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Conformal coating material is applied to electronic
circuitry to act as protection against moisture, dust, chemicals, and
temperature extremes that if uncoated (non-protected) could result in a
complete failure of the electronics.
Precision analog circuitry may suffer less
accuracy if insulating surfaces become contaminated with ionic
substances such as fingerprint residue, which can
weaken conductivity in the presence of
moisture. |
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The classic symptom of
micro-contamination on an analog circuit board is sudden changes in
performance at high humidity, for example when a technician breathes on
it. Furthermore, a suitably chosen material coating has proved to
actually reduce the effects of mechanical stress and vibrations on the
circuit.
For example, in a chip-on-board assembly process, a silicon die is
mounted on the board with an adhesive or a soldering process, then
electrically connected by wire bonding, typically with
.001-inch-diameter gold or aluminum wire. The chip and the wire are very
delicate, so they're encapsulated in a version of conformal coating
called "glob top." This prevents accidental contact from damaging the
wires or the chip. Another use of conformal coating is to increase the
voltage rating of a dense circuit assembly; an insulating coating can
withstand a much stronger electric field than air, particularly at high
altitude.
With the exception of Parylene, most organic
coatings are readily penetrated by water molecules. A coating preserves
the performance of precision electronics primarily by preventing
ionizable contaminants such as salts from reaching circuit nodes, and
combining there with water to form a microscopically thin electrolyte
film. For this reason, coating is far more effective if all surface
contamination is removed first, using a highly repeatable industrial
process such as vapor degreasing or semi-aqueous washing in a special
machine. Extreme cleanliness also greatly improves adhesion. Pinholes
would defeat the purpose of the coating, because a continuous
contaminant film would be able to make contact with the circuit nodes
and form undesired conductive paths between them.
Coating methods
The coating material can be applied by various
methods, from brushing, spraying and dipping, or, due to the increasing
complexities of the electronic boards being designed and with the
'process window' becoming smaller and smaller, by selectively coating
via robot. A typical robotic process involves a needle applicator that
can move above the circuit board and dispense the coating material. Flow
rates and material viscosity are programmed into the computer system
controlling the applicator such that desired coating thickness is
maintained. The process quality of dip or dam-and-fill coating can be
improved when necessary by applying and then releasing a vacuum while
the assembly is submerged in the liquid resin. This forces the liquid
resin into all crevices, eliminating uncoated surfaces in interior
cavities.
Choice of method is dependent on the complexity of the substrate to be
conformally coated, the required coating performance, and the throughput
requirements.
Coating material when dry (after curing) should ordinarily have a
thickness of between 50 and 100 μm for situations where direct
condensation of moisture does not occur. Thicker coatings are required
when liquid water is present due to microscopic pinhole formation when
the coating material thins on the sharp edges of components. Typically
enough material is applied to "pot" the components by completely
covering them to a depth equal to or greater than the highest metallic
conductor on the circuit board.
Another type of coating called Parylene is applied with a vacuum
deposition process versus a spray or needle application. The Parylene is
applied at the molecular level by a vacuum deposition process at ambient
temperature. Film coatings from 0.100 to 76 μm can be easily applied in
a single operation. The advantage of Parylene coatings is that they
cover hidden surfaces and other areas where spray and needle application
are not possible. Coating thickness is very uniform, even on irregular
surfaces. The disadvantage is any desired contact points such as battery
contacts or connectors must be carefully covered with an air-tight mask
to prevent the Parylene coating from masking the contacts.
Material considerations
Selection of the correct choice of coating material
(lacquer) is one of the process engineer's most critical decisions.
Criteria for selection must be based on answering many questions, which
will include:
- What is being protected against? (e.g., moisture,
chemicals)
- What temperature range will the electrical device
encounter?
- What are the physical, electrical, and chemical
requirements for the coating material itself?
- Electrical, chemical, and mechanical
compatibility with the parts and substances to be coated (for
instance, does it need to match the coefficient of expansion of chip
components?)
Answers will determine the suitability of a particular
material, be it acrylic, polyurethane, silicone, etc.
Process, production and commercial issues will then
enter the equation:
- How easy can the material be reworked once
applied?
- How fast does the material dry (cure)?
- How fast can the material be applied and dried
(throughput time)
- What type of process and equipment is necessary
to achieve the required coating quality
(uniformity and repeatability)?
- Price of the material per liter.
- Quality of the material supplier (two acrylic
material manufacturers will not make equal quality of material)
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