Numerical modeling of cyclic stress-strain behaviorudOf sn-pb solder joint during thermal fatigueud

摘要

This study examines the cyclic stress-strain response of solder joints in a surfaceudmounted electronic assembly due to temperature cycles. For this purpose, a threedimensionaludmodel of an electronic test package is analyzed using finite elementudmethod. The model consists of 92 solder joints arranged along the peripheral of aud24x24 solder array. The various different materials considered in the simulationudare Si-die, 60Sn-40Pb solder alloy, Cu-traces, Cu6Sn5 intermetallics, FR-4udsubstrate and PCB. The temperature- and strain-rate-dependent plastic stressstrainudcurves define the viscoplastic response of the near-eutectic solder alloys.udOrthotropic behavior of the FR-4 substrate and PCB is modeled. Other materialsudare assumed to behave elastically with temperature-dependent material properties.udTemperature loading of the package consists of an initial cooling down from theudre-flow temperature at 183 oC to 25 oC followed by thermal cycling between -40udto 125 oC. Results of the analysis show that the package warps with a magnitudeudof 93 µm at 25 oC after re-flow. In this process, the critical solder jointudaccumulated an inelastic strain of 0.856 percent. Faster temperature ramp rate atud370 oC/min (load case TR1) versus 33 oC/min (load case TC1) resulted in 12udpercent lower inelastic strain after completing 3 temperature cycles. However, theudinelastic strain magnitude is achieved in a much shorter time. The shear stressstrainudhysteresis loops display the largest strain ranges compared to other stressstrainudcomponents. The calculated shear strain range is 0.8 percent with theudcorresponding stress range of 34.0 MPa.