Operationally, conformal coatings are applied to the surface of PCBs and related electrical components, to insulate and protect them during use. Coatings improve PCBs’ performance under any circumstances, but are especially valuable for their functional tolerance to harsher working environments.
Liquid coatings are so-termed because they are applied to PCBs by means of wet methods, primarily by brush, dipping (component immersion) or automated/manual spraying. The primary liquid coatings are constituted from acrylic, epoxy, silicone or urethane resins. A fifth coating material, parylene, uses a chemical vapor deposition (CVD) method to apply the substance to substrates in a gaseous form.
Material selection is largely dependent on the type of PCB being coated and its function. Selecting an inappropriate coating material inevitably leads to product malfunction or breakdown. Equally as important is selection of the coating method most suitable to the film material and PCB-operation; a bad match can also cause poor performance and device failure. As well, application results are dependent upon process humidity and temperature, requiring further attention pre-coat and careful monitoring through application and curing.
Here, the selected wet coating is manually applied onto the PCB with a brush. This method has the benefits of:
- relative ease applying the coating to the designated regions of the PCB surface, and
- cost-effectiveness for small-batch production.
Manual brush-coating has several disadvantages, which can emerge regardless of the operator’s experience or level of skill. The surface of some PCBs may be complicated or irregular, making it difficult to apply an even coat across the designated PCB-area. Coats that are too-thin provide inadequate protection, leaving the component vulnerable to infiltration from external agents (moisture, oil, etc.). Overly thick conformal films are prone to cracking, similarly exposing the PCB; cracking of too-thick films occurs especially during thermal cycling. In addition, brushing allows only one side of a PCB to be coated at a time, slowing production.
Dipping immerses a PCB entirely in a fluid conformal solution; the film forms around the assembly while submerged. The dipping process can be done manually, but use of automated equipment is becoming more common. Typically, PCBs attached to a mechanical arm are lowered into a dip-tank containing the liquid coating; immersion rate is set by the quantity of assemblies being coated.
Advantages of the dip method include reliable coating penetration under components and generally fast processing, outside of intensive manual masking , which slows production time. Further disadvantages include:
- erratic edge/tip coverage despite thorough immersion in the dip tank, and
- often inconsistent coating thickness.
Also, coating problems can emerge if an assembly’s components are situated too-closely on the circuit board, limiting immersion’s efficiency reaching all areas of the assembly.
Spray applications can be exceptionally cost-effective. A well-trained operator can provide superior coating surface-quality, in comparison to other liquid application methods. At the same time, robotics has become a key component of high volume spray application jobs. Automated spray procedures generate enhanced project accuracy, resulting in better coatings produced faster. Dedicated spray booth applications offer efficient medium-level production. Manual benchtop spray coating is recommended for smaller-scale rework and repair assignments.
Spray coating provides these advantages:
- high-level production volume,
- enhanced edge/tip coverage,
- reduced masking, and
- better film uniformity.
- over-application, leading to thicker coats than recommended for optimal assembly performance,
- diminished film-penetration under components, and
This is the case with either a well-trained application technician or robotics –
automated aerosol spray procedures -- which
· generate enhanced project accuracy,
· very efficiently for high-volume spray assignments,
· leading to better coatings produced faster.
Chemical Vapor Disposition (CVD)
Chemically inert parylene uses a vapor-phase, chemical-vacuum polymerization process – CVD – to convert powdered parylene dimer into a gaseous form, in a vacuum. Conversion occurs on a molecule-by-molecule basis, allowing parylene to penetrate the most minute surface crevices of the substrate, tightening even multi-layer elements with uniform, pinhole-free coating. CVD’s primary benefits include:
- production of consistently high-quality conformal films
- that resist chemicals, corrosives, moisture and solvents,
- with outstanding dielectric properties and minimal thermal expansion.
- True, micron-thin conformance to substrate contours protects PCBs’
- function/performance through an exceptional range of operational conditions.
CVD-applied parylene coatings are successful in the nanometer range making them very useful for MEMS/nano-technologies.
CVD’s disadvantages include:
- greater expense than any liquid technology,
- small batch-size/greater turn-around time, and
- difficulty reworking/repairing coatings.
Selecting the best material/application method for your coating assignment prolongs assembly service-life and promotes optimal performance. Diamond MT’s 30 years’ experience combines with its staff of highly-trained personnel, quality materials and process solutions to generate optimal implementation of your conformal coating assignments.
To learn more about applying conformal coating, download our whitepaper now: