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FIGURE 27.33 Three different tooling schemes: (a) four-slot tooling; (b) four-slot/four-hole; (c) a tooling system with a full perimeter of holes placed.
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Therefore, careful analysis should be given to the product mix and technology types to be manufactured prior to choosing a scheme. The primary purpose of the tooling system in ML-PWB manufacturing is to facilitate layer-to-layer alignment during the lamination cycle while maintaining a positional reference for subsequent processing. The tooling hole arrangement is mirrored within the front-end tooling routines mentioned earlier. The master alignment of each part keys off the tooling scheme. The primary tooling locations may be used at all process steps or sometimes used to introduce secondary reference holes. Figure 27.33 shows three different tooling schemes. Figure 27.33a shows a system known as four-slot tooling. This is an excellent system, because the use of four slots allows for growth and shrinkage of the layers during processing. The four slots may be used for printing, inspecting, and lamination, or, if post-etch punch is used, they are used for inspection and lamination only. Figure 27.33b shows a system known as four-slot/four-hole. This is common in a prepunched system where the slots are used for image printing and the extra holes are engaged at lamination. Alternatively, punched slots can be used for innerlayer image and lamination, while post-lamination holes are drilled for plated holes drilling and outerlayer imaging. In this method, targets imaged in the details under the outerlayers are exposed by controlled depth boring; a camera then locates the targets and a spindle drills the locating holes for the secondary operations. Figure 27.33c shows a tooling system with a full perimeter of holes placed. This style would be considered overdetermined. It has a disadvantage at lamination because imaged panels typically have to be stretched over the alignment pins. A fourth tooling system not shown uses low-profile rivets to align layers. The rivets can be arranged outside the pattern area and do not penetrate into the caul plate. Rivet height is chosen based on the appropriate panel thickness.
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Imaging 27.4.7.1 Surface Preparation. The first step in the imaging operation is surface preparation to enhance photoresist adhesion. Double-treated foils (DTFs) and foils laminated with the tooth side up (RTFs) require minimum surface preparation. Generally, a process through a tacky roller machine to remove dust and foreign material is adequate. For cores with standard foil with the shiny side up (non-tooth or drum side), a more aggressive treatment is needed. Common options include a conveyorized chemical cleaning and/or an abrasive
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cleaning.Automated equipment is available to perform this function with pumice or aluminum oxide slurry. Scrubbing the layer by hand is acceptable but labor-intensive, and can provide inconsistent results. However, acidic 4F grit pumice is required for single-step cleaning. Standard copper foils are coated with a chromate conversion to eliminate formation of heavy copper oxidation during storage. This coating, and any oxides, must be removed to provide a microroughening of the surface. When using any of the mechanical cleaning methods, care must be taken to avoid mechanical deformation (stretching) of the thin laminates. Stresses induced will be relieved after etching of the image, causing positional movement. 27.4.7.2 Photoresist. Photoresist is supplied in both dry-film and liquid forms. Dry films are a popular choice because of the simplicity of application. For buried via innerlayers, dry films are preferred because liquids are difficult to use with through holes. The major weakness of dry-film resists is sensitivity to surface flaws and a tendency to lift on poorly prepared surfaces. 27.4.7.3 Dry Film. Resist film is typically 1.5 to 2.0 mil. thick. Both thinner and thicker resist is available as a specialty product. Dry-film resist is applied with a hot roll laminator. To aid in production of fine lines and spaces, the use of a wet nip applicator aids in filling surface imperfections. Precision flatness and uniform temperature are equally important as well. Popular resists for high-density applications include films formulated with high-speed sensitivity for laser technology. 27.4.7.4 Liquid Photoresist. Liquid resists work well in a print-and-etch process. They have excellent adhesion and a tolerance for surface flaws and are relatively low in cost. The disadvantage of a liquid photoresist is the need to produce a perfect coating. Foreign material, skips, thin spots, and dewetting all cause serious image problems. The use of a wet coating can also cause problems due to resist contamination of the transport system. 27.4.7.4.1 Roller Coating. The least expensive and most popular way to coat liquid resists is with a pair of roller coaters consisting of pinch rollers in which one or both rollers is used for coating. The coating roller has a closely spaced, precisely cut spiral grove. Liquid is metered onto this roller and then transferred to the panel. A good roller coater produces an extremely uniform coating. By carefully selecting coater parameters, it is possible to achieve a coating thickness control of 0.1 mil. Some roller coaters use the top roller as a coating roller, allowing the panel to be transported on a clean conveyor system. In these systems, the resist can be dried or the panel can be printed wet. Other roller coaters use both the top and bottom rollers for coating. These roller coaters require a handling system that holds the panel by its edge until the resist is dry. The coating roller is the weak link in a roller coating. Alignment problems result in nonuniform coatings. Flaws produce repeating defects in the coating. Worn or improperly cut grooves result in a low-quality coating. 27.4.7.4.2 Curtain Coating. The curtain coater operates by pumping a waterfall of liquid through a narrow slot. By carefully controlling the slot width, pumping pressure, and viscosity, the fabricator can create a well-controlled curtain of liquid. When a panel moves through the curtain, a thin coating is applied to one side. Curtain coaters produce a good-quality coating, but not all photoresists have the proper viscosity for curtain coating. 27.4.7.4.3 Electrodeposition. With electrodeposition, a polymer is deposited on a biased metal surface submerged in a liquid medium by a process that is analogous to plating. It gives a well-controlled, high-quality coating. Both curtain coating and electrodeposition are expensive and are not widely used for innerlayer processing. 27.4.7.5 Photo Print Exposure. The third step in imaging is photo printing. The three types of photo-printing machines are flood, collimation, and direct image. Most flood printers consist of high-UV (5,000 W) light sources housed within large reflectors to distribute the light uniformly across the imaged surface. Hard contact by means of a vacuum is required between the film and the innerlayer. The silver emulsion side of the polyester film tool makes direct contact with the resist. This allows good image reproduction without collimated light. The drawbacks of contact printing are defects associated with poor contact, poor productivity due to the time required to establish hard contact, and the potential for artwork damage.
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