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NAVSO P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines (NAVMAT P4855-1)
Complete
List of Guideline Documents | Table
of Contents

4.5 MANUFACTURING CONSIDERATIONS

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Quality manufacturing processes are crucial to producing reliable
power supplies. The following guidelines have been
developed to assist in this endeavor.

RECEIVING INSPECTION CONTROL ON COMPONENTS

The objective of the receiving/inspection process is to prevent
detrimental impact to both schedule and cost resulting
from receiving and installing defective components. It
is recommended that the tests and inspections listed in
the Component Screening and/or Special Controls section
of this document be performed on a 100% or sample basis
as indicated.

Testing of components includes but is not limited to the
following nondestructive and destructive tests and
inspections.

(1) Nondestructive Tests

a. Electrical
b. Hermeticity
c. X-ray
d. PIND
e.
Component marking permanency.

(2) Destructive Tests

a. De-lid/de-pot
b. Pull test on bonds, both internal and
external
c. Plating adhesion
d. Lead
solderability
e. Microscopic examination (preferably
with a scanning electron microscope).

Each lot that does not successfully pass all of the tests should
be returned to the component manufacturer with
notification as to the cause for rejection. Concurrence
with and positive corrective action relative to the
deficiency should be requested/required in writing by a
specific date. The test data and manufacturer's response
need to be recorded as component performance
history.

COMPONENT MOUNTING

It is recommended that components be inserted via automatic or
semi-automatic component insertion equipment whenever
possible. This ultimately prevents components from being
inserted backwards or in the wrong location. If such
equipment is used, the sequence and placement of
components must be validated by "first piece"
inspection. When using automatic polarity checking with this
equipment, the accidental application of high-voltage pulses
must be prevented.

Manufacturing personnel should verify their own work prior to
submitting an assembly to inspection.

Components that are sensitive to ESD or voltage transients should
be protected via appropriate containers and workstation
grounding.

SOLDER JOINTS AND SOLDER PROCESSES

A key precondition to a good solder joint is pre-tinning of all
leads/pins prior to assembly. For maximum reliability and
consistency, the solder joints should be made via
automated processes:

(1) Wave solder for pin-in-hole connections

(2) Semi-automatic for surface mounting (e.g.,
flatpacks).

All solder joints should be inspectable; this requires that
blind-hole solder joints be restricted in the design.

For pin-in-hole solder joints, the solder should penetrate
through the barrel of a PTH. This applies also to wires,
transformers, connectors, etc., and requires that the solder
joint be made from the side of the PWB/MIB opposite that
on which the component is located.

Make all wire connections to a PWB/MIB only on the component side
of the PWB/MIB to simplify assembly, rework and repair,
and to minimize handling damage.

Flux must not become entrapped due to a blockage at one end of
the PTH; this has a high potential for causing a poor
solder joint which will fail at a later time.

Cavities are a natural place for moisture, foreign material, or
solder to get trapped and/or wedged, resulting in an
electrical short to the frame. This can be prevented by
either:

(1) Designing the cavity out,

(2) Making the cavity sufficiently large so as to reduce the risk
of entrapment,

(3) Coating the area to prevent the short from occurring,
or

(4) A combination of the latter two techniques.

ASSEMBLY LEVEL

Loose Particles

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.

Wire Routing

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.

Handling Damage
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 rework/repair operations.

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 required.

INSPECTION

Manufacturing personnel should verify their own work prior to
submitting an assembly to inspection. Inspection and
assembly personnel should have as many visual aids as
possible. Examples of effective visual aids are blown-up
color photographs of the completed work at each stage of
assembly for each assembly and next higher level of
assembly.

REWORK

Workmanship errors found during the inspection process should be
corrected by the initial-build
personnel.

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Last Modified on :
Friday, 07-Mar-03
14:20:12