The thermally induced fiber alignment shifts of fiber-solder-ferrule (FSF) joints in laser module packaging have been studied experimentally and numerically. From direct measurements of the metallographic photos with and without temperature cycling, fiber displacement shifts of up to a 0.8 /spl mu/m were found after undergoing 500 temperature cycles. Experimental results show that the fiber shifts increase as the temperature cycle number and the initial fiber eccentric offset increase. The major cause of fiber shift may come from the plastic solder yielding introduced by the thermal stress variation and the redistribution of the residual stresses during temperature cycling. A finite-element method (FEM) analysis was performed to evaluate the variation of thermal stresses, the distribution of residual stresses, and fiber shifts of the FSF joints. Experimental measurements were in reasonable agreement with the numerical calculations. Both results indicate that the initial offset introduced in the fiber soldering process is a key parameter in causing the thermally-induced fiber shift of FSF joints in laser module packaging. The fiber shift, and hence fiber alignment shift under temperature cycling tests can be reduced significantly if the fiber can be located close to the center of the ferrule.