Printed circuit board assemblies often require protection from moisture, dust, or vibration. Common techniques include conformal coating, encapsulation, room temperature vulcanizing (RTV) silicone spot treatment and overmolding. Here is what to look for in a contract manufacturer’s (CM) capabilities and how your choice impacts lead time, cost, and quality.
Conformal coating. Photo courtesy: Z-Axis
Conformal coating techniques
In conformal coating, a thin layer of acrylic or urethane conforms to a board’s topography.
Applying conformal coating using a robotic system is a well-controlled and repeatable process. The robotic system sprays droplets of coating, like an ink jet printer, to create a uniform layer of protection n the board. The CM determines and programs the optimum spray pattern for each board design and stores it in memory.
Look for different nozzles. Fan spray nozzles are good for rapidly covering large areas. Precision needle applicators are needed to get even coverage around the edges of BGAs and multi-lead surface mount technology (SMT) components, or around the base of LEDs while leaving the light emitting surface exposed. They also preserve uncoated areas such as connector pads.
Your CM may use a manual sprayer rather than a robot. Manual spraying can yield acceptable throughput and quality for boards that are relatively flat, that is, all components are about the same height. Labor costs can rise substantially if any areas need to be left uncoated, as these areas must be masked before spraying and then masks removed afterwards.
Manual spraying is an uncontrolled process and the coating can vary depending on operator skill and consistency. If your CM will use manual conformal coating, ensure that the quote includes careful inspection for thin areas and gaps, and manual touch-up as needed.
Conformal coating materials
Conformal coating materials are urethane or acrylic, specially formulated for electronics. Traditional coatings contain a solvent which evaporates as boards air-dry. These coatings shrink as they harden, and there is some risk of developing thin spots or gaps. Air-drying can also add hours to the production cycle.
Solvent-less urethane is a major improvement. It is cured by brief exposure to ultraviolet (UV) light, hardening faster without shrinking. It is more expensive than solvent-based coatings and may show up as higher material costs. However, it usually results in lower overall costs, faster production, and reduced need for touch-ups.
Potting involves placing a printed circuit board assembly in a shell and filling the shell with epoxy or urethane. This protects the board from dust and moisture, and stabilizes tall components to protect the assembly from vibration damage. The potting material is also better than unmoving air at dissipating heat, further supporting reliability and giving a performance boost as well.
Molded plastic shells are available in a wide range of standard shapes and sizes.
RTV silicone spot treatment
For less expense and weight than potting, an electrical grade RTV silicone can be applied around only those components that need vibration protection. Like potting compounds, RTV silicone helps dissipate generated heat away from the components.
Plastic injection molding can be used to encapsulate and protect a circuit board. The board is placed in a mold and molten plastic is injected into the mold around it. For electronics, this is done at a relatively low melting temperature and injection pressure.
Overmolding materials for electronics include a low-temperature polyester that withstands alcohol wash for medical applications, and a polyamide that withstands oil and gas for industrial applications.
Overmolding requires an upfront expense for tooling, usually around $10,000 to $20,000 for an aluminum mold. Depending on production volume, this cost can be offset by eliminating the per-piece expense of a potting shell. Overmolding also offers greater flexibility to create custom shapes that are functional in the final assembly.
The best protection technique or your PCB assembly depends on environmental conditions, board design and component selection. A CM should be able to help your engineers evaluate cost-vs-performance tradeoffs including material and labor costs as well as non-recurring expenses for molds and tooling.
To learn more about these techniques, take a look at the video below:
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