Stripping or removal of unwanted conformal coating from a printed circuit board (PCB) can be a relatively simple process or a very messy difficult job. It does depend on several factors including the type of conformal coating you need to remove, where the coating is found and the type of components on the board.
Local Rework or Repair of Conformal Coating
It is possible to rework components that are coated with conformal coatings with a soldering Iron. The conformal coating volatizes away with the application of heat, leaving the joint clear of coating. Occasionally, with certain coatings, the material may char slightly but this is a cosmetic issue and you need to discuss this with your quality department.
The simplest to remove are the acrylic conformal coatings. They have little chemical resistance and therefore are the easiest to remove with stripping fluids. These conformal coatings generally re-dissolve back into solution so a combination of soaking and gentle mechanical abrasion works well.
The simplest process for local area rework around a device for instance is a cotton bud soaked in conformal coating stripping fluid and then rubbed gently across the area to be removed which will dissolve the coating. If the coating is fresh, it comes off in a matter of seconds whereas if the coating is old, having been coated many years ago, then it could take a little longer and patience is required!
Large Scale Repair or Removal of Conformal Coating
If the area to be removed is larger or the whole board is to be stripped then submersion in a tank of a conformal coating stripping system or handmade system with a correctly selected stripping fluid and abrasion using a soft bristle brush will also dissolve the coatings. A word of warning must be given here. First, when submerging in a stripping material check there are no compatibility issues with the PCB. Stripping fluid could attack components and or writing on the boards occasionally although for acrylic coatings the strippers tend not to be too aggressive.
The other issue that can be a major headache with full stripping of a board is that because the coating re-dissolves into the stripping fluid, there will now be coating residue all over the PCB even where you didn’t want it. This can be a real problem with certain components such as low profile connectors! To remove these residues you will need several tanks of stripping fluid and the PCB will need to be fully rinsed in each, gradually flushing the residue out of the wrong areas. Once completed the PCB should then be cleaned in a cleaning system to remove any unwanted ionics.
These two processes also work for coatings such as polyurethanes and silicones although since they have chemical resistance they are tougher to remove. Correct stripping solution selection is critical and this is why Conformal Coating manufacturers have several stripping products.This was a guest post by Dr. Lee Hitchens from Nexus3C.