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New Conformal Coatings Combine Protection with Environmental Safety
December 31, 1969 |Estimated reading time: 13 minutes
Providing Excellent dielectric resistance, these coatings feature insulative protection and resistance to various solvents and harsh environments.
Barry Ritchie
Lester Bennington
Aconformal coating is a thin layer of insulating material applied to the surface of a printed circuit board (PCB) to protect sensitive components from thermal shock, moisture, humidity, corrosion, dust, dirt and other damaging elements. When properly applied, these coatings provide a high degree of insulative protection and are usually resistant to many types of solvents and harsh environments. Coatings also provide excellent dielectric resistance. Demanding applications where conformal coatings are critical include automotive, consumer electronics and appliances, industrial controls, military/aerospace systems, and medical devices.
In the past, because of the cost of conformal coating materials and application processes, only the most expensive boards or those with special reliability requirements were coated. Recent advances in application technology and process ability have improved the economics of conformal coating use. Additionally, as circuit size diminishes and components become more delicate, protective barrier coatings are growing in importance.
While many available conformal coatings are still solvent-based, the market for solvent-free coatings in North America and other parts of the world is growing rapidly. In 1970, the U.S. government passed the Clean Air Act, which gave the Environmental Protection Agency (EPA) the authority to set national air quality standards to protect against common pollutants, including materials that release volatile organic compounds (VOC) and ozone-depleting chemicals (ODC). EPA and Occupational Health and Safety Administration (OSHA) standards, stringent state and local regulations, and emerging environmental awareness have combined to encourage both coatings formulators and electronics manufacturers to use solvent-free and low-ODC/VOC coatings wherever possible in their manufacturing operations.
While solvent-free coatings are more expensive on a per-unit basis than solvent-based materials, much less volume is used. Because solvent-free coatings are 100 percent solids, they do not evaporate as part of the curing process. Solvent-based materials are typically 60 to 70 percent solvents, all of which is wasted during evaporation.
Coatings Selection
Conformal coatings are generally classified according to the molecular structure of their polymer backbone. There are five traditional conformal coating chemistries: acrylic, epoxy, urethane and parylene (commonly grouped together as organics), and silicone (an inorganic). All except parylene were solvent-based until a decade ago, when increased environmental concerns and resulting government regulations dictated the reformulation of conformal coatings to solvent-free, low-ODC/ VOC materials and processes.
Many environmentally acceptable coatings are hybrid formulations that combine two or more coating chemistries (e.g., urethane acrylate and acrylic functional silicones) to improve performance properties, wetting, adhesion and cure requirements.
Solvent-free organic coatings are typically tough, abrasion-resistant materials that offer improved moisture and chemical resistance and operate at temperatures ranging from -40° to 125°C. Typical organic coating dielectric strength is 1,000 V per mil. Organic coatings in general, and acrylics and urethanes in particular, are resistant to a broad range of solvents. However, acrylics and urethanes may not be the best coating chemistries for environments exposed to wide fluctuations in temperature over short periods of time, as they tend to crack under thermal stress. Rework on acrylics and urethanes can be handled using mechanical abrasion or micro-sand blasting. Epoxies tend to be the least popular conformal coatings because of rework issues. Because most board substrates are made of epoxy, the manufacturer may destroy the board by removing the coating.
Silicone coatings are soft, flexible materials with a high coefficient of thermal expansion (CTE), which allows them to absorb expansion and contraction stress without harming protected components, and to function well in environments with extreme temperature cycling from -40° to 204°C. Silicones are very forgiving materials in production because they coat and adhere to just about any surface found on a PCB and offer good resistance to polar solvents, an attribute that makes them ideal for automotive electronics applications. Silicone`s dielectric strength is typically 500 V per mil.
Parylene coatings are deposited onto PCBs using gas-phase polymerization to provide a very thin uniform coating. Boards coated with Parylene must be processed in a batch operation using special high-vacuum equipment. An adhesion promotion process using silane and isopropyl alcohol (IPA) followed by a rinse and bake-out step is generally required as a pre-treatment to most electronic components bound for parylene deposition. Because this coating can find its way into gaps as small as 0.001", air-tight masking of interconnects is required to prevent leakage. Parylene is applied in the cured state during the chemical vapor-deposition process once the raw dimer material is sublimated.
Cure Methods
There are various methods available to achieve rapid cure or solidification of conformal coatings, including two-component mixing, heat, moisture and ultraviolet (UV)-light exposure. Each of these methods is appropriate for specific coating chemistries and has distinct advantages and disadvantages.
Traditional acrylic, urethane and epoxy coatings can cure or solidify in minutes using heat or two-component technology, which involves a room-temperature chemical reaction. Silicone coatings may be cured by exposure to heat, UV light or ambient moisture. Hybrid coating formulations, which incorporate multiple coating chemistries, are designed to be either UV curable or to rely upon dual-cure mechanisms such as UV light, heat or ambient catalyzation to enhance cure efficiency and increase in-line cure speeds.
