PCB Burlesque: Stripping Away Solder Mask -- Several safe and reliable ways of removing solder mask do exist!

Here's the scenario-you've asked your board supplier to apply solder mask only where you want it. Seems like a reasonable request, doesn't it? Now you're up against a deadline and your board supplier says, "We've got a problem." This problem could be any number of things, but, this time, it's solder mask covering some part of a circuit board where you don't want it. Often, it's not unusual to find test points, ground pads or even a component footprint inadvertently covered with solder mask. Don't lose your cool! Several safe and reliable ways to remove solder mask from the printed circuit board (PCB) surface exist. Grinding and scraping, milling, microblasting and chemical stripping are the most common methods used. Each method has advantages and disadvantages.

Several factors will help determine what method will be used to remove the coating. What type of solder mask is it? Where is the mask located on the board surface? How large is the area of the mask that needs to be removed? Are the circuit boards assembled or bare? These factors, and others, must be evaluated before deciding on the most appropriate removal technique.

Grinding and Scraping

Nothing fancy here, just lots of grunting and groaning. Often, the use of a knife, scraper or pick in the hands of a skilled technician is the most controlled technique for removing solder mask from undesirable areas. No special setup is needed, but operator fatigue can be a drawback on large projects. Mechanical erasers, similar to the kind a draftsman would use, can speed up the process. This technique is controlled and methodical and most commonly used when removing a thin layer of solder mask. This method can be used in conjunction with other removal techniques as a final surface preparation step.

Machining and Milling

Would you use a milling machine to remove solder mask? The concept may seem extreme, but proper use of a milling machine can be a very effective and precise technique for removing solder mask. Since sharp cutters will be used, precision depth control is essential. The milling system will need to be fitted with a microscope for visual aid. Carbide end mills are the most common type of cutter used. Since carbide cutters are so sharp, they will tend to pull into the coating and may penetrate into the board surface. Turning the cutter in the opposite direction can be an effective technique for controlling depth. Operator skill and experience are paramount in this operation.

Microblasting

Microblasting is an excellent technique for removing solder mask from large areas of the circuit board surface. Small benchtop systems, designed specifically for coating removal, are available from several sources. An abrasive material is propelled through a pencil-shaped handpiece and the abrasive material simply blasts away the coating. This process creates substantial friction and static charges in the process. When working on circuit boards containing static-sensitive devices, the microblasting system must be designed to eliminate potential electrostatic discharge (ESD) damage. Significant preparation time, including masking, is often needed to control the areas to be removed. A thorough cleaning will be required to flush away any blasting material from the circuit board. Operator skill and training are essential if you expect to obtain a reliable result.

Chemical Stripping

Chemical stripping is most effective when removing solder mask from copper planes or soldered surfaces. Masking tapes or other protective materials should be placed on the circuit board surface to isolate the area to be stripped. The chemical stripper is then applied with a brush or a swab. Since the stripper is liquid, it is often difficult to control. The chemical will attack the coating and break it down-just like a paint stripper on paint. The most common chemical strippers will contain methylene chloride, a strong solvent. Not only will methylene chloride-based strippers quickly remove the solder mask, but they will also deteriorate the base material if exposed a prolonged period. For this reason, chemical strippers should be used with great care, and only when alternatives prove too costly or time consuming.

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Jeff Ferry is CEO of Circuit Technology Center, 45 Research Drive, Haverhill, MA 01832; [phone]; fax: [phone]; www.circuitnet.com.

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