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TABLE 41.1 Conformal Coating Materials and Their Characteristics*
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Material type Solvent evap. x x x x x x x x Urethane Heat cure UV Cure RTV Silicone UV Cure Catalyzed Solvent evap. Epoxy Heat cure UV Cure Catalyzed Solvent evap. Acrylic Heat cure UV Cure XY Vapor deposited
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Cure methods hand paint dip hand spray automatic spray selective coating vapor deposited Solvent Thermal Mechanical Plasma etch Easy to rework moisture resistent simple cure Biologically compatable High temperature use Abrasion resistent Solvent Resistent Complete coverage Reversion resistent Cold temperature Use High VOC Requires strict viscosity control flammable aided by primer cure affected by thickness and mass cure inhibition or reversion High cure shrinkage not for low temp Incomplete cure in shadow Has pungent odor Brittle at high temp rework difficult UV wavelength affects cure potential for contamination difficult process Poor edge cure Short work life Moisture affect cure Long complete cure time Health and safety concern Violent reaction with water Requires moisture for cure Low abrasion resistance High CTE May contaminate other products Intermittant solvent resistance difficult adhesion batch process Full masking required
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*Disclaimer: Due to the widely varying nature of the different formulations even within a single column heading, the above information is general in nature and cannot reflect the nature of all available materials that fall under that heading.
Application method Removal Benefits Risks
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4.5 4 3.5 Modulus (GPa) 3 2.5 2 1.5 1 0.5 0 50 40 30 20 10 0
Hard Coating Medium Coating Soft Coating
10 20 30 40 50 60 70 80 Temp ( C)
FIGURE 41.1 Elastic modulus versus temperature for three different type UR coatings.
41.2.1.1 Chemistry. Urethanes are based on a di-isocyanate and polyol backbone. They are available in solvent evaporative cure, heat cure, and UV cure formulations. 41.2.1.2 Properties. Since urethanes are polymerized and cross-linked in place, they have excellent resistance to chemical, moisture and solvents. They are available in hardnesses ranging from tough, abrasion-resistant varieties to low-modulus versions suitable for extreme temperature ranges (see Fig. 41.1). 41.2.1.3 Advantages and Disadvantages. Urethanes have good adhesion to most materials, including epoxy part bodies, metals, and ceramics. As such, the coating process is fairly robust. Since urethanes are chemically resistant, they are also difficult to remove except by thermal or mechanical means. Urethanes can be soldered through, although this often results in a brownish discoloration that must then be removed. 41.2.2 Silicone (Type SR) Silicones are very flexible, and stay that way over a wide range of temperatures. They have good adhesion to a variety of surfaces, but contaminate the surface and, once applied, prevent other materials from adhering. Typical application is .002 to .008 in. thick. They are often used in automotive applications, where high temperature and moisture-resistance are needed. 41.2.2.1 Chemistry. The silicone polymer chain is based on an alternating silicon-oxygen backbone. SR coatings are available in three types, room temperature vulcanizing (RTV), UV cure, and catalyzing (addition) cure. 41.2.2.2 Properties. Silicones have relatively stable properties from 55 C to 200 C. Their coefficient of thermal expansion (CTE) is higher than that of urethanes, but this is mostly offset by their lower modulus, so that the level of stress exerted on parts is still relatively low. They have high resistance to moisture and humidity, as well as polar solvents. 41.2.2.3 Advantages and Disadvantages. Silicones are usable over a wide temperature range and are relatively easy to remove via mechanical or thermal means. If mishandled, silicones
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may contaminate the work area, causing adhesion problems on other CCAs. Careful process sequencing and process separation is necessary to prevent this. 41.2.3 Epoxy (Type ER) Epoxies are chemically stable and very resistant to chemical attack. Typical application thickness is .001 to .005 in. Epoxies are useful in extreme environments where chemical vapors or high temperatures are present. 41.2.3.1 Chemistry. Epoxy coatings are based on epoxy resin systems and come in four types: solvent evaporation, heat curing, UV curing, and catalyzed. 41.2.3.2 Properties. Epoxies have a low CTE that matches well with printed wiring board (PWB) epoxy resin, since they share very similar chemistry. They have a higher Tg than most of the other coating materials. They are also very tough and abrasion resistant, so that rework is very difficult; epoxy coating can form the basis of an antitampering coating. 41.2.3.3 Advantages and Disadvantages. Epoxies are useful at moderately high temperatures, up to about 150 C. Because of their strength, they also provide mechanical support for components. The disadvantages of epoxy coatings are their usually pungent odor and the possibility of skin irritation. They are difficult to rework. Some formulations of epoxies are chemically delicate and do not cure properly in the presence of inhibiting compounds. Cure shrinkage is also of concern for fragile components; a softer buffer coating should be applied locally before the epoxy, particularly for assemblies that need to withstand wide swings in temperature. 41.2.4 Acrylics (Type AR) Acrylic coatings are applied .001 to .005 in. thick and are often used in military and consumer electronics. 41.2.4.1 Chemistry. Acrylic coatings are usually supplied as dissolved pre-polymerized acrylic chains. The acrylic chemistry does not cure by polymerization and cross-linking as the other coating materials do, but instead hardens gradually as the solvent evaporates. Acrylics are also available in heat-curable and UV-cured formulations. 41.2.4.2 Properties. Acrylics are easy to apply, and are the easiest coating to remove, since relatively mild solvents soften and dissolve the acrylic coating while leaving the epoxy encapsulated parts and PWB unharmed. They can be cured quickly. 41.2.4.3 Advantages and Disadvantages. The greatest strengths of acrylic coating are the ease of rework and the fast room temperature cure.Acrylics provide good moisture resistance and fluoresce easily, aiding inspecting under UV lamps. Since they are so easy to rework, acrylics are also susceptible to inadvertent chemical attack from solvent splash during hand cleaning of solder joints elsewhere in the assembly. The high emission of solvent inherent in the acrylic process makes them less environmentally friendly than other materials. As the solvent evaporates, the coating shrinks and exerts stress on the components, so acrylics may not be suitable for all low-temperature applications. 41.2.5 Para-Xylylene (Type XY) Para-xylylene coatings are unique, since they are applied by vapor deposition (see Sec. 41.4.6) rather than as a liquid. They are applied .0005 to .002 in. thick. Para-xylylene coatings are often used in biomedical devices due to the inert character of the coating.
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