"Recycling" Paste?

I wouldn't mix it in a jar with new paste. But in another jar to be used on the next job, sure. I mean if your concern is contaminates, doesn't that mean its contaminated on the stencil? Unless the board is a hard one to print, or has really tiny apertures, I generally have my people mix the "old paste" with fresh past on the stencil at 1:4 or so.

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The people that like you to throw paste away every use are the paste manufacturers and people that don't understand.

If your shop environment is clean and you have temperature and humidity control, I concur that there is nothing wrong with re-using paste, but you need to do your due diligence based on common sense.

If you have customers that mandate you use new paste every time then make sure they are paying for the amount you pitch. That $hit is expensive.

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Reusing paste that has been on the stencil for long periods of time can be a little tricky. Depends on the formula, environment that its used in, quality issues, technology being printed.

Throughout the shift, new paste needs be added as its being used to maintain supply, which will "freshen" the paste on the stencil, slowing the rate of degradation, extending its stencil life. Solder paste is hygroscopic, it absorbs moisture when exposed to the environment, so it may become slumpy and bridge defects, solder balls, etc. may start to rise. Solvents in flux can also evaporate in dry environments and over time, and the opposite may occur, it starts to dry out, doesn't print well, and you get insufficient solder, poor release, etc. If you have inline SPI, you may see this degradation in real time in increased callouts, shape errors, washed boards, etc.

So as far as keeping old paste for the next job, or next shift, etc. assuming its still inside the performance window, no issues. Keep used paste in a separate jar from fresh paste. Add fresh to used and combine to freshen, maintain the proper volume of paste on the stencil to avoid excessive waste and optimum print roll. When paste is no longer performing or hits the established exposure limit, dispose of it in a chemical bin, hopefully you have a recycle company that will pick it up for recycling like with other solder waste and dross, maybe make a buck back.

Institute a standard based on good data and good practices.

This message was posted the Electronics Forum @ SMTA

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