CONFORMAL COATING BASICS
What is Conformal Coating?
Conformal coating is protective chemical coating of polymeric material applied in thin layers to printed circuit boards (PCBs) and electronic assemblies. Its purpose is to provide environmental and mechanical protection to significantly improve the longevity and reliability of the components and circuitry. Traditional methods used for applying conformal coating include brushing, spraying, and dipping. When applied correctly they form a thin film that ‘conforms’ to the contours of the PCB and its components. Typically, conformal coatings are applied in the 25-250 microns range. While Parylene films are applied much thinner and may be fractions of a micron in film thickness.
Why use Conformal Coating?
When today’s complex electronics are exposed to harsh conditions, added protection becomes necessary to ensure normal function. Conformal coatings provide proven protection against moisture, salt sprays, aggressive chemicals and solvents, vapors, dust, abrasion, and even organic attack (e.g. fungus). In addition, a suitably chosen material has been proven to reduce the effects of mechanical stress and vibrations on the circuit and its performance in extreme temperature ranges. Conformal coatings not only protect the product, but serve to enhance the reliability. Thus, diminishing damaging effects of early field failures and reducing the potential cost as well.
What are Conformal Coating Standards?
Conformal coating standards are a series of specifications and tests which a coating must pass in order to be considered for use in certain environments such as automotive or military applications. The most common standards for conformal coating are IPC A610, IPC-CC-830 and MIL-I-46058C. These standards list indications of good and bad coverage and describe various failure mechanisms. Conformal coating inspection is a critical factor in determining successful coating application and long term reliability of PCBs. Using the IPC and MIL standards allows the coating operator to monitor the coating application performance. This can be done manually by the operator in an inspection booth by examining the PCB under white and/or UV light.
What is Conformal Coating Masking?
In conformal coating many components and printed circuit board locations must remain uncoated due to the insulating nature of the coating. The purpose of the conformal coating masking materials is to prevent migration of the conformal coatings into components that need to clear and designated keep out areas. This applies to both liquid conformal coating and Parylene processing.
There are several different masking techniques using different materials such as masking tape, dots, shapes, boots and liquid latex in conformal coating processing. The masking requirements for liquid applications and Parylene coated PCBs differ in that Parylene masking must be ‘airtight’ and properly ‘sealed’ while liquid masking is basically a ‘shield’ from the liquid. An exception would be PCBs being processed via the dip method. These masked areas need to be sealed to the PCB to prevent the leakage of any unwanted coating into the connectors. However, if the masking fails or rework becomes necessary there are approved methods of coating removal.
Removal options include thermal, mechanical, chemical and abrasive. These methods can be used to remove coating from small isolated areas or for total coating removal.