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Single-sided coating (hand or flat bed) In this application, each side of the panel is fully coated (except for a border on each side) with solder mask, one side at a time. The panel may be tack dried between coating the two sides, or the second side may be coated immediately using a dimple plate to protect the coating on the first side. It is advantageous to coat both sides of the panel before tack drying so that both sides see the same tack-drying cycle. One concern about this technique is the amount of solder mask that is pushed into the holes. Since this is a single-sided process, there is nothing to restrain the amount of mask that gets pushed into the holes, so it may be more difficult to develop all of the mask out of the holes. Double-Sided Coating Vertical double-sided screen printing applies solder mask to both sides of the panel or panels simultaneously. The screening units used are available in various levels of automation, up to fully automated screening units that are integrated with tack drying ovens. Because they apply mask to both sides at the same time, the units put less mask into holes, making it easier to develop mask out of holes a particularly important consideration when working with small holes that must be free of solder mask. Front-to-back squeegee alignment is critical for putting the least amount of solder mask into the holes.
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A key variable in screen printing is the screen mesh. Most applications utilize a noncalendared, nylon mesh material. These products are designated with the number of threads per inch and the diameter of the threads in microns.Typical examples of commonly used mesh counts are 86-120, 83-100, 86-100, 92-100 or 110-80. Supplier literature will provide a theoretical ink volume for each type that can be used to determine the relative difference in coating weight to be applied to the panel. Table 33.1 provides an overview of the screen process key variables and the effect of operating a process at the extreme of that variable. Spray coating accounts for a limited percentage of the solder mask coating. There is renewed interest in spray coating since it can allow more latitude in removing solder mask from holes. As the holes become smaller, some solder masks and processes have difficulty achieving clean holes after development. At the same time, because the solder mask is sprayed at reduced viscosity, the amount of solvent (VOCs) that is exhausted to the environment is significantly higher than for screen printing, the use of spraying may be limited in some localities.
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TABLE 33.1 Screening Process Variables and Effect Operating at the Extremes Process variable Mesh Tension Effect of low Mesh sticking to screen Poor quality printing Low coating thickness Skips (areas with no solder mask) Lower circuit coverage Very sharp edge, lower coverage Lower coverage, Less ink in holes Screen sticking to panels Lower coverage, More flow or sagging if too low Effect of high Nonuniform coating across panel Low coverage over circuits Higher coverage Slightly rounded edge, higher coverage Higher coverage, More ink in holes Skips or difficult printing Higher coverage, Bubbles if too high
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Squeegee Pressure Squeegee Speed Squeegee Shape
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Squeegee Angle (from panel surface) Snap-off Ink Viscosity
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Two basic types of spray equipment have been used for spray coating PCBs with solder mask, HVLP (high volume, low pressure), and electrostatic. HVLP spray systems are most common. Single- and double-sided spray units are available. The solder mask is sprayed at a reduced percent solids compared to screen printing, so circuit coverage and tack drying can be more of a problem, especially if the ink and/or spray guns are not heated. Heating of the ink allows more solvent to evaporate between the nozzle and the panel, resulting in a higher viscosity liquid on the panel. The higher viscosity theoretically allows greater circuit coverage with less ink. The conventional HVLP spray process utilizes a low pressure stream of solder mask liquid that is atomized by a high volume stream of air. The atomized solder mask is directed to the PCB surface in a specific pattern so that as either the panel or the spray head(s) move, a uniform coating is applied to the panel. In some systems, the ink is heated just before spraying to lower its viscosity and aid in the evaporation of solvent between the spray nozzle and the panel. Typical process variables for HVLP spray include ink viscosity, ink (pot) pressure, atomization air pressure, conveyor speed, and ink and atomization air temperatures. In a typical electrostatic spray application, the ink is atomized and given an electrostatic charge while the PCB panel is grounded to attract the ink particles. The intent is to limit the amount of overspray and provide a more even coating. In reality, with the infinitely variable PCB designs to be coated, there are often many ongoing adjustments required to minimize Faraday effects that cause uneven distribution of the spray. Often solder mask formulations need to be modified for effective use in electrostatic spray applications. The main process variables in electrostatic spraying are solder mask temperature and viscosity (dilution), bell speed and voltage, back plane voltage, and shaping air pressures. These variables are adjusted to provide a uniform coating of the desired thickness or weight. Table 33.2 lists the process variables and the effect of operating at the extremes of that variable. Curtain coating is an application method where the panel to be coated is rapidly passed through a continuous falling curtain of solder mask. The curtain is created by pumping the solder mask through a die with a slot of controlled opening. The curtain falls vertically through an opening between two conveyors that carry the panels horizontally through the curtain, thus coating the panel on its upper side. Solder mask that is not coated onto the circuit board panels falls into a trough between the conveyors and is recirculated. The pump and conveyor speeds and the gap in the die control the coating thickness. Solder mask used in curtain coating applications must be coated at reduced viscosity or percent solids (as in spray applications), so the increased VOCs emitted to the atmosphere may limit curtain coating s acceptability in some localities.
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TABLE 33.2 Key Variables for Spray Coating and Effect of Low and High Levels for Each Variable Process variable Ink Viscosity Effect of low Low circuit coverage, Tack drying difficulties Effect of high Mottled or orange peel coating, excessive atomization, and overspray Possible clogging of lines or nozzle Thick coating, drying problems Excessive overspray Thick coating, insufficient drying, and/or bubbling
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