Every cure method has its own set of advantages and disadvantages. Two- component mixing offers wide latitude in adjusting a coating`s cure speed and pot life. However, this technology is often considered undesirable because it requires the user to inventory and mix, in the proper ratio, two different materials. Catalyzed coatings use two-component room-temperature cure. Properly formulated, these materials have a 1:1 mix ratio and a pot life of 8 to 10 hours (one shift) or up to five days depending on the chemistry, which makes them good candidates for robotic applications. These coatings wet and adhere well in the no-clean process, and have a very effective shadow cure. Recent advances in static mixing and meter mix equipment make it possible to mate two-component materials with both atomized spray and selective application equipment.
While heat cure improves wetting and lowers viscosity, some heat-cure coatings, particularly platinum-catalyzed silicones, are subject to cure inhibition. This occurs when the coating comes into contact with various sulfur, amine or organometallic compounds that are sometimes found on boards as residual contaminates from the integrated circuit (IC) chip demolding or solder flux process. Additionally, achieving very rapid cure (less than two minutes) requires a temperature in excess of 150°C, which may be high enough to damage some components.
Moisture-cure coatings solidify rapidly on exposure to ambient or induced moisture. Extremely moisture-sensitive materials may cure inconveniently (e.g., in the feed line, on the surface of the supply reservoir or at the dispense nozzle). To control a potential rise in viscosity, the coating should be exposed to as little moisture as possible prior to application.
UV-light cure is an efficient process (Figure 1). UV-light-cure materials contain a photo initiator that cures the coating in seconds when exposed to the proper UV light wavelength. One major problem encountered with UV materials is their inability to cure in areas not exposed to UV light. To overcome this problem, UV coatings have been formulated with a secondary cure mechanism to ensure full cure in areas that are not directly exposed to UV energy. Full cure in shadowed areas is extremely important for board performance with all coating chemistries, as elevated operating or test temperatures may cause uncured material to expand, rupturing the coating fillet and cracking solder joints or IC packages.
UV-light-curing materials also are subject to oxygen inhibition, a process that occurs at the coating-air interface when oxygen reacts competitively with free radicals generated during UV light exposure. Oxygen inhibition can be overcome by increasing UV light intensity or by reducing oxygen concentration with a nitrogen blanket.
Prior to selecting a coating chemistry, the end-use environment of the PCB should be reviewed, assessing the potential for exposure to solvents, temperature extremes, dramatic temperature gradients and physical stress. No one coating is right for all boards and conditions. Working closely with reputable coating material suppliers that team up with equipment suppliers is the best way to ensure the selection of an appropriate material and process.
Coating Application/ Dispense Methods
Normally the final manufacturing step on a PCB assembly line, conformal coatings can be applied manually or with semi- or fully automated techniques. Preparation for conformal coating is a four-step process. First, board cleanliness is established and, if necessary, the board is cleaned. Next, connectors on the PCB are masked off as necessary to protect interconnects from the coating process. Coatings are then applied to the board and cured. Finally, any protective masks on the board are removed.
Conformal coating dispense techniques can be selective or nonselective. Nonselective systems apply the coating uniformly at a very fast pace, but add substantial time and cost in material waste and manual masking/demasking operations. Examples of nonselective dispense techniques are dip, atomized spray, brush, and wave or flow coating.
- Dip - Boards are immersed into liquid conformal coatings and withdrawn. Because most dipped coatings will not penetrate very narrow gaps, the dipping process is decreasing in popularity (Figure 2).
- Atomized spray - Performed with a standard automotive paint spraying gun, atomized spray is the most popular method of manual coating as it provides fast, uniform coverage.
- Brush - Typically used for small boards or localized repair and touch-up coating application, brushing is the least-used application method and is best suited for low-volume production. Uniform thickness and bubbling are the main problems with brush application.
- Wave or flow coating - Boards are indexed over a wave or pumped tide of coating material that is applied to one side at a time.
Computer-controlled selective coating systems apply coatings to designated areas of the PCB. Because coatings are precisely dispensed in defined areas, masking and demasking operations as well as other off-line batch activities are significantly reduced or eliminated. Selective coating systems offer substantial savings in resin conservation and off-line masking and demasking labor operations.
Some popular selective coating systems are:
- An airless process using a nozzle to dispense a shaped curtain of coating material, one automated process was designed for use with solvent-based materials.* Evaporation helps control cured film thickness and migration of wet material (Figures 3 and 4).
- Designed to dispense high-viscosity, solvent-free materials, a venturi-type air-assisted dispense head produces a wide (1" plus) coating curtain from the head and stretches it over the board`s surface.** Overspray is dependent on the viscosity of the material being applied and the on/off times programmed into the dispense profile.
