Andy Price, National Sales Manager
Peter Vigneau, Vice President
Circuit Technology Center, Inc.
Haverhill, MA USA
You're up against a deadline and you hear "we've got a problem". It could be any number of things. This time it's solder mask covering some part of the circuit board surface where you don't want it.
It's certainly not unusual to find test points, ground pads, or even a component footprint inadvertently covered with solder mask. But all is not lost! There are several safe and reliable ways to remove solder mask.
Several factors will help determine the method to use including the type of solder mask, where it's located, the amount of solder mask needing removal and whether the circuit boards are assembled or bare?
We'll start with method number 1, 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.
No special setup is needed, but operator fatigue can be a drawback on large projects. Mechanical erasers, like the type an old draftsman would use, can speed up the process. This technique is controlled and methodical, most commonly used when removing a thin layer of solder mask.
Would you use a milling machine to remove solder mask? Seems extreme, but it can be a very effective and a precise technique for removing solder mask as explained here in method number 2. Since high speed cutters are used when milling, precision depth control is essential since the cutters will tend to pull into the coating and may penetrate into the board surface.
Turning the milling cutter in the opposite direction can be an effective technique for controlling the depth. Operator skill and experience are paramount.
Method number 3 uses chemical stripping and is most effective when removing solder mask from copper or soldered surfaces. Masking tapes or other protective materials should be placed on the circuit board surface to isolate the area before the chemical stripper is applied. The chemical will attack the coating and break it down just like a paint stripper.
Not only will the stripper remove the solder mask, but it may also attack the base material surface if exposed to it for a prolonged period. For this reason chemical strippers should be used with great care and only when alternatives prove too costly or time consuming.
Our final method uses micro blasting. This is an excellent technique for removing solder masks from large areas. An abrasive material is propelled through a pencil shaped hand piece to 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 micro blasting system must be designed to eliminate potential 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 and operator skill and training are essential.
There you go, four separate methods for removing solder mask.