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60 FLEX007 MAGAZINE I JANUARY 2019 the incidence of foreign objects getting under the coverlayer, which would result in scrap (Figure 2). Laminating the Coverlayer and Bondply When the flexible layers are complete with the pieces of coverlayer and bondply in place, the flexible panels are placed in a "lamination book" with a release film and steel separators. The materials are then laminated at tempera- ture under pressure and a vacuum to press and adhere the coverlayer and bondply to the flex layers. This creates the cut-back coverlayer fea- ture into your design for high-reliability pack- aging (IPC 2223, 5.2.2.2). The lamination cycle is typically more con- servative than for hardboards and/or flexible circuits as the temperature and thermal ramps are designed to reduce dimensional movement as much as possible. The layup folks in lami- nation will typically use materials that con- form easily and help to drive the coverlayer and bondply into the circuits to create intimate contact and complete encapsulation. Fabricate Glass-reinforced Laminate Layers The hardboard layers run in parallel with the flexible layers and follow much the same processing as you would see in rigid-board manufacturing. Key differences are that, for consistency, we use chemical cleaning here as well. Using a chemical clean line on the flex layers and a scrubber on the glass-reinforced layers would just induce more issues with mis- registration of the dissimilar layers. Another key difference is that, after imaging and etch- ing the circuitry, the windows in the layers are routed out, corresponding to the flexible areas of your board. Similar to the no-flow prepreg, designs that are very small or have a lot of flex arms have a significant amount of the mate- rial removed, which can make them difficult to register because they have lost much of the material that comprises their support. Final Lamination All of the components—flexible layers, glass- reinforced layers, and prepreg—are kitted and ready for final lamination. This applies to a straightforward rigid-flex process sequence, but other designs that are more complicated will be covered in a future column. The layers are then layed up in the lamina- tion book again, with the no-flow prepreg, in the order of your layer stackup. At this stage, we are laminating a board that is 0.040", 0.062", 0.093", etc., and now eight, 10, or 20 layers deep. The conformal materials become even more essential for getting the resin to flow enough to encapsulate the circuits in the whole book. Often, there will be so much extra conformal material in each opening that only one panel per press opening can be lami- nated. Hardboard manufacturers will typically get eight to 12 panels per opening; thus, our capacity can be significantly reduced per press load. Again, the lamination process cycle is very conservative and is meant to get the no- flow prepreg to flow enough to encapsulate the circuits without flowing out onto the flexible areas of the board. There are three things to know about lami- nation and no-flow prepregs. The first is that the prepreg is designed to not flow, which can make encapsulating thicker copper circuitry and planes more challenging. Half-, one-, and two-ounce copper layers are usually fine; how- ever, with the latter, more prepreg may need to be added to encapsulate the two-ounce copper circuitry fully. Three-ounce and greater copper thicknesses can be very difficult to encapsulate Figure 2: Cleanroom used for coverlayer and bondply application.