Cleaning Machine and Chemistry

Hello Mark,

I wish it were that simple so I could direct you to a particular manufacturer and stencil cleaner model. Unfortunately, there are several issues complicating the process of cleaning all these applications in one machine.

First and foremost is; what chemistry do you use that will clean both solder paste and adhesive? There are chemistries that will clean both (Smart Sonic�s 440-R SMT Detergent being one) but these chemistries usually require different conditions: different temperatures, concentrations, filtration requirements and possibly different mechanical agitations (machines).

Another major concern is cleaning populated misprinted PCBs in the same machine as stencils. There is an article on our website by Richard Clouthier (formerly from AMTX and Photo Stencil and currently with the Rochester Institute of Technology) that addresses this application (www.smartsonic.com and click on the "Recommended Reading" button or see the July '96 issue of EP&P). Mr. Clouthier presents his opinion of different types of stencil cleaners, chemistries and overall stencil cleaning processes. He references the use of a �spray� type stencil cleaner for cleaning populated (double-sided) misprints as a �solder ball nightmare.� The explanation is that when cleaning stencils, literally millions of solder balls are broadcast throughout the cleaning chamber and that these solder balls will become entrapped within the populated side (A-side) of the PCB. Furthermore, the filtration systems used to recirculate the wash solution are not adequate to filter out 100% of the solder balls. Assuming the filters are 99.9% efficient when filtering millions of solder balls, literally thousands of solder balls will be �bombarding� the components on side �A� of the PCB (this condition can expect to worsen as the filters become loaded). For this reason, spray-type stencil cleaners that are used for cleaning raw solder paste should never be used to clean a populated assembly.

Ultrasonic stencil cleaners do not use sprays and therefore do not �broadcast� the solder balls. If the misprinted side is oriented downward in the ultrasonic bath, the solder balls will fall away from the populated side by gravity and not contaminate side �A�. However, when using ultrasonics to clean populated misprinted PCBs, it should be noted that the ultrasonic agitation is capable of delivering the wash solution into tight tolerance areas where sprays cannot penetrate. Therefore, spray rinsing may not be effective in removing wash solution from under or around low standoff components. An ultrasonic rinse with DI water or fresh solvent may be required to thoroughly remove the wash solution.

I would recommend an ultrasonic stencil cleaner that has two wash tanks; one for cleaning solder paste and one for cleaning adhesives. That way the wash solution may be kept at the temperature and concentration for cleaning each application. Otherwise, waiting to heat up and cool down to change from solder paste to adhesive and back would be very cumbersome. The contaminants will also remain separated by using two ultrasonic wash baths.

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