||NAVSO P-3641A: More Power For The Dollar
3.3.1 Manufacturing Process
Component Receiving Inspection
Effective receiving inspections prevent detrimental impact
to performance, schedule and cost resulting from installing defective
components. Testing and inspection of components may include the
- Particle Impact Noise Detection (PIND)
- Sample Destructive Physical Analysis (DPA) and
Each lot that does not successfully pass all required
tests and inspections should be returned to the component manufacturer with
written notification as to the cause for rejection. A formal reply stating
the coffective action intended should be required by a specific date. The
test data, manufacturer's notification and his response should be recorded as
component performance history.
Components should be inserted via automatic or
semiautomatic insertion equipment whenever possible to prevent improper
insertion of components. If such equipment is used, the sequence and
placement of components should be validated by "first piece" inspection.
The accidental application of high voltage pulses must be prevented when using
this equipment for automatic polarity checking. Components that are
sensitive to ESD or voltage transients should be protected as described later in
Mounting of power devices requires special consideration. Potential
reliability gains by utilizing thermally 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. Too little pressure will result in poor thermal conductivity and,
hence, a higher semiconductor junction
temperature, Great care must be taken in selection and
installation of shoulder washers to obtain proper pressure. Manufacturer's
guidelines can be used to establish torque 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.
Solder Joints and Solder
Solder joints and processes should be performed in
accordance with IPC-A-610 or ANSI/J-STD-001.
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 such as:
- wave solder for pin-in-hole connections, and
- semi-automatic for surface mounting (e.g.,
solderjoints should be inspectable. This requires that blind-hole
solderjoints be minimized in the design. For pin-in-hole solder joints,
the solder should penetrate through the barrel of a Plated Through Hole
(PTH). This applies also to wires, transformers, connectors, and other
related items, and requires that the solder joint be made from the side of the
Printed Wiring Board/Multi-layer Interconnect Board (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:
- designing the cavity out,
- making the cavity sufficiently large so as to reduce the risk of entrapment,
- coating the area to prevent the short from occurring, or
- a combination of the latter two
controlled properly, ESD can destroy electronic devices instantly or have latent
effects which can lower the life expectancy of a device. Static discharge
can vaporize conductors and rupture insulators within electronic
components. This can result in an open or short circuit. In a lab or
factory environment, the goal is to prevent charge from building up to a
damaging level. In designing circuits, the goal is to minimize
vulnerability to static discharges and to slow the rate of static discharge to
ensure no harm is done.
There are numerous ways ESD can be controlled in
a lab or factory environment. Personal ground straps (wrist, leg, or
ankle) should always be wom when handling circuit boards and devices.
These straps provide ESD protection by providing a path to ground for
potentially dangerous charges. The workbench also should have an ESD
protective (static dissipative) work surface over the total area where boards or
devices will be placed. This surface should be connected to ground.
It is recommended that each workstation have monitoring equipment, which
continually tests the ground integrity of the work surface and wrist
straps. If gloves must be wom, cotton or ESD protective materials are
preferred. Keeping the level of relative humidity above 40% will also help
in controlling ESD.
Devices should be stored such that all exposed leads
are held at a common potential. Devices inserted into conductive foam are
a common way to store such items. Items should be placed in antistatic
bags or other appropriate containers. Items should be removed from their
protective packages within the confines of an ESD work area. Prior to
removing or handling sensitive components, neutralizing the ESD package by
placing it on a grounded workbench or by touching the package while grounded is
also good practice.
Uninsulated hand tools or static controlled hand
tools are preferred for use in an ESD safe work area. Insulated hand tools
should be discharged to a conductive mat before being used. When using
multimeters, momentarily ground the probes prior to circuit
Do not insert or remove static sensitive devices with power applied.
This is especially true for MOS devices. Additional MOS precautions
- Do not apply an input signal while the MOS power is off
- When testing MOS devices, connect all unused input leads to either ground, Vdd, or Vss, whichever is appropriate for the circuit involved.
- Prior to performance of dielectric or insulation resistance tests, remove MOS devices from the equipment if possible.
- When designing circuits with MOS devices, a small
amount of series resistance can slow down the rate of static
While in the vicinity of static sensitive
items, personnel should avoid physical activities which are static
producing. Such activities include wiping feet, removing or putting on
smocks, or pulling tape from a tape dispenser. The workstation should be
kept as free as possible from static generators. Work instructions, test
procedures, drawings, and similar documents used in ESD protected areas should
be stored in anti-static covers. Common plastics, untreated styrofoam, and
similar static generators shall be kept a safe distance from the work station
area. Static generators which are essential to the activities being
performed should be used with proper precautions taken.
The following assembly level
manufacturing processes require special consideration:
- Verify the compatibility of components with solvents used during the manufacturing process.
- Care must be taken in the decisions to use a wire (or harness) and its placement and routing to avoid broken and/or pinched wires, especially during or after a rework/repair operation and to prevent crosstalk.
- Consideration of fixture and container design and the use
of adequately trained personnel can reduce handling damage substantially during
assembly and rework/repair operations.
- Inspection and assembly personnel should have
adequate visual aids for use in verifying their completed work at each stage
of assembly. The initial build personnel should ensure workmanship
errors found during the inspection process are corrected.