Once moisture and/or humid air enters into the equipment housing, it follows that there can be moisture condensing on, or in, the subassemblies. Table 4-6 lists the subassemblies which are susceptible to moisture and fluid intrusion. Free water may run onto these subassemblies, and there are even cases of standing water inundating interior surfaces. This moisture within the housing penetrates into the subassemblies in the manner described below.
TABLE 4-6 DESIGN CONSIDERATIONS TO IMPROVE SUBASSEMBLY SEALING INTEGRITY
penetration through coatings and/or under coatings
MIL-I-46058 conformal coating. Paraxylylene is preferred. Coat edges,
mask lines and drilled holes.|
intrusion by "breathing"
||Use of "O" rings
and preformed seals of fluorocarbon material|
intrusion through "breathing"
||Use of more
stable materials such as: silicones, fluorocarbons and fluorosilicones
||Use of desiccant
system with visual indicator. Pressurization of system.|
||Eliminate the use
of hygroscopic materials.|
THROUGH PRINTED WIRING BOARD COATINGS
None of the polymeric coatings is totally impervious
to moisture. The coatings retard the penetration of moisture, but
corrosion frequently can be seen under the transparent conformal coatings and
is the cause of many microminiature repair actions. Cracks, fluid
attacks (such as solvents, silicate esters, etc.), poor adhesion and minute
voids all reduce the integrity of coatings. Clean and dry boards as well
as careful mixing and application of coating materials are crucial to
minimizing voids and achieving proper adhesion and curing. Figure 4-6
shows an example of contamination as a result of poor cleaning of the board before application of the conformal coating. Selection of the type of MIL-I-46058 conformal coating to be applied involves performance of the coating, ease of application, maintainability, cost and other parameters. "Insulating Compound Electrical (Printed Circuit Assemblies)," MIL-I-46058, provides information on acceptable conformal coatings.16
Currently, the preferred coating for application during production is paraxylylene. This very thin, tough, fairly flexible, transparent coating flows well over sharp projections and edges, and is exceptionally impermeable to moisture. Although MIL-I-46058 requires the use of a clear (transparent) coating, a waiver to this requirement is sometimes obtained on the basis of safeguarding the underlying circuit design. While an opaque coating does not preclude serious circuit analysis, it does make fleet or depot PWB repair much more difficult.
UNDER COATINGS AT PRINTED WIRING BOARD EDGES (along mask lines, at drilled holes, etc.)
Of necessity, electrical leads of PWB edge connectors must emerge from the protection of the board and a conformal coating. This leaves an edge or exposed area of the PWB/conformal coating interface. Similarly, holes for the mounting fasteners are drilled after a conformal coating is applied, which means the interface of the PWB and the coating is exposed at the drilled hole. Moisture can penetrate at the exposed PWB/conformal coating interface much more readily than it can penetrate directly through the coating itself. Any poor adhesion of the coating obviously accelerates the spread of moisture under a coating. Extreme cleanliness and dryness are necessary prior to coating application if good adhesion is to be achieved. Consideration should be given to the potential effect of each PWB assembly production procedure on the final moisture resistance configuration of a new design.
PAST RF CAVITY SEALS
Tuned or resonant cavities are utilized in both the generation and transmission of r.f. energy. In most aircraft these items are not in sealed pressurized systems. Although there are seals to prevent gross water intrusion, these seals frequently deteriorate in the presence of some of the aggressive fluids, such as a silicate ester dielectric coolant. Then the moist ambient air that "breathes" in and out of the cavities with pressure changes leaves a moist atmosphere in the cavities that deposits condensation with temperature cycling. Many units rejected from service are corrosion induced failures. It is recommended that "O" rings and preformed seals be fabricated from a fluorocarbon material to minimize deterioration and for dimensional stability.
PAST DAMAGED ELASTOMERIC SEALS
Some of the elastomeric seals are susceptible to accelerated aging due to exposure to ozone, ultraviolet, certain fluids, unusual temperature cycling or other conditions that result in a shrinkage or embrittlement. Another form of seal damage is wear that is caused by the ingestion of fine particulate matter into areas of relative motion in a dynamic seal. As seals are damaged or deteriorate the "breathing" past the seal increases. Use of the more stable seal materials, such as fluorocarbons and fluorosilicones greatly delays such deterioration.
PAST ROTARY JOINT AND FLANGE SEALS
It is particularly difficult to seal rotary joints and waveguide flange seals. Indeed, moisture will penetrate into waveguides via the rotary joints and flanges in non-pressurized systems. In such systems, one should design to minimize the moisture intrusion, but also include a desiccant system wherein one element of a desiccator is being dried (by a heater) while another element is absorbing moisture from the waveguide atmosphere. These elements should be automatically alternated to prevent saturation of a desiccant. The system should include a visual indicator, located for easy viewing, to permit maintenance personnel to monitor the effectiveness of the desiccator. Figure 4-7 is an example of moisture intrusion and resulting corrosion on a waveguide flange.
ABSORPTION BY HYGROSCOPIC MATERIAL
Some gaskets are hygroscopic, e.g., felt, leather, cork, asbestos. Some packaging and cushioning materials are hygroscopic. Even the cut edges of laminated PWBs absorb moisture if not sealed. Ceramics generally are hygroscopic, as are plastics. The worse problem, however, is due to the hygroscopic nature of dust, lint and dirt. These contaminants hold moisture on the surface and cause surface corrosion. This effect is a principal reason for the importance of maintaining cleanliness throughout equipment.
Prior to being finalized, the list of design materials should be reviewed to identify any hygroscopic materials. Placing such cushioning material in heat-sealed waterproof bags, sealing laminated PWB edges, selection of non-hygroscopic gaskets and similar actions can control the potential problem posed by hygroscopic materials.