PCB fact(9) - BGA Resist: “ON PAD” Design
In the field of modern electronic manufacturing, BGA (ball grid array) package is popular because of its high density and high performance. However, have you ever encountered the problem of irregular solder resist next to BGA ball in your PCB production? Do you apply all the designs of your solder mask to soldermask opening?
In fact, not all pad parts are suitable for soldermask opening. As shown in the figure, this is a typical case of irregular solder resist caused by alignment problem, and Solder Mask Opening is adopted:
According to the engineer's analysis, it is mainly caused by solder resist misalignment and line etching. In fact, the most important reason is that soldermask opening may not be able to adapt to BGA pads with such a small density.
How do we solve it effectively and quickly?
Designing the surrounding solder mask in “ON PAD” mode
We use a simple metaphor to explain this problem.
Imagine that you have a piece of paper with many small holes, which are like window areas on BGA pads. Now, you want to fill these small holes with water, but it is easy for water to leak out from the edges of the holes.
It's like drawing an irregular circle around a small hole in paper, trying to stop water from leaking out. However, because the distance between holes is very close, it is easy to shake hands when drawing a circle, resulting in irregular drawing of the circle, and water will still leak from the edge of the hole. This is just like on BGA pads, because the distance between pads is very close, it is difficult to draw a perfect window on each pad accurately, and solder is easy to leak from these irregular window edges, forming irregular solder resist.
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It's like putting a small cover on every small hole, so that no matter how irregular the paper around the hole is, water won't leak out. On BGA pads, On Pad is like covering each pad with a complete cover, so that no matter how close the pads are, the solder will not leak from the edge of the pad, because each pad is completely covered.
Therefore, the method of On Pad can solve the problem of irregular solder mask opening on BGA pads, because it provides a complete protection for each pad, rather than relying on drawing a perfect circle around each pad. In this way, even if the distance between pads is very close, the integrity and uniformity of solder mask can be ensured.
After the change, you will get regular solder resist every time!
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Should you have any questions regarding the 'ON PAD' process or any other PCB-related inquiries, or if you wish to discuss further how to improve the quality of your PCB designs and manufacturing, please don't hesitate to reach out to me via email.
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