Polyurethanes (sometimes abbreviated as “PU”) overcome some of the limitations of acrylics by being usable over a much wider range of temperatures. Though typically not as hard, polyurethanes tend to be tough, and they add relatively low stress to sensitive solder joints and wire bonds on a circuit board.
These advantages come with their own trade-offs. Polyurethanes are more difficult to use, since circuit boards often must first be dried before they are applied. There are also toxicity concerns with some urethane formulations, which must be carefully addressed before they can be used.
Silicones (sometimes abbreviated as “SR”) have the widest usable temperature range and are very easy to handle. Most of them are solventless and add very little stress. However, these superior properties do come at a cost, which is typically a higher price per kilogram.
Yet, several factors are influencing engineers to choose silicone conformal coatings over other options to provide greater reliability and durability to electronic devices. These factors are low stress, lower production costs, tin whisker issues associated with lead-free solder, high-temperature processing issues associated with lead-free solder, higher reliability, and faster processing.
A controlled-pattern silicone conformal coating has been spray-applied to automotive controllers, and fluoresces under UV light to aid visual inspection of the coating's coverage.
(Source: Dow Corning)
Electronics are shrinking in size even as they expand in functionality. This results in boards with higher component density, narrower spacing, shrinking lead sizes, and smaller-gauge wires. All of these -- board, components, and wires -- are more sensitive to movement caused by the mismatch in thermal expansion of different board, component, and wire materials.
A coating must protect without over-stressing a board’s solder joints and wire bonds. Soft conformal coatings with a very low modulus are sometimes the only effective protection that does not cause damage during thermal movement, mechanical shock, or harsh vibration. Silicones are well-known and recognized as providing great environmental protection coupled with low modulus and high elongation. These features of silicone conformal coatings, in turn, deliver low stress to the boards and components over which they are applied.
Thorough analysis of the total cost of ownership has proven to electronic device suppliers that silicones often provide the lowest cost-coating solution. Manufacturers are finding that lower-priced coatings do not necessarily yield lower total production costs.
There are several reasons for this finding. The handling of flammable, odorous, or even toxic solvents can become very costly once volumes rise. High-volume production lines demand materials that are very easy to handle and use, cure quickly, and provide minimal environmental impact before, during, and after use. The need to dry parts before they are coated or other special handling procedures adds cost, as do slowed cycle times or higher reject rates.
Emerging initiatives and regulations have forced device manufacturers to find alternative materials that are qualified as containing either a minimum amount of lead or no lead at all. Lead-free solder alloys meet the new requirements, but come with their own issues.
Many new alloys contain tin, and with it comes the rising concern of tin whiskers that can bridge between components to cause electrical arcs and shorts. Silicone conformal coatings can delay and slow the formation of tin whiskers. Tin whiskers typically grow in straight lines out from the metal source. The shortest distance to bridge will always be a straight line. Since silicone coatings cause whisker growth to bend and curve, they can deflect the whiskers’ typical growth direction to lessen potential arcing and shorts.