What will the PCB’s Operating Environment be?
Coatings are applied in order to protect the PCB from corrosion in its operating environment. It is important then to consider the coatings ability to protect the board from its environment. The main questions which should be considered are:
What temperature range will the board be exposed to?
How rapidly will the operating temperature change (see thermal shock)
What humidity will the board be operating in?
Is condensation a consideration?
Will there be salt present?
Will it be immersed in water?
What solvents will be present?
What will be the level of UV present (daylight exposure)?
The coating must be suitable for use under the required operating conditions. Technical data sheets (TDS) are provided based on a range of internal and external tests according to international standards. Tests must also take place to ensure that the coating retains all the required properties throughout the duration of use. This is due to the differing conditions exhibited as a result of varying PCB materials and designs.
In addition to the performance characteristics of the cured coating, the application process plays an important role in the success (or otherwise) of the coating application and the following considerations should be made during the selection process:
Cure time available
Maximum cure temperature possible
Volume of boards to be produced
Available cycle time or TAKT time
Existing application equipment or new process?
What are the Electrical Requirements?
Conformal coatings form a protective, insulating layer. The most common electrical parameter tested is the Surface Insulation Resistance (SIR). This measurement is often taken before and after coating and exposure to harsh conditions, thus ensuring the coating continuously provides the level of insulation required. The coating should also exhibit high dielectric strength; the minimum required can be determined from the intertrack separation and the potential difference between adjacent tracks.
What will the PCB Board Layout be?
The design of the board should include consideration of the placement of components that should not be coated. Selective spray equipment or the application of a peelable coating mask can be used to help avoid such areas. Alternatively, gel materials can be used to form a ‘dam’ to contain the coating and avoid capillary effects transferring material to unwanted areas, such as connectors. Another aspect to consider is how close together the components are. If there is not sufficient gap between the components, it can lead to the coating bridging the gap rather than correctly conforming to the board, this can have a number of effects including the coating building up to become too thick and crack, or it can leave pockets under the coating where solvent can gather and cause corrosion.
Will it require Rework and Repair?
If the assembly requires repair then consideration must be given to the ease of removal of the coating. Electrolube offer products for the effective removal of conformal coatings, including those that are solvent resistant.
Removing Conformal Coating
Electrolube have a number of complementary products to assist with the removal of the coating, please see the section of the website here:- along with the TDS instructions.
What will the volume of production be?
The volume of production will have effect on a multiple of the other considerations mentioned in these articles. The most obvious being the application method and the cure times, for example automated applications will be preferable for mass production, whereas hand spray coating can be used on small runs.
What will be the final enclosure be like?
The mounting of the board in its final position can have an effect on the effectiveness of the coating for a number of reasons – for example, if the housing is open it will let more environmental elements in than a fully enclosed case. Whether the board is mounted horizontally or vertically is also very important, if possible it should be mounted vertically to avoid moisture from condensation sitting on the board.
