Numerical Modeling of Cyclic Stress-strain Behviour 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 consist of 92 solder joints arranged along the peripheral of a 24x24 solder array. The various different material considered in the simulation are Si-die, 60Sn-40Pb solder alloy, Cu-traces, Cu6 Sn5 intermetallics, FR- substrate and printed circuit board(PCB). The temperature and strain rate de[endent plastic stress-strain curves define the viscoplastic response of the near-eutectic solder alloys. Orthotropic behaviour of the FR-4 substrate and PCB is modeled. Other materials are assumed to behave elastically with temperatire dependent material properties. Temperature loading of the package consist of an initial cooling down from the re-flow temperature at 183 degree C to 25 degree C followed by thermal cycling between -40 degree C to 125 degree C. Results of the analysis showed that the package warps with a magnitude fo 93 micrometer at 25 degree 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 degree C/min (load case TR1)versus 33 degree C/min (load case TC1)resulted in 12 percent lower inelastic strain after completing 3 temperature cycles. However, the inelastic strain magnitudeis 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