Conformal PCB Coating: The Process Matters

Many purchasing managers are aware of the different processes used in contract manufacturing for printed circuit boards, but they aren't always clear about when and why certain capabilities matter. Conformal coating is one of those processes. It can save your boards from an early death, but only if it's done right.

Conformal coating means applying an acrylic or other specialized material to protect boards from contamination that can lead to failure. Usually, we're trying to block moisture from condensation, which is a concern in any environment where temperatures fluctuate -- that is, just about any outdoor application (especially military and automotive).

You've undoubtedly seen condensation first hand -- perhaps last summer as moisture from the warm air condensed on the surface of your cool beverage can. The same thing happens to electronics -- for example, as airborne systems descend from cold altitudes to warmer ground elevations.

Conformal coating can be applied manually with a brush or sprayer, but this has serious drawbacks. Because brush application is labor-intensive and yields inconsistent results, it is rarely used for production boards. Spray application is more common, but even skilled operators have a hard time getting consistent coverage. The coating thickness will vary, and areas such as the corners of surface mount ICs may be left exposed.

Manual spraying is sometimes acceptable when the entire board is to be coated. However, most boards have LEDs, switches, connection pads, or other features that must remain free of coating. So you need to make a mask to cover these features before spraying. With the mask in place, it's hard to get spray into critical areas around the edges of the mask -- for example, the solder connections at the base of a masked LED. It gets even worse as the mask is used on successive boards and accumulates a buildup of coating. The mask must be periodically cleaned, and the resulting hazardous mixture of coating and cleaning solvents must be disposed of safely.

Programmable automated robotic coating systems eliminate the problems of manual coating. In a robotic system, a spray nozzle travels along a pre-programmed path above the board, automatically dispensing an even layer of coating on the desired areas. It is fast, repeatable, and precise, and there is no need for masking.

The benefits of robotic coating include:

• Consistent, high-quality coating for higher yields and better protection
• Higher throughput for lower production costs and faster delivery
• A more environmentally friendly process -- no wasted spray and no masks to clean

Robotic systems can be fitted with a range of spray nozzles. Wider nozzles cover large areas at high speed. Narrower nozzles provide more precision. Needle applicators apply fine lines precisely where needed, such as along the edge of a row of switches or at the legs of an LED. (The needle serves the same function as the small brush you use to cut in the corners of a room before painting the walls.) Each nozzle is a considerable investment in quality.

For best results on production-volume boards, a contract manufacturer should use a robotic conformal coating system with both spray and needle applicators. In addition, many OEMs request 100 percent visual inspection of coated boards before shipping. Many coatings contain a UV tracer that fluoresces under UV lighting, making it easy to spot any unlikely gaps in the finish.

Work with your contract manufacturing partner to understand which coatings they can use, how they are applied, and what inspection tools are used to make sure your boards have the protection you need.