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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.

9 comments on “Conformal PCB Coating: The Process Matters

  1. prabhakar_deosthali
    November 21, 2012

    This is quite an informative article on conformal coating. The conformal coating has assumed a significance not only in Industrial and military products but in consumer products such as home appliances – washing machines , dish washers and such things.

     

    One of the disadvantage of conformal coating that I have experienced is that once the conformal coating is done, the boards become difficult to service and repair.  So such board design should take into account .

     

  2. FLYINGSCOT
    November 21, 2012

    Thanks for writing this article as I never thought too much before about conformal coating.  How much does it cost as a percentage of the average base board material and how environmentally sound are the coating materials?

  3. David G Allen
    November 21, 2012

     Prabhakar,

    You are most certainly correct, but there are ways to mitigate this concern. Depending on the requirements for coating and the coating material chosen, there are ways to manage any rework or repair after coating.

    Urethane, being the more chemically resistant of the coatings may require a micro abrasion system for removing the coating within the suspect area where acrylic is VOC based and therefore will soften with the appropriate solvent.

    David

  4. bobwillis
    November 21, 2012

    Many thanks for the overview on coating and the issues to consider. Readers may be interested in the video on quality control for coatings processes see
     my video on measurement

    IPC sell inspection standards wall charts for conformal coating, interactive CD ROM and photo CD ROMs used for creating training content. I am not sure if anyone else offers this range of training products? We have been running webinars on coating for the last couple of years which look like a good training medium for worldwide consumption see webinars

    Many thanks again

    Bob Willis

     

  5. prabhakar_deosthali
    November 21, 2012

    Thanks David for enlightening me on this issue of repair and rework. As far automated tools for surface coating is concerned, I designed a robotic work station for surface hardening by chemical spray by a robot. This robot had a teach and learn function whichwas very useful in determining the exact depth of the coat with precision and repeatability. I guess similar tools can be employed for conformal coating

  6. _hm
    November 22, 2012

    Some application like defense, industrial control etc. do require conformal coating. However, some of electronics used in office and home may not require conformal coating.

  7. Ed Branco
    November 22, 2012

    Over the years I have found printed circuit assemblies have pushed the limits of all of the materials used to and processes used to apply them. I have researched several of the CNC machines on the market and found they fall short of for filling all the needs for all designs. Many designs have to lower the acceptance criteria for the sake of full automation. We have several automated machines the coat selected areas, but the high end parts at this point still requires very skilled sprayers, and maskers. Most designs we see today have conformal coating is an after thought.

  8. elctrnx_lyf
    November 24, 2012

    Many contract manufacturing companies have been making investments for the process of conformal coating. As there is robotic technology becomes more and more matured, there will be more improvements in the reliability of conformal coatings.

  9. Michael Allen
    November 25, 2012

    A lot of good questions and comments here.  I typically see the cost of conformal coating individual boards below $5.00 in 100 to 1K quantity.  Boards that can be conformally coated in arrays obviously have a much lower cost depending on the piece size.  We usually use our robotic coating equipment so having keep outs does not raise the cost.  Many of today's coatings such as acrylic are easily repairable.  UV tracers make it very easy to see where you have adequate coverage.  The increased life of the board depends greatly on the environment that the boards will see.  I have seen boards that were used in a machining application that by design were in dust proof enclosure.  These boards had not been conformally coated and covered in metallic dust and filings leading to the boards failure.  How often does a company inspect and repair seals on a 20 year old machine?  This machine could have not been taken down if these boards were coated.  In addition to protecting the boards the conformal coating can provide additional mechanical protection.  All of the parts are held to the board by their bodies via the coating, not just by the solder.  This can certainly enhance the reliability of boards used in applications like aviation .

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