The formation process of BGA solder joints is unique and has distinctive features → solder paste and solder ball fusion, two times slumps, and automatic centering, as shown in Figure a. In the BGA reflow preheating phase, the solvent evaporates, the flux wets the solder ball and removes the oxide on the surface of the solder ball, as shown in Figure b.
As the temperature increases, the solder melts and the BGA sinks, which is called a slump, as shown in Figure c. As the temperature increases and the time increases, the BGA solder ball melts and fuses with the molten solder, and the BGA sinks and automatically centers, as shown in Figure d. The BGA sinking caused by the melting of the BGA solder ball and fusion with the molten solder is called secondary slump.It is a condition to realize the automatic centering function of the BGA. Only when the solder ball fuses with the molten solder and completes the second slump can it complete the automatic centering. Therefore, if automatic centering of the BGA is required, the temperature curve must be set to ensure that the BGA solder ball fuses with the molten solder. The power of the automatic centering comes from the surface tension of the molten solder.
The BGA was soldered with the same solder paste composition as the solder ball, and the formation process of the BGA solder joint was recorded with a high-speed camera. You can see the two times collapse processes of the BGA, the automatic centering process and the temperature point. The secondary slump temperature is 11 ~ 12 ° C higher than the primary slump. Why is the temperature different?
Theoretically, the solder alloy temperature depends on the composition. Therefore, the time when the solder paste melts is also when the solder balls melt. The reason for seeing the second slump is later than the melting of the solder paste.This is mainly because the oxide film on the surface of the solder ball prevents the fusion of the molten solder ball and the solder. In fact, the solder ball has also melted. There is only an oxide film on the surface, and the temperature needs to be raised to remove the oxide on the surface of the solder ball. As long as the oxide is broken by the molten solder (as long as a small opening is broken), the molten solder will immediately fuse with the molten solder ball, which is easy to observe in the test.
The above process shows that BGA welding does not need too high position accuracy, so it becomes simple for rework, and the smt process can be placed according to the silk screen frame at the corner of the BGA.