Introduction Conformal coatings are thin layers of synthetic resins or polymers which are applied to electronic devices for protection against a variety of environmental, mechanical, electrical and chemical problems such as:
• Humidity and moisture
• Stress, mechanical shock, vibration and thermal cycling
While most conformal coatings are mostly used on populated printed wiring boards (PWBs), they are also used to protect components such as transistors, diodes, rectifiers, resistors, integrated circuits (ICs) and hybrid circuits including multi-chip modules (MCMs) and chip on board (COB). The development of conformal coating technology was fostered to a large degree by the requirements of military and aerospace industries.
Types of Conformal Coating. Conformal coatings are generally classified according to the molecular structure of their polymer backbone. There are five basic types of conformal coatings:
• Acrylic (Type AR)
• Epoxy (Type ER)
• Silicone (Type SR)
• Parylene (Type XY)
• Urethane (Type UR)
The coatings are further subdivided by the mechanism of their cure, e.g., heat, moisture, UV light, etc. Type AR Acrylic, epoxy and urethane coatings can be either solvent based lacquers which cure via solvent evaporation to give thermoplastic materials or two component materials which cure upon mixing to give thermoset materials. Traditional solvent-based AR materials are easily removed. Thus, they do not provide good protection against solvents. The new hybrid AR/UR combinations have excellent resistance to solvents, therefore, rework is more difficult. Type ER Epoxies are used when an extremely tough coating is required. They are becoming less prominent because of rework issues and thermal coefficient of expansion (TCE) mismatch with surface mount components and materials. The solvents used to dissolve the cured epoxy do readily attack printed wiring board (PWB) laminates and component packages. Type SR Silicone coatings generally have a much lower modulus than other coatings and have a wider useful temperature range. Silicones have a relatively high TCE. However, because they are soft and flexible, they tend to withstand relatively higher mechanical abrasion and stress. The rework of silicone coatings is relatively difficult. Type XY Poly-para-xylelene or parylene is an extremely reliable and pin-hole free material which is applied by a vacuum deposition process. These coatings provide a very thin and uniform surface coverage. Since parylene deposition is a dry film process with no liquid phase, it does not pool, bridge or exhibit meniscus properties during application. After the coating has been applied, parylene has essentially no undesirable physical or mechanical impact on underlying devices even with wide temperature excursions. As a dry non-solvent based coating it is not affected by VOC restrictions. The rework of parylene is easy for a local repair site, however, the removal from the entire assembly can be time consuming and difficult. Type UR Urethane coatings have been the most widely used. They offer very good moisture, solvent and mechanical abrasion resistance. They also have excellent dielectric properties and are UV curable. The rework of urethane coatings is relatively difficult.
The hybrid acrylated-urethanes (AR/UR) form the backbone of most of today’s UV curable coatings. These coatings contain photoinitiators which, when irradiated by UV exposure, cause the monomers to produce free-radicals that cross-link the polymer chains. This causes the coating to cure instantly.
