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46 The PCB Design Magazine • May 2014 For many years, the demand for polyimide and PTFE materials was very low, and very spe- cialized. Demand for polyimides was confined almost exclusively to high-temperature or high- voltage applications and in the case of the PTFE laminates, such as the Rogers duroids, micro- wave circuits. Designers are increasingly turning to those laminates, perhaps for good reasons based on the thermal or electrical attributes listed in data sheets, but without regard to certain man- ufacturing characteristics that set them apart from the fabrication processes for conventional FR-4. Many of those manufacturing characteris- tics are not apparent from data sheets. Wise designers consult manufacturers before developing hybrid stackups, because combining laminates with dissimilar mechanical properties can complicate fabrica- tion and therefore, bear on cost, especially with respect to yield. By the time a design is ready to prototype, it's often too late and too expen- sive to recast in a way that would achieve the design objectives and yet be easier to build. My company, which is devoted to prototype manufacture and up to medium scale produc- tion, often takes on challenging projects that might have been better architected had the designers reached out to us at the stackup stage. Not Covered in Data Sheets Let me focus on one aspect of hybrid builds brought to mind by a recent conversation with the engineer who supervises drilling operations. "The problem is," he emphasized, "there can be very different feed and speed requirements for drilling one material compared to another." Several designs we recently built involved three, four, even five different materials. For example, we had a project with two different polyimide materials: FR-4, and flex material, combined. The in-feed setting for the drills (how quick- ly they descend) and their spindle speed for drilling the polyimides is completely different than the in-feed and spindle speed for the flex material. Polyimide laminates are hard materi- als that fracture easily. Therefore, they must be drilled at a relatively low in-feed rate and a high spindle speed. Flex material is just the opposite, requiring a high in-feed rate and a slow spindle speed, because the slower the in-feed and the higher the spindle speed, the more heat that will be generated. "It's difficult when those materials are combined, because the polyimide can't be drilled using the flex parameters, or vice versa, or the boards will be compromised," the engineer pointed out. "The softer the material—the duroids and flex materials are by Amit Bahl sierrA CirCuiTs DESIGN FOR MANuFACTuRING Consult With Your Fabricator— Especially for Hybrid Designs feature column

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