The Thin Polymer Line
Joe Gizmo rubs his hands together and waits impatiently for the train. Another drizzly, damp November evening in Chicago. Suddenly, his pager goes off. It’s the boss’ secretary. Joe switches off his portable MP3 player and calls her back on his cell phone. She tells Joe that tomorrow’s meeting has been moved up an hour. Joe thanks her for the news, takes out his personal digital assistant and enters the new time. When’s that train going to arrive?
Chances are, Joe isn’t thinking about whether the printed circuit boards (PCBs) inside his portable electronic devices are protected from the unpleasant reality of fall in Chicago. That’s because those PCBs have been coated with a thin layer of polymer to protect them from moisture, chemicals, dust and thermal shock. The coating is usually just a few mils thick.
Once reserved for high-reliability automotive and aerospace applications, conformal coatings are increasingly being applied to PCBs inside more mundane products, like portable CD players and washing machines. Not only do conformal coatings protect PCBs from the elements, but they also temper the effects of corrosion, stress, vibration and electrical noise. Conformal coatings immobilize particulates on the surface of the PCB, and they prevent arcing in highly humid operating conditions, especially with fine-pitch assemblies.
"You need to apply a conformal coating to any circuit board that’s going to see the environment," says Chris Marinelli, director of application engineering at Henkel Loctite Corp. (Rocky Hill, CT). "As circuitry has become smaller, it needs more protection."
The MaterialsThe most commonly used chemistries for conformal coatings are acrylic, epoxy, urethane, silicone and parylene. Some new coatings combine two or more of these chemistries. Which coating to use depends on such factors as:
- the conditions the board will be exposed to,
- the materials and components used to assemble the board,
- whether the board will be repaired if it fails in service,
- how much of the board will be coated, and
- the production volume.
Acrylics are usually solvent-borne coatings, but formulations are available that do not contain volatile organic compounds. Because they are solventborne, acrylics should not be used on boards that might be exposed to solvents. On the other hand, this "disadvantage" is also an advantage, in that acrylic coatings are easily removed if repairs are necessary. Waterborne acrylics and urethane-acrylic hybrids are available, but these are not as easy to repair.
Epoxies are very hard and usually opaque. They are most often supplied as two-part materials, but one-part materials that cure with ultraviolet (UV) light are available.
Epoxies provide good resistance to humidity, abrasion and chemicals. Because epoxies are so rigid, they can stress components when they expand or contract during thermal extremes. Moreover, they are virtually impossible to remove chemically for rework, because any stripper that can dissolve the coating can also attack components on the board and the board, itself. The only way to remove an epoxy-coated component is to burn through the coating with a soldering iron.
"Epoxies will shrink while curing," warns Marinelli. "They don’t shrink as much as other materials, but because of their high modulus of elasticity, the shrinking could cause damage."
Urethane coatings are available as two-part materials, and as one-part waterborne or UV-curing materials. Urethanes resist abrasion, humidity and solvents, and they have outstanding dielectric properties. They stay flexible in cold temperatures, but they can be adversely affected by high temperatures.
One-component urethanes require close control of the coating and curing process, Marinelli advises. Moisture-curing urethanes may form minute bubbles if the coating is too thick or the coating environment is too humid. Air-curing urethanes are tack-free in less than an hour, but without heat, they may take as much as a month to fully cure. In addition, air-curing urethanes may crack or form "alligator skin" surfaces if they are applied too thick.
UV-curing urethanes require a secondary cure mechanism to harden the coating beneath components and other areas shielded from the light. Waterborne formulations are slower drying than solvent-borne coatings. They have lower chemical resistance than other urethanes, and they don’t wet surfaces as well.
Two-component urethanes require tight control of humidity during application. And, both one- and two-part materials contain isocyanates, which can sensitize the skin.
Urethane conformal coatings can be burned through with a soldering iron, making component replacement fairly easy. Chemical stripping of these coatings varies from easy to difficult, depending on the formulation.
Silicones are excellent for coating PCBs that must endure extreme temperature cycling, such as automotive and aerospace applications. Silicones are also desirable for PCBs with heat-dissipating components, such as power resistors. The operating range of silicones is -55 C to 200 C.
Silicones have a high coefficient of thermal expansion, says Marinelli. However, because they are soft, flexible materials, they can withstand the stress of expansion and contraction without transmitting this stress to components. These coatings resist humidity, corrosion and polar solvents, but they are susceptible to abrasion.
Silicone coatings are available in solvent-borne formulations, or in 100 percent solids formulations that cure with heat, moisture or UV light.
In the past, repair of assemblies coated with silicones was difficult, because they were not readily soluble and they could not be vaporized with a soldering iron. However, chemical strippers are now available that can dissolve these coatings.
