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A critical function of conformal coatings is to protect PCBs and electronics from damage caused by moisture and humidity. In this discussion, we will attempt to answer some of the more common questions we often hear regarding the level of protection you can expect from a conformal coating.

Are conformal coatings waterproof?

The above is probably the most common question and most misunderstood subject in the area of PCB protection

In previous articles, we discussed common defects in conformal coatings related to surface energy issues. These included issues such as

• fish eyes
• lack of coating coverage
• poor bonding
• delamination

We received significant feedback, and the most common response was to ask for more specifics about just what surface energy is anyway.

The PCB industry continues to evolve with ever increasing demands for efficiency and manufacturing cost reductions. From the design phase onward, suppliers and contract manufacturers are being asked by their OEM overlords for increased performance at lower and lower costs. An obvious potential source for cost reduction is the use of alternative cheaper raw materials or their elimination where possible. At some point in the discussion, it is sometimes asked,  “Why don’t we just eliminate the conformal coating?” or "Are conformal coatings necessary".

Conformal coating of printed circuit boards (PCBs) has resulted in a steady evolution in performance and protection for modern electronics. It has allowed for lightweight and reliable controls in industries such as automotive and aerospace among others. This has brought about the possibility of increased length of warranties as well as fewer failures and claims. Nevertheless, original equipment manufacturers (OEMs) and consumers continue to raise their standards and expectations for longevity and reliability to levels unimagined just 10 years ago. In the automotive industry, as an example, thermal cycling testing might have involved as few as 10 or 50 cycles in the recent past. That standard is now often 500 or even 1000 cycles with no apparent ending in sight.

So what do I do when conformal coating is not enough?

Nobody likes masking, nevertheless it can be a necessary part of your PCB production process.

“But I thought selective conformal coating equipment had eliminated the need for masking in most cases.”

The cost of selective conformal coating equipment can easily run into hundreds of thousands of dollars. Even those who invest in the latest selective coating technology find, at times, that masking in certain areas is the only solution.

You have completed your search for an ideal conformal coating chemistry, one that will provide your PCBs the protection and reliability demanded by your industry and customers. After all that work, you probably assume your job is finished. Our apologies for being the bearer of bad news: There is still work to be done. In this basic presentation and discussion, we will talk about the proper choice and implementation of thinners for your coating. In many cases, a mistake in thinner can harm or even completely negate the protective properties of your conformal coating when applied to your PCB.

The past few years have seen the introduction of a number of thinner-film circuit board protection products of various chemistries, many of which have been generically termed “nano-coatings” by manufacturers despite confusion regarding what this really means. There is no clear definition of a nano-coating as it relates to printed circuit boards, and the product offerings vary from vacuum-deposited films and microparticles of single digit nanometer thicknesses to more robust performing products with thicknesses in the hundreds of nanometers. It is critical to manufacturers that the properties of true nano-coatings be compared to more traditional protection technologies so an accurate evaluation can be made.

Xylene and Toluene. Two words that can evoke concern and add to paperwork for environmental health and safety departments and manufacturing professionals. Aromatic solvents have been safely used for decades by employing basic safety equipment including gloves, masks, and proper workplace ventilation. Nevertheless, concerns for maximizing employee safety and minimizing effects on the environment have led electronics manufacturers to seek out alternatives. For over 20 years, HumiSeal® has supplied a wide-ranging group of aromatic-free products across all traditional chemistries to manufacturers for whom these concerns are critical.

The evolution of conformal coatings for protection of PCBs and electronics has accelerated over the past few years. One development in particular has increased in popularity due to a promising array of advantages over its predecessors. These are LED Curable Coatings.

A partial list of existing technologies for conformal coatings would include:

• acrylics (both solvent and water-borne)
• solvent-borne polyurethanes
• silicone-based polymers
• synthetic rubber (solvent-borne)
• Parylene technology
• UV-curable acrylated urethanes (arc and microwave bulb source)

and most recently . . .

Picking a Conformal Coating

You and your organization have spent weeks or months researching conformal coatings to meet the demanding specifications of your customers. These can include factors such as dielectric strength, thermal and mechanical shock properties, processing considerations, and so on. At some point, the critical consideration of price and cost must be added to the equation. The amalgamation of cost and performance could be considered a measure of the true value of any specific coating.