Limited openings in the equipment housing. To reduce moisture intrusion, keep the number of penetrations into the equipment to a minimum. When penetration is required, use “O” rings and gaskets for sealing. For wiring entry, use “L” type electrical connectors and mount them horizontally (through the vertical sides), and well above the bottom of the housing.
Proper electrical connector mounting. Electrical and coaxial connectors should be mounted horizontally (through the vertical sides). When electrical and coaxial connectors are mounted on the top of the equipment, there should be a raised area on the upper side where the connector is mounted and an “L” type should be used. Electrical wiring should incorporate a drip loop into the wiring harness so the wiring is leading up to the connector.
Proper printed circuit board mounting. Printed circuit board (PCB) should be mounted vertically with the electrical connection also in a vertical position.
Low point drains. Low point drains should be incorporated so that any moisture will drain from the equipment when the aircraft is in normal flight and when it is parked on the ground.
Eliminating moisture traps. Avoid moisture traps or “bathtub” areas on the interior areas of the equipment. Identify drain paths to the low point drains. Avoid moisture traps in electrical wire bundles where anti-chaffing material or boots are incorporated.
Cooling air systems. Cooling air systems should incorporate a means to remove moisture and particulate matter from the conditioned air. This is especially important when the air is forced directly toward active electronic elements.
Proper bonding and grounding. Electrical bonding and grounding should be accomplished using straps rather than “sliding housing-to-rack” or tapered pin on housing-to-rack electrical contacts. Straps should be located for ease of maintenance and properly sealed because of the dissimilar metal (galvanic) couple.
Proper equipment mounting. Avionics equipment should be mounted in such a manner that will allow sufficient airflow and should be kept at least 1/2 inch above the compartment floor.
Proper material selection
Proper material selection is critical for protecting avionics equipment against the environment. Many types of corrosion that occur in electronic devices also occur in the airframe structure. However, the range of material called out in avionics equipment is usually greater than in the airframe. Several new types of degenerative symptoms are unique to avionics equipment. The following indicate the uses of different materials in the construction of various electrical and electronic components:
Copper and copper based alloys are generally used in avionics systems as contacts, springs, leads, connectors, printed circuit boards (PCB), conductors, and wire. Iron and steel are used as component leads, magnetic shields, transformer cores, brackets, racks, and general hardware. Magnesium alloys are used extensively throughout avionics systems as antenna structures, chassis, supports, and frames (radar). Nickel and tin plating are used for protective coatings and for material compatibility purposes. Tin is also one of the components of solder. Tin plating is used on radio frequency (RF) shields, filters, and automatic switching devices. Silver is used as a protective plating material over copper in wave guides, miniature and micro-miniature circuit boards, tank circuits, and RF shielding. Aluminum and aluminum alloys are widely chosen, because of their light weight, in equipment housings, chassis, mounting racks, supports, and electrical connector shells. Cadmium is used as a sacrificial coating on ferrous hardware, such as bolts, nuts, washers, and screws. Ion vapor deposition (IVD) of aluminum is also used as a sacrificial coating on hardware and is a nonhazardous replacement for cadmium. Gold is commonly used on electrical connectors, contacts, and edge connectors where the lowest electrical resistance is required.
Due to the complexity of the material process used in modern electronic assemblies, it is sometimes difficult to predict if potential problems will be created by the reaction between two or more nonmetallic materials in a circuit assembly. Incompatibility of materials can result in the release of chemicals or gases that will react with other circuit components. In some cases, the incompatibility of cleaning solutions will cause reactions in substances that are corrosive to associated circuitry. The following list contains several of the potential problems:
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