COATINGS FOR AUTOMOTIVE APPLICATIONS
As the usage of electronics in automobiles increases, the protection of these electronics to assure reliable performance under very harsh environments is critical. Three general environments are well known in the automotive industry:
• Passenger compartment
• Under the hood
• On or near the engine
Of these, the least severe ambient occurs in the passenger compartment while the most severe is on or near the engine. Temperatures inside the car can range from -40° to 85°C and relative humidities can vary from near 0 to 100%. Water can condense on electronic surfaces and, combined with water-soluble contaminants, can cause momentary malfunctions and ultimately catastrophic failures. Generally, electronics inside the vehicle are not coated because of the sparsity of proven failures due to unprotected circuits and the added cost of coating. However, this situation is changing with the increasing use of sensors, advanced devices, PCBs having finer conductor lines and closer spacings, and the use of thin-film metallization— all of which are more susceptible to corrosion and transient electrical effects. Where organic coatings are used, those that meet the industry conformal coating specifications, such as acrylics, polyurethanes, epoxies, and silicones are the favorites. Silicones, because of their high thermal stabilities are used in the near-engine circuits, while polyurethanes and epoxies are used for passenger compartment electronics. Applications include sensor circuits for aid in parking, belt-locks, side airbags, and the central locking mechanism. Schenectady-Beck’s Bectron PK43, a polyure-thane, is used to selectively coat a portion of an airbag electronic module installed in the sidedoor of an automobile.[89]
The under-the-hood environment is much more severe than the passenger environment. The maximum temperature there can reach 105°C and temperatures can rise quickly from outdoor freezing to super hot. Concurrently, the electronics may be exposed to corrosive materials including gasoline, motor oil, antifreeze, battery acid, brake fluid, and numerous road contaminants. For this environment, thin conformal coatings may not suffice as adequate protection. Thick coatings or coatings combined with encapsulants or potting compounds are generally required. Silicones and
epoxies are favored because of their high temperature and chemical resistances.
An even more severe environment exists on or near the engine where
temperatures may reach 125°C and where extremely rapid temperature
changes occur. Electronics in this environment must be protected by
potting with very high temperature resistant materials such as
silicones.[90]
Even harsher environments occur near the exhaust manifold and catalytic
converter where temperatures reach 150°C, near the brake system where
maximum temperatures may reach 205°C, and where electronic modules, if
used, are exposed to hydraulic oil and greases.