributions and thus, it leads to wrong failure assumptions and lifetime predictions. IntroductionThe solder materials are key factors for the reliability of 2 nd level interconnections. The solder balls transmit electrical signals and have to cover high mechanical stresses resulting from the different behaviors of PCB and components during thermal or mechanical loadings. Due to these permanent stresses solder interconnections are prone to fail and cause reliability problems. That is why reliability experiments are more and more often analyzed by FEM simulations, in order to gain inside into the complex stress creation and distribution during these tests. Those analyses demand precise material properties and an adequate material model of solder which is the most challenging input for FEM simulations.