Power supplies should be designed for ease of maintenance at the Organization (O-level), Intermediate (I-level) and Depot (D-level) levels. A key objective is to minimize the amount of handling, teardown and reassembly required to perform a maintenance action. The design of a power supply may initially be cost- effective, and it may be assembled correctly the first time. However, any repairs or rework should be equally cost-effective and performable by qualified personnel.
PRINTED WIRING BOARDS
Towards the objective of simplicity in the design of the packaging, it is highly recommended that printed wiring boards (PWBs) and/or Multi layer Interconnection Boards (MIBs) be used instead of wiring harnesses and point-to-point wiring. Interconnect these boards using keyed connectors.
The top and bottom surfaces of a PWB/MIB should contain only the soldering surfaces/pads for components. Circuit lines should not be exposed but should be covered ("buried"). It is recommended that a glass-epoxy layer be laminated to the top and bottom surfaces, exposing only the soldering surfaces. This prevents shorts between and damage to the circuit lines.
Printed wiring boards should be conformal-coated. The coating should be a continuous, homogeneous and fully cured material which covers all components, leads, and circuitry, except grounding surfaces.
The design of MIBs should provide for the elimination of large heat-sinking to internal ground/voltage planes via the Plated Through Holes (PTHs) during soldering operations. This improves the solder penetration into the PTHs. All interfaces (input/output) with a PTH/MIB should be via a keyed connector. This improves manufacturability, maintenance, rework and handling.
BONDING OF PRINTED WIRING BOARDS
It is recommended that PWBs/MIBs be bonded to their frame(s) with an adhesive that is put on via spraying or screening. This technique has excellent moisture resistance. The use of sheet-bond material is to be avoided, since it has imbedded threads that absorb moisture.