Loose particles can cause shorts. Prevent them from getting into the power
supply during the assembly cycle. The mechanical design can reduce the problem
of loose particles by eliminating convenient places (e.g., cavities) into
which they may settle. The manufacturing processes should provide facilities
(e.g., Plexiglass shields on lead-clipping benches) to prevent the particles
from getting into an assembly. As a final assembly procedure, the power supply
as well as the individual assemblies should be turned upside down and shaken
in order to get rid of any particles.
Susceptibility to Solvents
There are several cleaning solutions and/or processes that can cause an
electrical failure at a later time. One mechanism is chloride contamination
that evolves into an electrical short; e.g., chemical reaction with the
aluminum foil inside an aluminum electrolytic capacitor. (Refer to paragraph
4.2 for a method of preventing this interaction.) "Circuit coolant" used to
test portions of a circuit at low temperature can cause damage from
electrostatic charge and chemical contamination. Semiconductor isolating
washers may not all have the same resistance to chemical solvents. Solvents
such as methyl ethyl ketone (MEK) should be avoided.
Wires are susceptible to being broken and/or pinched, especially during or
after a rework/repair operation. To avoid such problems, as well as to prevent
crosstalk, care must be taken in the decisions to use a wire (or harness) and
its placement and routing.
The reliability obtained with good design and
manufacturing processes can be degraded by poorly designed fixtures and
containers, and damage caused by inadequately trained personnel. Consideration
of these factors can reduce handling damage substantially during assembly and
Power Device Mounting
The potential reliability gain by eliminating the use of thermal grease and
using the thermal conductive plastic impregnated fiberglass isolators can be
destroyed if proper torquing requirements are not precisely followed. Too much
pressure can rupture the isolator, thus allowing an electrical short to
develop between the "hot-case" device and heat sink, while too little pressure
will result in poor thermal conductivity and, hence, a higher junction
temperature. Great care must be taken in selection and installation of
shoulder washers to obtain proper pressure.
Manufacturer's tables can be used as a guide to establishing torquing
requirements; however, caution should be exercised because of the wide
variation in pressure obtained as a function of the mechanical resistance
between the screw head, or nut and the washer or other bearing surface.
Experimental results may be required to obtain the optimum torque