Under certain operating conditions, such as high heat and low air flow, silicones can give off "cyclics" that form a thin layer of silica on nearby surfaces. This silica layer can increase electrical resistance and interfere with mechanical relays and playback heads. As a result, silicones should be used with caution in areas where mechanical contacts and tape decks reside directly over the coating.
Parylene produces a very thin, uniform coating with excellent coverage of all parts. The material is applied by vapor deposition in a vacuum environment. As a result, the coating can only be applied in a batch operation.
This tough, transparent, biocompatible coating has outstanding dielectric strength, and it provides excellent protection against organic and inorganic solvents. Because it’s so strong, the coating is not easily removed. Lasers and microabrasive blasting are two methods that work.
"Parylene is very expensive, but in the end, it’s probably the best coating you can get," says Marinelli.
Manual DispensingConformal coatings can be applied in several ways.
For low-volume production, prototyping or repair work, conformal coatings can simply be brushed on. This is the least expensive method of applying conformal coatings, but it’s difficult to achieve uniform results. Only one or two brush strokes should be used, or small air bubbles will form in the wet coating, creating microvoids and cosmetic defects. In addition, care should be taken to avoid depositing brush fibers in the coating. If the material is applied over an existing coating, the new coating should overlap the edges of the old one by 0.125 inch.
For medium-volume production, a handheld spray gun is simple and cost-effective. This method works best with low-viscosity coatings. High-viscosity materials tend to sputter. Because this method covers the entire board, it’s necessary to mask components that should not be coated, such as contacts, connectors, heat sinks, LED lenses, speakers, terminals, test points and unsealed components.
"On the component side of the board, you typically only want to coat certain sections," explains Frank R. Hart, director of marketing for PVA (Halfmoon, NY). "On the bottom side, the solder side, you want complete coverage."
Coatings should be sprayed using compressed air or nitrogen at the minimum pressure necessary to provide good atomization. If compressed air is used, steps should be taken to keep oil and water out of the line, says Jim Victoria, application specialist with EFD Inc. (East Providence, RI).
"Any moisture might cause the ‘orange peel’ effect, where you see a lot of small air bubbles in the coating," he says.
The process should be performed in a well-lit and well-ventilated spray booth. The boards should be placed on a turntable and rotated 90 degrees after each pass back and forth.
Because spray coating is operator-dependent, the results can be inconsistent. Oversprayed material is wasted, and operators are exposed to the coating, which can be a health issue.
Another way to apply conformal coatings is by submerging the board in a tank of material. This method was more popular when PCBs were mostly populated with through-hole components. Masking is difficult, and the results depend on the viscosity of the material, how fast the board is submerged and withdrawn, and how much time the coating is allowed to drip from the board.
Immersion and withdrawal speeds range from 2 to 12 inches per minute, depending on the size and complexity of the board. The goal is to submerge the board slowly enough to displace the air surrounding components. Because the material in the tank will get thicker as solvent evaporates, thinner should be added periodically. If the coating is moisture-sensitive, a blanket of dry nitrogen above the tank’s surface will keep the material from skinning over.
Automated DispensingFor high-volume production, automated dispensing equipment is required. A major advantage of automated dispensing is that the coating can be applied selectively. This often obviates the need for masking. Automated dispensers also produce more consistent results and waste less material than other coating methods.
"An automated process can be justified by looking at labor savings from masking and manual spraying," says Hector Pulido, product specialist for conformal coating with Asymtek (Carlsbad, CA). "Savings can also be found in material usage and reduced ventilation needs."
Available in benchtop or conveyorized models, automated equipment consists of an X-Y-Z positioning system and one or more valves. The material, the valve and the gantry speed determine how thick the coating is applied and how much of the board is covered at one time.
Valves for conformal coating can be classified as atomizing or nonatomizing. Atomizing valves use air to deliver the material in a mist or a pattern, such as a swirl. "Air enters the nozzle at the exit point, mixes with the fluid, and breaks it up," Victoria explains. "But, it’s not like an automotive spray painting device, where there’s a lot overspray. It’s a low-volume, low-pressure spray valve."
Nonatomizing valves deliver a column or curtain of material. "Nonatomizing valves usually provide the best results in edge definition and film thickness buildup," says Pulido. "Atomizing valves yield a lower film thickness per pass, and they are less forgiving on edge definition."
PVA’s Hart says it’s not uncommon to equip an automated dispenser with both valve types. An atomizing valve can do most of the work, while a nonatomizing needle valve on a tilting actuator reaches areas that a spray head cannot, such as between or beneath components.
"Different valves give you different characteristics," he adds. "Many contract manufacturers use more than one type of coating. Atomizing valves are good for the new, 100 percent solids materials. Nonatomizing valves work very well with solvent-based coatings."
Even with automated equipment, conformal coating often remains a batch process. For example, the PCBs may need to be cleaned and dried before coating. "True in-line applications, where a board enters one side and a complete, coated assembly comes out the other side, are not common," says Pulido. "Usually, there’s some handling before and after the coating operation."