PCB Tech - PCB problem Will the printed offset of the solder mask on the circuit board cause a BGA short circuit?

PCB problem Will the printed offset of the solder mask on the circuit board cause a BGA short circuit?

RD's requirements for board layout are becoming more and more stringent, because the boards are made smaller and smaller, and the requirements for solder mask size are also relatively reduced, but the manufacturer's process capability can't keep up, and those who have the ability to say that they have to increase the price, one Hearing the need to increase money, everyone began to shrink their hands and continue to use the existing PCBA board factory's manufacturing process. As a result, the solder mask printing offset exceeded the solder pad/pad.

What are the problems caused by the offset of the solder mask printing?

If the pad/pad of the BGA is offset, it will cause the pad of the BGA solder ball to become smaller, and finally cause a solder short (solder short). How can the smaller solder pad cause a short circuit? The opening on the original steel plate (stencil) is fixed, that is, the amount of solder paste in the same opening on the steel plate is theoretically fixed. If the size of the solder pad of each circuit board BGA is the same, the steel plate can be based on the actual solder pad size To give the appropriate opening size and solder paste volume, but if different batches of circuit boards, some solder pads are maintained at the original size, but some board solder pads are reduced, but the solder paste volume remains the same, it will become Too much solder paste causes an overflow phenomenon. In severe cases, it overflows to the adjacent solder pads and forms a solder short circuit.

But how can the solder pad (pad) become smaller?

It’s like wearing a headgear with only two eyes exposed. If the headgear is not placed in the correct position, if you put it slightly off, your eyes will be covered by the headgear, covering half of your eyes. The eyes can be thought of as solder pads, and the solder mask is the headgear. Maybe some people don’t know how to get it right now, so I’m talking about it, this solder mask is green paint! Understood! If you don’t understand anymore, just take out the board and look at the large green area! These green paints will cover the copper foils and circuits that do not need to be exposed on the circuit board to avoid unnecessary contact short circuits or oxidation. (Note: The solder mask of some boards will be printed in black or red, but most of them are green)

Because the company's new circuit board design has the tolerance of the solder mask at +/-1mil (+/-0.0254mm), but the process capability of the solder mask of the board factory is +/-2mils (+/-0.05mm), so The actual printing deviation of the solder mask is covered to the originally exposed solder pad, which makes the originally exposed solder pad smaller, so the problem has caused the above problems. Another reason is that in order to prevent the occurrence of HIP, we printed a larger amount of solder paste on the BGA outer ring, so the solder pads that short-circuited are almost concentrated on the BGA outer ring solder balls. (Related reading: How to solve the HIP (Head-In-Pillow) virtual soldering problem of BGA solder balls)

The next solution:

1. The circuit board factory is required to locally modify the position and size of the solder mask openings of the problematic solder pads. In principle, all solder pads under the BGA are required to have the same size.

2. Reopen the stencil to reduce the opening of the BGA outer ring solder pad that is prone to short-circuit, in order to reduce the amount of solder paste printing.