Recently, a customer contacted the EMPF asking how to remove a silicone conformal coating so that they could determine the functionality of the underlying circuit.

There are four methods recommended by the IPC (7711B, 2.3.1) for removing thin silicone coatings. In order of preference, they are 1) solvent, 2) thermal, 3) scraping, and 4) micro-blasting.

Solvent Method
Solvents should only be used in a closely controlled process. Even short term exposure may cause damage to component parts, solder joints, and plated-through holes. To ensure the surrounding area is not damaged by the solvent, the area must be masked to only expose a localized area.

Typically, most silicone solvents do not actually dissolve the silicone, but cause it to swell and detach from the surface. By applying the solvent using a saturated foam swab or a cotton patch cut to the same size as the coated area to be removed, solvent evaporation will be inhibited and the overall exposure time reduced. The softened surface can then be gently removed using a brush or a wooden stick. After removal, the circuit board assembly should be carefully inspected to ensure no damage has occurred.

Thermal Method
This method removes coatings by using a controlled heat source such as hot air or a heated tip. The heat will soften the silicone to allow removal using gentle pressure with a wooden stick or other non-marring tool. The tip or air temperature should be adjusted to prevent charring or scorching of the silicone, or reflow of the solder.

Scraping
A knife or dental style scraper can be used to carefully remove silicone coatings using a side to side motion holding the blade perpendicular to the board. This is frequently used in conjunction with the solvent or thermal methods.

Micro-Blasting
Micro-blasting propels a fine soft abrasive powder through a small nozzle at the area where the coating needs to be removed. Typical powders used are ground walnut shells, beads (glass or plastic), and sodium bicarbonate powder. When using this method, the entire circuit board should be masked, exposing only the area to be removed. The board should be electrically grounded and the area flooded with ionized air to guard against the static charges that can be generated.

The detailed procedures for each technique can be found in IPC 7711B 2.3.2 through 2.3.6. Regardless of the procedure, the board should be optically inspected to verify complete coating removal and any evidence of damage. The affected area should be rinsed clean with a chemically neutral material such as deionized water. Remaining residues can contain corrosive agents which can cause electrical failures if they are not removed. The EMPF can provide help in selecting the appropriate coating removal method, perform the removal of conformal coatings, and clean circuit card assemblies.