- Another process employs a dispense head shaped like a probe. During selective dispense, air assist "twists" the coating stream into a bead, corkscrew or conical mist pattern on the substrate. Though designed to dispense low-viscosity coatings, this head is not as sensitive to viscosity or chemistry as other technologies, and offers three different dispense patterns that can be programmed for specific board geometries.
- Selective atomized spray head systems have been found to work well with a wide range of viscosity coatings. Multiple tools and spray heads can be incorporated to perform varied dispense tasks (Figure 5).
All selective systems can be configured with indexers, board inverters and various cure systems to create an automated process that can be added to a conventional assembly line.
Process Issues
The degree of soldermask cure is important to conformal coating performance. If not cured completely, ingredients such as glycols, bromides and ionic compounds can exit the film during subsequent solder excursions, leaving residues that may ultimately affect the wetting characteristics and adhesion of the conformal coating. Some of these residues can also contribute to electrochemical migration (ECM) and subsequent dendrite formation. Also, if the soldermask materials are not applied to clean, uncontaminated substrates, many problems can occur during conformal coating. While contaminated boards fail less quickly with a conformal coating, they can still fail because of trapped ionic and corrosive species between the substrate and the coating.
Not long ago, board substrates were G-10 or FR-4 materials using RMA flux chemistries and CFC-113 solvent-cleaning methods. These limited variables made tracking compatibility problems relatively simple. Today`s wide selection of soldermasks, flux chemistries, solder processes and alternative cleaning processes has greatly complicated compatibility issues.
By performing cleanliness and adhesion testing, board manufacturers can assess the baseline contamination of the PCB and determine whether further cleaning is necessary to ensure conformal coating adhesion, integrity and reliability. Two effective methods of testing boards are ionic cleanliness testing, which determines levels of chloride per sq. in., and surface insulation, an electrical test of solder joints. Although conventional ionic cleanliness testing methods are not as accurate as ion chromatography, they are excellent cleanliness monitors for the production environment.
If contamination levels are higher than recommended (5.7 µg/NaCl/cm2), board manufacturers have a number of choices for cleaning the assembly. However, there is still no single, easy, environmentally safe answer to board cleaning. When considering replacement-cleaning chemistries, manufacturers should note the cleaning capability, evaporation times, residue and odor of the process. With any of the available cleaning processes, a chemical reaction may take place that could affect board performance and reliability.
No-clean/low-solids fluxes are not usually removed by cleaning prior to coating application. The type and level of remaining residue is dependent on the board design and the solder profile. Inadequate preheat temperatures and dwell time duration can affect the success of a no-clean process. If the profile is not done correctly, ionic and corrosive species from the flux can cause a variety of performance problems. The flux application method and soldering environment also have an impact on how much unvolatized flux species will remain on the assembly.
Because a conformal coating can retard corrosion but not prevent it, manufacturers using these materials must benchmark and maintain proper processes to prevent corrosion and ensure reliability. Excessive residues can be coated, but entrapment of the ionic and corrosive species will cause a variety of problems over time. Conventional ionic testing is not routinely performed on low solids because evaporation of the IPA in the test solution causes a visible, white residue to form around the solder fillets, creating aesthetic concerns.
Water-soluble fluxes work well when used in conjunction with conformal coating operations. As these materials are very aggressive, they must be washed off within minutes of soldering operations. As a result, board surfaces are generally well cleaned and few coating problems are encountered. However, water-soluble fluxes can outgas residual absorbed contaminants and water during heat excursion, causing dendrite formation or coating blisters in areas of flux stains. The drying process must be as controlled as the actual cleaning process. Again, proper process controls will greatly reduce potential problems.
For effective, long-term conformal coating solutions, board manufacturers should work closely with conformal coatings formulators to determine effective board cleaning methods. Once the sole responsibility of board manufacturers, coatings suppliers will now assess and test process-ready assemblies for cleanliness, and coat them with application-appropriate coatings to determine their effectiveness in the assembly process. Coatings companies also will work closely with equipment manufacturers to pre-qualify adhesives and dispense equipment, ensuring that materials and the application systems will run smoothly on the customer`s production line.
* SelectCoat
** ControlCoat
SwirlCoat
PVA 1000 and SCS 4398
BARRY RITCHIE, application engineer, and LESTER BENNINGTON, development scientist, may be contacted at Loctite Corp., 1001 Trout Brook Crossing, Rocky Hill, CT 06067; (860) 571-5100; Fax: (860) 571-5358 or (860) 571-2511; E-mail: barry.ritchie@loctite.com and lester.bennington@loctite.com.
Figure 1. Conformal coatings cure in seconds when passed under a UV light, speeding up the assembly process and eliminating the need for gated assemblies.
Figure 2. During dip coating, boards are immersed into liquid conformal coatings and withdrawn.
Figure 3. Coatings applied using a machine.
Figure 4. One airless process uses a nozzle to dispense a shaped curtain of coating material to a board.
Figure 5. This selective atomized spray head works well with a range of coating viscosities.