AuSn alloy is widely used in silicon-based chip welding application because of its good thermal and electrical conductivity. The relationship of welding process parameters on the diffusion of silicon atoms to AuSn alloy was studied. The mass percentage of silicon atoms in AuSn solder increased by an order of magnitude when the welding temperature increased from 310 ℃ to 340 ℃. The diffusion mechanism of AuSn solder was analyzed. The influence of silicon atom diffusing into solder on product reliability and control measures were discussed.