Application Methods. Conformal coating application techniques have progressed significantly during the last decade. A number of conformal coating application techniques may be considered. They include dipping, brush application, spray application, flow or wave coating, vacuum deposition, etc. The type of coating material selected may dictate the method of application. The PWAs must be thoroughly cleaned before coating application to prevent any flux residues or other contaminants being trapped under the coating. Each of the conformal coating application techniques is characterized by unique benefits and limitations as described below. Dipping This method has been used since the early stages of conformal coating technology. PCBs are masked and submerged in the dip tanks which contain coating material and are withdrawn for cure. Immersion and withdrawal rates are critical to the thickness and to the elimination of air bubbles. Dip systems can be manual or automatic. When choosing a dip process, the following factors should be thoroughly considered: • contamination in the tank resulting from the surface of PWAs • specific gravity of the solvent to monitor solvent loss due to evaporation • use of an inert gas blanket for moisture sensitive materials • excessive dripping of solvent Brush Application This method is generally used for touch-up and repair operations, small parts and low volume applications. While providing good material utilization and minimizing or eliminating the need to mask, brush applications can be labor intensive and can result in an inconsistency in coating thickness. Materials used with this method are generally "air dryable" solvent-based or moisture curable. Spray Application This is the most common method of applying conformal coatings. It can be as simple as a hand-held spray gun in a spray booth to as complicated as an elaborate automated application system. Masking is still a requirement for this type of process. Material usage is high and significant amount of coating is wasted. Air spray systems can also be labor intensive as they require regular cleaning and waste disposal. Variations in the coating thickness uniformity and surface coverage is a major drawback to this method. Flow Coating A method of bottom-side flow coating (for meniscus coating) works similar to the wave soldering operations. A board is passed over a "wave" of coating material at a specific angle. The thickness of coating is controlled by the viscosity of the material and the speed with which it passes over the wave. This process only coats the bottom-side of the board and is limited to flat surfaces. Masking is usually required. Selective Applications The selective coating process has become a method of choice for many medium and high volume users. Selective coating can virtually eliminate masking/demasking and provide excellent material utilization. Selective coating systems can apply most materials (including solvent-based and solvent-free) and use sophisticated robotic systems to provide high consistency and transfer efficiency. The following selective applications are commonly used: • Selective spray • Non-atomized curtain coating • Air-assisted curtain coating • Ultrasonic dispense coating Chemical Vapor Deposition This dry, solvent-free coating process is designed to coat PWAs and other assemblies with a thin, inert and highly conformal polymer film known as parylene. The raw material for the process is di-para-xylelene dimer, a dry powder. Parylene C is the prominent family of parylenes used in electronic applications (Parylene C has a chlorine atom on the benzene ring). Since parylene deposition is a dry film process with no liquid phase, it does not pool, bridge or exhibit meniscus properties during application. Substrate temperature remains nearly at ambient during the coating process. After it has been applied, it has essentially no undesirable physical or mechanical impact on underlying devices, even with wide temperature excursions. This is a batch operation and the systems are relatively expensive.
Defect Characteristics Regardless of the application method selected, a conformal coating must be free of defects which can be a cause for rejection or rework. The following are some of those typical defect characteristics: • Tackiness, soft spots, lifting or peeling • Excessive fileting or running • Bridging of stress relief areas thereby negating stress relief • Bridges between the printed wiring board and the bottom of DIPs and flatpacks potentially causing lifting or contact problems • Conformal coating material used after the shelf life expiration • Bubbles or bare spots bridging two electrically conductive elements • Discoloration • Pinholes, blistering, scratches, wrinkling or cracking • Any sign of contamination (i.e., flux, loose particles or foreign material)
Processes Affecting Conformal Coating Many new electronic manufacturing technologies have emerged as a result of CFC/ODS/VOC emissions. The impact of processes associated with the use of new solder masks, no-clean (NC) or low-solids (LS) fluxes, water soluble fluxes and alternate cleaning technologies should be studied with respect to the application of conformal coating. These processes include: Solder Mask Compatibility of solder mask and conformal coating is essential in maximizing the quality of the final assembly. Adhesion and wetting performance are the most critical factors affecting the compatibility issue. Improper adhesion can lead to peeling and cracking of the conformal coating, especially during thermal cycling. PCB manufacturers should check with their conformal coating supplier to ensure their compatibility especially in relation to the processes/materials used in solder mask operation. Water Soluble Flux Residue Water soluble flux is very corrosive and must be removed immediately after the soldering operation. The PWAs must be thoroughly dry prior to conformal coating application. The conformal coating shows poor adhesion and inferior quality in presence of any moisture during the coating application. No-clean/Low-solids Residues With increasing use of no-clean/low-solids fluxes, it is very important that the PWAs do not exhibit any of the flux residues. Small traces of flux residues left on the PWAs after soldering operation can cause dewetting and poor coverage of the conformal coating. Residues from Alternate Cleaning Technologies The residues from the CFC-alternate cleaning agents (hydrocarbons, terpenes, esters, etc.) can be absorbed into the solder mask and can cause outgassing at elevated temperatures resulting in coating defects such as blisters, vesication etc. Product Handling Even in the continuous flow manufacturing (CFM) environment, process/product handling during masking, conformal coating or inspection operations can result in contamination on the PCBAs. PCBAs which are handled during masking may contain oil residues from skin contact. Adhesive from tape used in masking can cause dewetting problems during coating.
