How to improve the BGA (Ball Grid Array) package solderability is always a big challenge for SMT (Surface Mount Technology) process. Especially for the fine pitch (ball pitch less than 0.8mm) BGA is more difficult to be soldered on the PCB. Besides, it’s soldering quality is also hard to be inspected since its solder hide under its body and sometimes even can’t be inspected by 2D X-Ray inspector. (5Dx or angle rotation X-Ray maybe ok to inspect BGA soldering quality but expensive)
As experience tell us that most of BGA solderability fail is caused by the HIP (Head-In-Pillow) soldering defect and over 90% of the HIP defect show on four corners of BGA package. Because of the diagonal distance is longest and greatest impact from board warpage and BGA deformation. There are many senior SMT engineer had done some experiments and prove that Increasing the solder paste volume on the pads under BGA can reduce the HIP defect risk.
So, below is the idea how to partially increase the solder paste volume under and BGA and not increase the solder paste volume for all balls. Be careful that give too much of the solder paste volume under the BGA will bring solder short.
Here WorkingBear modify the stencil aperture size for BGA package to control the solder paste volume. Ideally, the outer rows of the BGA ball shall give more solder paste volume and other inner BGA balls shall give less solder paste. This is because HIP always happen at 4 corners of the BGA and rarely show around inner row of BGA balls.
Below pictures show the BGA ball size is less than 0.4mm of diameter and gives circumscribed circle pad method . So you will find the 4 sides of outer row BGA ball size are bigger than the others.
Below picture show the actual stencil aperture result for BGA packing with ball size less than 0.4mm diameter.