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December 10, 2007

Pre clean boards before printing

What is the best solution for cleaning dust and debris from printed circuit boards before screen printing in an automatic SMT line?

I currently use paper roll wipes, but upon inspection I still find particles on the boards.



Carlo Galbiati

Experts Comments

Most companies do not pre-clean their boards before soldering, assuming the boards and clean and fingerprint-free. However, that is often a false assumption, and if your yields are not as high as they should be (95%++) then pre-cleaning might be an option.

The fibers you describe often come from cheap wipes made of cheap paper. While they are cost-effective to buy, they leave residues and lint and end up costing more, when rework is included in the equation. You have a couple of different options:

  1. My favorite option: If you have access to a dip-tank cleaner, especially one with ultrasonic cleaning capability, give the boards a quick precleaning in a water-based cleaning solution. My company sells a solvent for this application, called "BGA Stencil Cleaner". It's water-based, non-foaming (many less expensive solvents foam up in ultrasonics), powerful cleaning and dries without residues.
    (Special note: If you have a requirement to clean many, many boards – thousands per week – a better choice would be to preclean in a vapor degreaser. Vapor degreasers have very high capacity, fast throughput, and use less electricity than less sophisticated cleaning systems. But if you're only cleaning 10-100 boards per day, the aqueous system will work fine.)
  2. Not a bad option: If you don't want to clean in a dip-tank or vapor degreaser, then you're going to have to hand-wipe the boards. Get rid of the cheap paper wipes and switch to a high-purity, lint-free presaturated wipe. My company sells a product for this application, called "Economy Wipes". Presaturated wipes are desirable because they remove operator errors from the wiping process… every wipe is exactly the same, and carries exactly the same amount of solvent, so the cleaning can be very consistent.
  3. Upgrade Your Current Process: If you don't want to use presaturated wipes, at least switch to a high-purity, electronics grade stencil wiping paper with high absorbancy and high strength. The use of good paper will, by itself, make a big difference in the quality of the cleaning. Again, my company also provides products for this application, for example the 15cm x 15cm "MicroWipe FP" package that's very affordable, very clean and leaves the minimum residues on the board. The photos below show some of the differences between cheaper paper and high-quality paper:

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The paper on the left is made from polyester fibers and glue. It is a sample from a very common stencil wiping paper. The fibers are short and fragile, easily breaking in use. Notice how the glue fills up the spaces between the fibers, and coats the fibers with a solvent-soluble residue. All of this is undesirable.

On the right is a sample of a high-purity stencil wiping paper. The fibers are long, strong and (although you can't see it on the phone) uncompressible, so they scrub rather than break away into lint. Paper made with these fibers will scrub the bare board or the stencil and leave almost no residues behind. This gives you a much cleaner board, a cleaner stencil, better printing, higher yields and more profit at the end of the day.

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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

You did not describe the environment so I am speculating here based on experience. I believe this is a material handling problem. If you review the packaging coming from your PWB vendor and be sure it is properly sealed and then keep them sealed from dust and dirt you stop the dust and particles before they get there. Then put out on the line only as many are needed to keep the line running.

This is all preventative to eliminate the problem rather that fixing it as you go. I hope this is helpful. I realize that you asked about how to clean them not how to stop the problem.

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Ken Bliss
President & CEO - Retired
Bliss Industries, Inc.
Retired - Mr. Bliss has 20+ years experience creating process methods that improve profitability by maximizing hidden unused capacity and throughput. Ken has expertise in all areas of manufacturing specializing in electronics assembly.

Most PCBs are cleaned and then packed in moisture proof packaging. The key here would be to keep them in their original packaging until they are ready for use.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

One solution for cleaning is also the use of an ionizer. The ionizer with fan's creating electrostatic charge for discharge your dust.

If you have more question you can send me an email.

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Hartmut Berndt
President
B.E.STAT group
Mr. Burndt has been in Electronics & Semiconductor Technology since 1980 and work in the area of Electrostatic (ESD) and electronic devices. He is President of the B.E.STAT group (Germany); expert in ESD audits, trainings, failure analysis and ESD control programs.

PROMATION offers a different style of PCB cleaner before screen printing. Our EDR-700 uses an ESD brush, vacuum and ionizer bar to gently sweep and vacuum dust and debris form the top of the PCB prior to printing.

The system accepts PCBs from a board loader and then cleans the PCB. SMEMA protocol is used to control line flow between the loader and screen printer.

The height of the cleaning brush is adjustable (Z) to allow for varying PCB thickness. The system has a self contained vacuum that provides air suction away from the PCB surface while also capturing debris picked up by the brush.

Visit our web site for more information.

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Gary Goldberg
President and CEO
PROMATION, Inc.
Mr Goldberg has practical experience in production line layout, process flow and cycle rate analysis. He knows how to avoid bottle necks and most related PCB or pallet handling questions.

There are several different cleaning methods available, depending on the contamination and the location in the SMD line.

One solution is called the "sticky roller" approach, where the PCBs are fed underneath a sticky roller and the dirt will be picked up from the PCB onto the sticky roller and then transferred onto another roller not to contaminate the PCB again. Another solution is a brush with an ionized airknife and vacuum on the other side. This solution provides also excellent cleaning results without the cost of the consumerable. If you don't want to "touch" the PCBs, you can also opt for just an ionized airknife with a vacuum system, however your efficiency will be reduced.

ASYS can offer a variety of different cleaning systems. Please contact us if you want to learn more.

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Markus Wilkens
President
ASYS Inc.
Mr. Wilkens has been working in the SMD industry for over 10 years. He came to the USA in 1999 to start up the operations for ASYS, a global company with a product portfolio including Handling Systems, Marking Systems, Depaneling Systems, Screen Printers and end of line Automation.
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