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expect conformal coating to adhere well to a dirty board. These coatings were not meant to be applied over a no-clean flux, but over a clean substrate.
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Application Method for Installed Equipment Choose solder mask that is compatible with the existing or proposed application method. Not all solder masks can be screen printed, sprayed, and curtain-coated. Consult with the solder mask supplier for the appropriate solder mask for your application method.
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Gloss Solder masks are available in different levels of gloss. Typical terminology for finishes includes gloss, semigloss, satin or semimatte, and matte. These finishes represent a certain level of reflectivity of light and microroughness. Gloss level is generally specified by end users for assembly or cosmetic reasons.
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Color Likewise, different levels of color may be available. Most solder mask is green, but many other colors, such as red, blue, black, yellow, and white, are often specified. Some solder masks are specified to be clear.
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Packaging versus PCB When solder mask is used on substrates that will become part of a semiconductor package. The JEDEC Solid State Technology Association (once known as the Joint Electron Device Engineering Council) specifications apply in addition to the IPC specifications. JEDEC specifications include additional, more aggressive testing that generally requires a more robust solder mask. Low moisture absorption is important for a mask used in packaging applications.
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Surface Finish Compatibility The choice of the final surface finish for the copper has an effect upon how the solder mask performs. And, with the elimination of lead in solder, the compatibility of new final finishes needs to be confirmed before their implementation. Most of these finishes are applied after the solder mask so the process for mask application can have a significant impact upon final quality of the PCB:
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Hot air solder leveling (HASL) This has been the most common final finish, utilizing 63/37 eutectic or 60/40 tin/lead (Sn/Pb) alloys. The PCB is coated with flux, immersed into molten solder, and withdrawn through air knives to remove excess solder. Lead-free solders are expected to require solder temperatures significantly hotter than what has been required for eutectic solder. The higher temperatures are expected to place higher stress on the solder mask and may also present more difficult post-HASL cleaning challenges. Electroless nickel/immersion gold (ENIG) ENIG is typically applied to the metal surfaces of the PCB after the solder mask process. Each plating chemistry supplier has its own ENIG bath, and each has a different level of aggressiveness to the solder mask. Consequently, the solder mask and ENIG chemistry must be qualified together to avoid breakdown of the solder mask over circuits or around openings on metal. A thicker coating of
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solder mask may alleviate solder mask breakdown problems. Excessive microetching before plating can undermine the solder mask and induce tape test failure as the overhanging mask is broken off. Immersion tin The same comments from ENIG also apply to immersion tin. Immersion silver Immersion silver is relatively benign to the solder mask, and few, if any, issues are known as of the writing of this book. Organic solderability preservative (OSP) This is another process and chemistry that has little impact on the solder mask.
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SOLDER MASK APPLICATION AND PROCESSING
Surface Preparation Solder mask should be applied to a clean, dry surface. The cleaning step should remove all particulates, oils and greases, and unwanted oxides. In the case of bare copper, this step should provide some tooth to the copper to enhance the mechanical bond of the mask to the copper. The cleaning step is followed by a rinse and drying step that leaves the panels ready for coating with mask. The surface preparation technology chosen is dependent upon the metallization of the circuitry and, to a lesser extent, the base laminate material. 33.5.1.1 Copper Circuitry. A large majority of panels produced have solder mask coated over bare copper (SMOBC). The goals of the surface preparation process are to remove all unwanted oxides and contaminants from the copper and to impart a microroughness or tooth to the surface, and to leave a clean, dry surface for coating. 33.5.1.1.1 Incoming Copper. The quality of the surface of the copper is very important for achieving good solder mask adhesion. All traces of the etch resist metallization, typically tin or solder, must be removed prior to the solder mask surface preparation process. Solder mask does not adhere well to tin or solder residues or tin-copper intermetallic surfaces. 33.5.1.1.2 Preparation Methods. The most common surface preparation method is pumice scrubbing with either a silica or aluminum oxide media. Most equipment in use is brush pumice, but jet pumice may be used also. With this method, the main considerations are keeping the pumice concentration within recommended levels, periodic changing of the media, and good equipment maintenance. A prepared surface from a proper pumice cleaning will have a uniform rosy pink, matted surface that is free from stains. Considerations in pumice-based preparation techniques include the following:
Brush pumice versus jet pumice Both techniques utilize a slurry of pumice in water. With brush pumice the slurry is pumped onto the panel surface under low pressure, and rotating brushes scrub the pumice against the panel surface to clean and roughen it.With jet pumice, the slurry is sprayed through nozzles at high pressure onto the panel surface; the impact of the particles against the panel surface provides the cleaning and roughening. Silica versus aluminum oxide The media used in pumice scrubbing degrades with usage. Silica pumice is softer than aluminum oxide and must be changed more frequently, but it is much less expensive; thus relative cost differences in operation may not be significant. Silica pumice particles degrade by fracturing into smaller pieces and not providing the needed impact to roughen the surface. Aluminum oxide particles become rounded with age and will peen the surface instead of roughening it. Both conditions lead to a less than optimum solder mask adhesion.
Chemical cleaning, usually in the form of a microetch, may be used to prepare the copper surface. Not all microetch chemistries will provide the proper tooth for best adhesion, so a
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