Non-catastrophic damage can be caused by a static discharge as low as 30V in some MOSFET technologies where the gate oxide is very thin. Junctions, gates and contacts are the critical areas within a device where electrostatic discharge damage can occur.
Organic coatings are extensively used to protect electronic assemblies from moisture, high humidity and high temperature environments. Some coatings have been specifically engineered and formulated to remove large amounts of heat when subjected to high temperatures for short periods of time. Among these are the thermally ablative coatings that are used to protect components during missile and rocket liftoffs. These coatings, when applied to heat-sensitive parts such as electronic enclosures or nozzle control units, protect the inside surfaces from temperatures as high as 3000°F generated by the rocket engines during blast-off.
At the temperature of ablation, the exterior layers of coatings decompose and char. Thus the coating absorbs considerable heat (heat of ablation) while the inside surfaces remain sufficiently cool. The coating must be applied at a sufficient thickness and must ablate at a controlled rate, so that sufficient coating remains at the end of thermal exposure.
Ablative coatings consisting of filled epoxies and silicones were used to protect the nozzle control unit of Minuteman missile during take-off. A similar coating was used on Apollo capsule to protect astronauts from the intense heat of re-entry into the Earth’s atmosphere. Intumescent coatings are used to protect electronic boxes and structures from fire. These contain fillers or molecular structures that decompose and evolve into flame-retardant gases such as carbon dioxide.
Among the best non-metallic, high-thermally conductive fillers are beryllia, aluminium nitride, boron nitride and diamond. At the same time, conformal coatings don’t provide protection unless the surface is completely clean from flux residues, fingerprints as well as any other chemical used in the production of electronic assemblies.
Apart from temperature and humidity, electronic assemblies must be protected from marine environment (where salt spray/fog dominates), handling, abrasion, friction, radiation, micro-organism, etc.
Shavinder Singla is an IPC, USA certified interconnect designer, working as a technical officer at Centre for Development of Advanced Computing, Mohali.