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FIGURE 41.4 Selective coating using a continuous flow nozzle. (Courtesy of Chipco. Inc.)
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(Fig. 41.4), producing a well-defined boundary between the coated and uncoated areas and eliminating the need for masking. In the same way as the semiautomatic sprayer, the board is tack-cured and then turned over and coated on the other side.An automated board flipper may be used to accomplish this to automating the entire coating process. 41.4.6 Vapor Deposition Vapor deposition is only used with XY coatings, and requires specialized coating equipment. In the evaporator, a measured amount of dimer is placed in a crucible. The coating cycle is then started, and the chamber is pumped down to an almost complete vacuum (see Fig. 41.5).
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Split into active monomers 680 C, 0.5 Torr Dimer powder vaporized 150 C, 1 Torr
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FIGURE 41.5 Para-xylylene vapor deposition process.
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Condenses and polymerizes onto boards 25 C, 0.1 Torr
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CONFORMAL COATING
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During the deposition process, the dimer powder is vaporized and recondenses onto the batch of CCAs, forming a uniform coating covering all corners and edges evenly. The vapor deposition process drives the vapor into any crevices and openings.Vapor deposition has the most stringent masking requirements, since leaks in the maskant will still admit vapor that will contaminate the surface behind, and air pockets inside masking boots or tape may dislodge the boot in the vacuum and create leaks as the air escapes.
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CURE, INSPECTION, AND FINISHING
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The finishing steps require most of the labor involved in conformal coating process. Three finishing operations are:
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Curing the coating Inspecting under UV light to verify complete coverage and touch-up Removing mask materials
Cure Each of the four liquid chemistries is available in a formulation offering different cure mechanisms, as shown in Table 41.1. 41.5.1.1 Solvent Evaporation. Systems that are cured by solvent evaporation can be air dried, or slightly heated to drive the solvent out faster. Care must be taken not to overheat the freshly coated assemblies; the coating will skin over and cause bubbles. Long-term exposure to the unevaporated solvents trapped in the coating may also be deleterious to the materials in the parts and PWB. Drying of solvent cure assemblies must be done under a ventilation hood or other properly ventilated area. 41.5.1.2 Room Temperature Vulcanization (RTV). Some silicone-based coatings use the room temperature vulcanization (RTV) cure process, which consumes moisture from the surrounding environment as part of the cure process. Some means of humidity control is required; if cure is attempted in a dry oven, the material will not cure properly, if at all. 41.5.1.3 Heat Cure. Oven-curable systems use an extended bake, ranging from 15 min. to several hours or longer, for the material to cure fully. Thermally activated systems require the increased temperature to start the curing process, whereas thermally accelerated systems will cure eventually at room temperature but are baked to speed up the process and improve material properties. Generally, the hotter the oven, the quicker the cure, and the harder the finished material. 41.5.1.4 UV Cure. For high-volume production, UV-curable materials are used. Immediately after the coating machine is a booth with high-intensity ultraviolet lamps to illuminate the assembly. After several seconds, the assembly has cured enough to be able to be handled at the next stage of assembly, such as coating the opposite side. UV-curable materials do not cure fully under components, where the coating is not illuminated.Therefore, most UV-curable materials also contain a secondary curing mechanism, such as heat cure, so that at the end of the line the CCA can be fully cured in a single baking step or naturally come to full cure in ambient conditions inside the finished product. Since different coatings are sensitive to different wavelengths of UV light, it is important to coordinate the purchase of the light source with the supplier of the coating.
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41.5.1.5 Catalyzed. The catalyzed process involves mixing two parts together just before application, to initiate polymerization and cross-linking. This process occurs slowly at room temperature but may be accelerated by elevating the temperature in an oven. Catalyzed coatings may be affected by contaminants present in the PWB or parts being coated, resulting in uncured areas or peeling coatings. Since the pot life of catalyzed materials tends to be short, it is imperative that the mixed material be cleaned from sprayers, tubing, and so on, before it thickens and hardens. 41.5.2 UV Inspection Coatings contain a fluorescent dye that will glow under UV light and show where the coating has (or has not) been applied. Areas without coating can be touched up at this time. The areas most susceptible to insufficient coating are corners of parts, where the coating can flow away and leave little behind; the protruding ends of through-hole part leads; and shadowed areas under or behind larger components. If large flat areas are uncoated and show beads of coating, there is a wetting problem due to improper surface preparation or a material incompatibility. The coating should also be inspected for improper thickness, bubbles, trapped material or debris, and incomplete curing (see Fig. 41.6).
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