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Soldermask bubbles inmediately after screen printing

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Soldermask bubbles inmediately after screen printing | 26 July, 2020

Hello to everyone,

Have you ever experienced soldermask bubbles inmediately after printed the soldermask itself? (for screen printing method of LPISM). I am not talking here about blistering or any outgassing defects after exposing to high temperatures, like soldering process, etc.

I will try to explain the scenario: inmediately after the printing squeegee rises above the soldermasked board, you take the board out of the machine and you can see an almost perfect uniform and beautiful coating of the soldermask all over the board. However, after a few seconds (maybe half a minute) you can see the defects start. Small points (like a pin diameter) appear in some random areas of the board. They tend to grow a little and after a very small bubbling effect they "explode" or "bubble" leaving the pin center with a small ring around. The area into the ring seems with reduced soldermask coating since you can see the copper tonality. This defect occurs in maybe a minute or so.

Although these defects do not affect soldering in any way, the aesthetics of the boards are very compromised.

We are taking all precautions to avoid introducing air into the soldermask itself (like very slow mixing the soldermask and the hardener, etc.). However, we can see that the rising and lowering of the squeegees itself flooding and printing the boards introduce bubbles into the soldermask.

Could be a viscosity issue? We also had caution to mantain required viscosity and temperature of the soldermask. We also tried two passes of printing mask over the board (that is, flood/print/print) but again the defects arise.

Thank you in advance for any information.

Regards.

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