PCB Technical - The formation mechanism and solution of BGA underfilled solder joints

The formation mechanism and solution of BGA underfilled solder joints
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Insufficient solder joints in BGA rework refer to insufficient volume of solder joints. BGA solder joints with reliable connections cannot be formed in BGA soldering. The characteristic of under-filled solder joints is that the appearance of the solder joints is significantly smaller than others during AXI inspection. Solder joints. For this BGA problem, the root cause is insufficient solder paste.

Another common cause of underfilled solder joints encountered in BGA rework is the wicking phenomenon of the solder. BGA solder flows into the through hole to form information due to the capillary effect. The deviation of the patch or the deviation of the printed tin and the absence of solder mask isolation between the BGA pad and the betrayal via may cause wicking, resulting in insufficient BGA solder joints. Special attention should be paid to the fact that if the solder mask is damaged during the rework process of BGA devices, the wicking phenomenon will be aggravated, which will lead to the formation of underfilled solder joints.

Incorrect design can also lead to underfilled solder joints. If a hole in the disk is designed on the BGA pad, a large part of the solder will flow into the hole. If the amount of solder paste provided at this time is insufficient, a low Standoff solder joint will be formed. The way to make up is to increase the amount of solder paste printed. When designing the stencil, consider the amount of solder paste absorbed by the holes in the plate, and increase the thickness of the stencil or increase the size of the opening of the stencil to ensure sufficient solder paste; One solution is to use micro-via PCB technology to replace the hole in the disk design, thereby reducing the loss of solder.

Another factor that produces underfilled solder joints is the poor coplanarity between the device and the PCB. If the amount of solder paste printing is sufficient. However, the gap between BGA and PCB is inconsistent, that is, poor coplanarity will cause insufficient solder joints. This situation is especially common in CBGA.

Therefore, the main measures to solve the insufficient solder joints in BGA welding are as follows:

1. Print enough solder paste;

2. Cover the vias with solder mask to avoid solder loss;

3. Avoid damaging the solder mask during the BGA rework stage;

4. Accurate alignment when printing solder paste;

5. The accuracy of BGA placement;

6. Correctly operate BGA components during the repair phase;

7. Meet the coplanarity requirements of PCBA and BGA, avoid the occurrence of warpage, for example, proper preheating can be adopted in the rework stage;

8. Use micro-hole technology to replace the hole in the disk design to reduce the loss of solder.