Conformal coating

From Wikipedia, the free encyclopedia

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 electronic system. Precision analog circuitry may suffer degraded accuracy if insulating surfaces become contaminated with ionic substances such as fingerprint residues, which can become weakly conductive in the presence of moisture. (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.

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 micrometers 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 micrometres can be easily applied in a single operation. The advantage of parylene coatings is 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.

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 are we protecting against? (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 I re-work the material 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 litre.

- Quality of the material supplier (two acrylic material manufacturers will not make equal quality of material)