The cause and mechanism of tin spattering in the free flux laser jet solder ball bonding (LJSBB) process were analyzed using a right-angle Au/Sn-3.0Ag-0.5Cu/Au micro solder joint by removing the surface contamination and the volatiles on the Au bonding pads and observing their inner microdefects of Au layer. Results reveal that the prompt expansion of the gas in the microvoids distributed randomly in the Au layer is the key factor leading to spattering. Moreover, if the positions of the microvoids are different, the shape of defects are different. When the microvoids are near the edge of the Au bonding pad, tin spattering occurs easily, whereas the microvoids far away from the edge lead to a gas hole, evenly bring to the tin spattering and the gas hole simultaneously. Because of the solidification time and the limited floating speed of the gas bubble, the larger initial gas bubble easily results in the gas hole or tin spattering, and the smaller one leads to the inner gas hole.