It is important to consider how the choice of a conformal coating material affects the rework and repair issues. The need for rework or repair of a conformal coating can arise at any time after completion of an assembly due to a variety of process/product requirements or component replacement issues. Hence, rework of conformal coatings needs to be addressed up front when choosing a coating chemistry.
Thermal The thermal removal method using a soldering iron is the least recommended method. Most conformal coatings require a very high temperature and long exposure times. These, in turn, can cause discoloration, leave residues and adversely affect solder joints and other materials/components used in the fabrication of assemblies.
The process must be monitored to ensure that excessive temperatures do not cause delamination, lifting pads or overheat surrounding temperature sensitive devices. Extreme caution must be taken when burning rt conformal coating because some coatings emit toxic vapors which are hazardous.
Chemical Chemical methods are the most popular for the removal of conformal coatings. As long as the solvents used do not adversely affect the PWB or components and there are no environmental issues this technique works well. However, there is no one perfect solvent for all applications and in some cases it may be difficult to find a suitable solvent.
The following sections discuss the chemical removal methods for various types of coating:
Urethane: There are several solvents which provide a wide range of speed and selectivity that can be matched to a specific application. These solvents include:
Silicone: Methylene chloride based systems are very effective in removing silicone conformal coatings. Several hydrocarbon-based solvents are also used as alternatives. While not as fast as the methylene chloride, the hydrocarbon based solvents are more selective, and where not contaminated by water, will not attack epoxy-glass PCBs, components, metals and plastics.
Acrylic: The chemical removal of acrylic coatings was done in the past with highly volatile and flammable solvents such as methylene chloride, trichloroethane or ketones. A relatively safe alternative based on butyrolactone has been used for the removal of most of the acrylic coatings.
Epoxy: The complete removal of epoxy coatings for repair is nearly impossible by chemical means. The solvent can’t discriminate between the epoxy coating, epoxy-glass PCB and any epoxy-coated or potted components. However, if done carefully spot removal may be accomplished by the application of methylene chloride and an acid activator with a cotton tipped swab.
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