SOLDER MASK in Software

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SOLDER MASK
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TABLE 33.4 Solder Mask Drying Ovens and Their Associated Advantages and Disadvantages Oven type Batch Pros Productivity large quantities with minimal manpower Low investment Not dependent on panel design, materials, or construction Consistency Consistency Uniformity Not dependent on panel design, materials, or construction Very fast cycle time Productivity Cons Potential for nonuniform temperature Opening oven doors causes temperature to drop Cure time may be oven-loading dependent
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More handling Higher investment Thin panels may need a fixture to prevent them from bending Multiple recipes usually required to compensate for different materials, designs, and constructions Cannot cure every PCB design/construction Requires automation or extra handling High investment
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Exposure equipment must provide a means of holding a phototool in intimate contact with the surface of the panel being exposed and provide the required amount of light (millijoules/cm2) of the needed wavelength distribution (which may depend on the solder mask) at the appropriate intensity (mW/cm2). Unfortunately, many exposure devices that are used for exposing solder mask are substandard, including many units that were designed many years ago for exposing primary imaging photoresist. Modern units have high-intensity lamps (7 kW or higher) and sufficient lamp and exposure frame cooling to provide a highquality image very quickly. Lamp intensity and energy output should be monitored routinely with a radiometer sensitive to the appropriate wavelength to verify that the lamps are performing as expected. Intensity will decrease with lamp hours, making exposure times become longer. At the same time, the infrared output will increase, potentially causing frame cooling issues. Hot exposure frames cause artwork to stick to the mask, resulting in artwork marking or mask to transfer to the phototool. Laser units are available for exposing photoresist that have been used for exposing solder mask. These solder masks may be specially formulated to be sensitive to the laser wavelength. Laser imaging eliminates solder mask registration problems, but is not as productive as conventional exposure. Equipment cost is very high compared to conventional lamp exposure equipment. Underexposure will result is insufficient polymerization to withstand the development process properly without damage to the solder mask. Typical symptoms may include a dull or chalky surface, lifting of small features (e.g., solder dams), excessively undercut sidewalls, or lifted mask. Overexposure causes image growth where the exposed feature is larger than on the artwork and openings in the solder mask will be smaller. If the growth is excessive, the mask may grow onto the pads.
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33.5.2.1.4 Development. Development of the unexposed solder mask material must be sufficient to clean out completely the smallest holes that must be free of mask. Photoimageable solder masks may be developed in either aqueous or solvent chemistries. The developer chemistry needed depends on the mask formulation. In the development process, solder mask that has not been exposed to UV light and polymerized is washed away, or developed, from the PCB panel to reveal the desired pattern of solder mask. For aqueous development, the chemistry is a warm, mildly alkaline (pH 10.6 11.3) solution of sodium carbonate (Na2CO3) or potassium carbonate (K2CO3). Proprietary chemistries are also available. Antifoam is generally used to control foaming as the development solution loads with developed mask. Solvent-developing chemistry typically uses gamma-butyrolactone (also known as GBL or butyrolactone) or butyl carbitol (2-(2-Butoxyethoxy)ethanol) solvent. The development process must be set up to ensure that the smallest holes are free of undeveloped solder mask. This is very different from photoresist because the breakpoint will often be 10 to 15 percent of the development chamber instead of nearly 50 percent with photoresist. Key process variables for the development process are the time in the developer and rinse sections; solution temperature, pressure, and nozzle type; flow rate; and configuration. It is very important to have an inspection step immediately after development to check for registration, mask in holes, feature resolution, dam retention, lifting, and so on. Once the panels have been cured. it is significantly more difficult to strip and rework the solder mask, and the rework process is more likely to cause the panels to be scrapped. 33.5.2.2 Dry-Film Solder Mask (DFSM) Application. The process for DFSM differs from LPI solder mask only in the application step. Surface preparation, exposure development, and cure are the same as for LPI solder masks. 33.5.2.2.1 Application with Vacuum Lamination. DFSMs typically have no solvent in them to enable them to conform to circuits without entrapping significant amounts of air along or between circuit traces.To apply these films to circuit topography, a laminator must be able to laminate panels in a vacuum chamber. The predominant type of vacuum laminator has heated upper and lower platens. The panel or boards to be coated will have DFSM gently applied (without any pressure) to one or both sides and are placed into the vacuum chamber formed between the upper and lower platens. The platens are closed together and a seal is formed. Vacuum is drawn in the chamber. When a specified time has been reached (determined by the board type and thermal mass, DFSM type and thickness, and board/panel plated circuit height), the flexible diaphragm on the upper platen (which had been held tightly against the upper platen surface by vacuum) is released to slap down onto the panel surface. The heat will have made the DFSM plastic so that it will conform to the circuits, and the vacuum prevents poor lamination along circuits. The lamination is completed when the platens separate and atmospheric pressure forces the DFSM into all voids. 33.5.2.2.2 Roll Lamination. Lamination of DFSM with a standard hot roll laminator is unacceptable for all except the least demanding applications. It is impossible to laminate a flat film of solder mask to PCB topography without trapping air along circuits. There are vacuum hot roll laminators that will work satisfactorily, but these are not widely available. 33.5.2.3 Non-Imageable 33.5.2.3.1 UV and Thermally Curable Solder Masks. Because these products are not photoimageable, they must be applied with patterned screen printing in the exact pattern that is needed on the finished PCB. After their application, they are cured by the appropriate cure recipe as dictated by the supplier. There is no opportunity to develop off solder mask from areas where it is unwanted, so registration of the screen to the panel is very important. 33.5.2.3.2 Ink Jet Application. This technique utilizes digital data and ink jet technology to coat solder mask only in areas where it is wanted. This eliminates the photoimaging steps and should reduce or eliminate registration problems. As of the writing of this book, no such machines are being used in production for solder mask application.
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