Removal of Conformal Coating It is important to consider how the choice of a conformal coating material affects the rework and repair issues. The need for rework or repair of a conformal coating can arise at any time after completion of an assembly due to a variety of process/product requirements or component replacement issues. Hence, rework of conformal coatings needs to be addressed up front when choosing a coating chemistry. A number of methods are available for rework of conformal coatings. These include thermal, chemical, mechanical, abrasive, plasma and laser-based systems. The commonly used methods are described below. Thermal The thermal removal method using a soldering iron is the least recommended method. Most conformal coatings require a very high temperature and long exposure times. These, in turn, can cause discoloration, leave residues and adversely affect solder joints and other materials/components used in the fabrication of assemblies.The process must be monitored to ensure that excessive temperatures do not cause delamination, lifting pads or overheat surrounding temperature-sensitive devices. Extreme caution must be taken when burning through conformal coating because some coatings emit toxic vapors which are hazardous. Chemical Until a few years ago, chemical methods were the most popular for the removal of conformal coatings. As long as the solvents used do not adversely affect the PWB or components and there are no environmental issues this technique works well. However, there is no one perfect solvent for all applications and in some cases it may be difficult to find a suitable solvent.
The following sections discuss the chemical removal methods for various types of coating: Urethane There are several solvents which provide a wide range of speed and selectivity that can be matched to a specific application. These solvents include: • methanol base/alkaline activator solvents which provide a range in the dissolution power and selectivity • ethylene glycol ether base/alkaline activator solvents which are relatively the fastest and least selective Silicone Methylene chloride based systems are very effective in removing silicone conformal coatings. Several hydrocarbon-based solvents are also used as alternatives. While not as fast as the methylene chloride, the hydrocarbon based solvents are more selective, and where not contaminated by water, will not attack epoxy-glass PCBs, components, metals and plastics. Acrylic The chemical removal of acrylic coatings was done in the past with highly volatile and flammable solvents such as methylene chloride, trichloroethane or ketones. A relatively safe alternative based on butyrolactone has been used for the removal of most of the acrylic coatings. Epoxy The complete removal of epoxy coatings for repair is nearly impossible by chemical means. The solvent can’t discriminate between the epoxy coating, epoxy-glass PCB and any epoxy-coated or potted components. However, if done carefully spot removal may be accomplished by the application of methylene chloride and an acid activator with a cotton tipped swab. Parylene Parylene coatings cannot be dissolved but can be removed using tetrahydrofuran solvent. The coating separates from the PCB after immersion in the solvent for a period of two to four hours. The complete removal of the coating in most cases is accomplished by immersion of the PCB into the solvent at room temperature. The amount of time required for coating removal will vary with the type of the solvent, type of the coating and coating thickness. Spot removal may be done by application of the solvent with a brush, cotton pad or cotton-tipped swab. Some of the solvents are available in a gel form for spot removal applications. Mechanical This method of removal incorporates mini-grinding wheels, stiff brushes and other abrasive instruments commonly used in electronics rework operations. The coating is abraded enough to facilitate de-soldering and replacement of the component. However, extreme care should be taken to insure that damage to the substrate or surrounding devices does not occur. The effects of electrostatic discharge (ESD) generated during processing on parts and the assemblies should also be addressed. Micro-Abrasive Micro-abrasive systems used for conformal coating removal are very similar to small sandblasting machines which were originally designed for metal deburring and etching. Basically, a cutting media is introduced into a compressed air stream and is ejected through a handheld nozzle. This is directed at a component or surface area on PCB where the conformal coating has to be removed. This system can remove conformal coating from a single test node, an axial leaded component, a through-hole IC, a SMT component or an entire PCB without any modification to the system for a variety of coating materials. This method provides the most practical and environmentally friendly means for removing conformal coating from PWAs. Plasma The use of plasma based systems for coating removal is practical for conformal coating which has very good thickness uniformity. Since the plasma process removes the coating from all areas at the same rate, the non-uniformity in the coating thickness may result in damage to the substrate or underlying films. This method is suitable for coatings like parylene since the parylene deposition process yields a high uniformity in coating thickness. Discoloration in certain types of metals can be prevented by masking the exposed metal surface.