There are several answers to your question about cleaning mis-printed bare PCBs. The first is to not make many misprints to start with. To that end, make sure your stencils are properly designed, in good condition, and thoroughly cleaned after use. I hear from the industry the best squeegees are the new designs made from nickel, which are less prone to nicks and damage and so provide better prints.

Make sure your solder paste is fresh and properly stored. And, perhaps most importantly, don't use cheap stencil rolls but higher quality paper that delivers better wiping.

There is a DuPont fabric coming into use right now called the "FP" paper. This is the only paper in the industry that can clean stencils without using any solvent. This has been proven to speed the printing process, reduce defects and enhance yield. This is truly a case of getting what you pay for; cheap paper causes a lot of problems down-stream.

Now, if you have a stack of misprinted boards, the best cleaning option is a small aqueous cleaning machine. There are many of these on the market today, Branson, JNJ and Smartsonic are three common vendors. These machines MUST be used with nonflammable, aqueous-based solvents, such as the "BGA Stencil Cleaner" from my company.

Look for solvents with higher (better) toxicity ratings, lower (better) NFPA ratings, low aromas, and high Kb values or flux-loading values. Which ever vendor you select, this combination should give you more than satisfactory cleaning results at the lowest possible cost.

But if you don't have a board cleaner, you're going to have to clean them by hand. To this end, I'd recommend a presaturated wipe and compressed air. My company sells a presat wipe, alcohol-and-water mix, 100% polyester paper, 100 sheets per tub, about 5 inches x 8 inches in size, for $14 or so. This is widely used for stencil and board cleaning. Other vendors also feature their products on the web, a Google-search should find many of them.

Use the presat wipe to remove the solder paste; then the compressed air to blow residual solder paste from any apertures. Then, repeat the process to ensure the board is perfectly clean. The total process should take an experienced operator 2-3 mins. per board.

The IPC has just published IPC-7526, Stencil and Misprinted Board Cleaning Handbook. Section 4.0 provides a wealth of information for cleaning misprinted PCBs. The Handbook is a free download. It is also available directly from the IPC.

I also recommend the article by Richard S. Clouthier, “SMT Stencil Cleaning: A Decision That Could Impact Production (Improving Screen Print Yields)”. This article is also available via the same link in PDF format and addresses specific concerns about solder ball contamination when cleaning double-sided misprints.

We have found that cleaning a misprinted board is best done with a saponifier DI water cleaning that removes any oxide that is formed or would be formed during drying.

The use of EnviroGold 816 at 3-5% concentration in a batch Tsunami cleaner from Envirosense-inc.com has worked well for meeting these surface cleaning needs and getting all the paste and flux out of any via’s. contact me directly and we can clean some for you and send them back.

Misprint cleaning is best accomplished with the use of an ultrasonic stencil cleaning system. Hand cleaning frequently pushes solder paste in to vias and exposes operators to solder paste (lead or other non-desirable metals) and chemicals. Spray-in-air cleaning technologies, although better than hand cleaning may also force solder paste into vias.

The use of ultrasonic technologies (the use of sound waves) causes solder paste to literally "fall off" the stencil. No positive pressure is used, preventing solder paste from entering vias.