Thermal cycling induced plastic deformation in solder joints. II. Accumulated deformation in through-hole joints

Abstract

A thermal-cycled through-hole solder joint is studied. Nonlinear elastic/plastic solder properties at different temperatures and a steady-state creep law are used to characterize the deformation of the eutectic Pb-Sn solder. Large geometry changes in the solder and pin structure are observed experimentally. The deformation observed is much larger than expected from a simple thermal expansion mismatch calculation. This phenomenon is explained as a combination of plastic and creep deformations which accumulate during the thermal cycling. Because of the complexity of multiaxial stresses in the joint due to thermal expansion mismatch, finite-element analysis is required to characterize stress and strain in the solder joint. The large plastic deformation observed in the solder fillet is quantitatively simulated by the analysis. The majority of the deformation is a result of the time-dependent creep, while deformation occurring during the time-independent temperature change is minor. The displacement of the IC pins is recorded after each cycle and approaches the value observed in an actual joint after 1000 cycles. Thermal cycling fatigue life prediction based on uniform shear and the Coffin-Manson equation is found to be insufficient in dealing with the complex deformation mechanism of solder. >