Solder paste relative clearance

Manufacturing
1

Hello

I ordered some PCBs from JLC along with a stencil. I populated the board manually after applying T4 paste with the stencil and reflowed in a small oven. The stencil has a thickness of 0.12 mm.

I set -5% solder paste relative clearance in board setup but the results were not that great. I had to rework a lot of bridges on an LQFP-48 part with .5 mm pitch. The remainder of the board are parts with a far coarser pitch (SOT-223, SOIC-16W, 0805) and those reflowed flawlessly. The rework was easy to do but I am convinced that I can improve my process.

Do you recommend reducing the relative paste clearance further? To what number? Is the percentage I enter in “board setup” relative to the area/volume or to the length of the edges of the pad? Also, I tried to set it to -50% to answer that and then the paste aperture completely vanishes, which has me confused.

I am happy for any advice. Thank you and best regards!

Matt

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2

How did you place the LQFP? Any slight misalignment will lead to solder bridging.

3

Thank you for your response. We have one of those electronic suction machine thingies that pull a slight vacuum when you push a pedal and with that, you can pick up the part and drop it in the appropriate place. That said i guess the answer to your question is “I did it manually” and any potential misalignment is on me. I thought I was pretty careful when placing the part but then again, I have little experience with this process.

I did some research before posting here and bridges on fine-pitched part are primarily attributed to too much paste (see, e.g. here, although this post also claims that 0.5 mm QFP is impossible without stencil and I have done that in the past and it was easy… Then again this seems to be common advice, see e.g. here and here [I am not allowed to add another link as new forum user] but of course, too much paste is not the only reason for bridges. Still it somehow makes sense for me to start there.)

I am still confused about the -50% relative aperture thing that shrinks the aperture down to zero…

4

It’s all of the above, plus the size of the solder balls and the thickness of the flux.
If you plan on doing a lot of these types of assemblies, you should make a test board with various sized openings, and see which works best for your process.

Note that even professionals with finely-tuned processes get the occasional solder bridge on QFPs.

5

And the age of the solder paste and how it has been stored and how long it has been left on the board, etc, etc