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subcore that further ratchets up pricing, as they are also time-intensive operations.) The lamination process involves two distinct yet linked operations: lay-up and laminating. 27.5.1 Lay-Up and Materials Lay-up occurs in a clean and controlled room environment. The level of environmental control depends on the circuit feature technology being fabricated. The lay-up process for standard multilayer processing is relatively forgiving. However, working with material that is hygroscopic will require additional measures. The innerlayer details require a bake cycle (typically 120 C for 1 hour minimum) to remove moisture. A reduced bake cycle duration is indicated when certain oxide reduction chemistries are employed. Once layers are readied for lay-up, they should be processed as soon as possible. Should extended hold times become necessary, storage in a nitrogen-purged dry box is recommended. The lay-up operation is often referred to as building up a book. The operation follows a guide, often referred to as a stack-up sheet. The guide sheet, which depicts the engineering design of the ML-PWB, is highly recommended to minimize error. The written and illustrated guide to the book build-up process is made a part of the job planning/traveler. Due to the complexity of some products, the operator follows this guide in a recipe-type manner to perform the systematic construction. The lay-up may consist of some four or more PWB material components or other subassemblies. The lay-up operation produces a large assembly when complete that consists of the tooling plates, consumable press materials, and the ML-PWB detail. Refer to Fig. 27.34 and 27.35 for illustrations of the following.
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FIGURE 27.34
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Typical ML-PWB stack-up for hydraulic lamination.
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FIGURE 27.35
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Press stack-up using release sheets and a separator plate.
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27.5.1.1 Tooling Plates. The outermost item in the stack is termed a caul plate or carrier plate. These are thick, oversized metal plates, generally made of 3/8 in. (9.525 mm/0.375 in.) thick steel. The 4130 alloy of steel is often chosen due to its precision machining capability for placement of the holing holes. Sometimes a hardened aluminum alloy is chosen, but is not highly recommended due to its high in-plane expansion. The purpose of the caul plate is to provide a stable base to transport the ML-PWB stack. 27.5.1.2 Separator Plates. The multilayer boards in the stack are isolated from each other by metal separator plates. The separator plates provide a mold surface for the laminated MLPWB. It is extremely important that the separator plates be clean and free of debris. Both aluminum and steel separator plates are used. Separator plates should be cleaned regularly. The surface finish of the plates becomes very critical when laminating against microfoils that will be printed with fine lines. The most common type of steel is one of the 400 series stainless steels that have a very durable surface. Hardened 300 series steels are also occasionally used. Separator plate thicknesses range from as thin as 0.015 in. to as thick as 0.062 in. The thicker plates are more rigid and resist the tendency of internal layer features to print through from one ML-PWB to another.Thin aluminum plates have the advantage of being disposable, eliminating the need for cleaning. 27.5.1.3 Consumable Press Materials. The purposes of consumable press materials placed between the tooling plate and the top layer are to ensure that pressure is applied uniformly and to provide thermal lagging to correct heat rise.The purposes of consumable release sheets placed in contact with the foils are to protect the outerlayer foils, keeping them resin-free, and to allow easy separation of the foil from the separator sheet.
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Among the materials used to provide for pressure and temperature uniformity are multiple sheets of Kraft paper, silicone rubber pads, expanded mat paper, and composite board. Kraft paper and composite board have the advantage of low cost, but produce an odor that some press operators find objectionable. Silicone rubber pads have the advantage of being reusable, but are limited by the number of heat cycles they can withstand. Silicone rubber pads, when close to the end of their life, go through reversion and leach out silicone oil, which can become a source of contamination. They also have limited success in controlling edge voids. Excellent results are reported with expanded paper mat products that are commercially available. Mat paper press pad materials come in different thicknesses. Some are even produced with a sandwich layer of release material cohered. Other products exist that flow and thus act as a stop for resin during lamination of subcores with predrilled blind vias. Some form of release material is required against the outer foil surface of the ML-PWB.The release material acts as a nonstick slip sheet to keep the copper surface smooth, minimize the plate cleanup, and catch the resin run-out. Figure 27.35 shows the location of the release sheets. Alternatively, oversized foil can be used to catch the resin run-out. Or, a product called C-A-C (for copper-aluminum-copper) is used that consists of two sheets of foil laminated to either side of an aluminum carrier sheet. In this configuration, the one sheet of C-A-C is between two PCBs with the bottom copper foil over prepreg to form the top side of the bottom PCB, and the top copper foil under prepreg to form the bottom side of the top PCB. Figure 27.36 shows the location of the C-A-C sheet.The aluminum serves to replace the steel or aluminum separator plates and has the added benefits of reducing the stack height (which can sometimes translate to increased productivity by enabling another board to be made in the book ) and providing excellent protection to the surface of the outerlayer foils. Since they are not handled as foils but as a more rigid composite, there is no propensity to wrinkle, and since they are covered for the entire book-building operation, they are not subject to scratching or having errant debris introduction cause a pit during lamination. Finally, when the book is broken down, the aluminum between the ML-PWBs is easily separated from the foils, leaving the distinct ML-PWBs. The aluminum itself is sometimes sent to the drilling department to be used as an entry material, and then sent to reclamation. 27.5.2 Lamination Stack-Up The lamination process is key to building a reliable ML-PWB. In the lamination process, the board is subjected to heat and pressure that melts the B stage (bonding sheets) and causes it to flow.This encapsulates the circuits and fills any buried vias.The B stage then cures, establishing a good mechanical bond to the inner detail layers.A variety of materials can be utilized in standard laminating press cycles. (Refer to the preceding section 27.3.5 for a detailed discussion of the materials and prefill process.) A typical ML-PWB stack-up is shown in Fig. 27.35. For productivity reasons, multiple stack heights are repeated within each opening of the press.The various components and their placement within the book are shown. A standard lamination uses tooling pins that go from caul plate to caul plate, passing through each board and all of the separator sheets. Since the CTE of stainless steel roughly matches the in-plane CTE of a multilayer board, a tight fit to the pin is possible.Aluminum has a much higher CTE, so if aluminum is used, a loose fit to the tooling pin is required. Some fabricators use as few as four tooling pins. Others may use 20 or more. In addition to easy stacking, the four-slotted pin system minimizes problems arising from material growth and shrinkage in an overdetermined tooling system. On the other hand, users of systems with a large number of pins believe that firmly anchoring the ML-PWB to stainless steel obtains better dimensional stability. 27.5.3 Lamination Breakdown Once the ML-PWB is fully cured and then cooled to room temperature per the manufacturer s guidelines, the book is taken apart; this process is called breakdown. This is usually a
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