Numerical Modeling of Cyclic Stress-strain Behavior of Sn-Pb Solder Joint During Thermal Fatigue

This study examines the cyclic stress-strain response of solder joints in a surface mounted electronic assembly due to temperature cycles. For this purpose, a three dimensional model of an electronic test package is analyzed using finite element method.The model consists of 92 solder joints arranged along the peripheral of a 24 x 24 solder array.The various different materials considered in the simulation are Si-die, 60Sn-40Pb solder alloy, Cu-traces, Cu6Sn5 intermetallics, FR-4 substrate and printed circuit board (PCB).The temperature and strain rate dependent plastic stress-strain curves define the viscoplastic response of the near-eutectic solder alloys.Orthotropic behavior of the FR-4 substrate and PCB is modeled. Other materials are assumed to behave elastically with temperature dependent material properties. Temperature loading of the package consists of an initial cooling down from the re-flow temperature at 183 °C to 25 °C followed by thermal cycling between -40 °C to 125 °C Results of the analysis showed that the package warps with a magnitude of 93 |jm at 25 °C after re-flow. In this process, the critical solder joint accumulated an inelastic strain of 0.856 percent. Faster temperature ramp rate at 370 °C /min (load caseTRI) versus 33 °C /min (load caseTCI) resulted in 12 percent lower inelastic strain after completing 3 temperature cycles. However,the inelastic strain magnitude is achieved in a much shorter time.The shear stress-strain hysteresis loops display the largest strain ranges compared to other stress-strain components. The calculated shear strain range is 0.8 percent with the corresponding stress range of 34.0 MPa.