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48 The PCB Design Magazine • May 2014 about the same—the easier it is to drill, but the more susceptible it is to heat. So if you also have a harder material in combination to drill, it's easy to distort the softer material if you go in at too high an in-feed. The drill can essentially pull the material out of the hole wall and then the material snaps back, but not all the way, leaving what appears to be negative etchback in the hole wall of the softer mate- rial." What we are doing in such rigid-flex cases is "peck" drill- ing. We control the machines to drill a just certain distance and then withdraw the drills to let them cool, and then drill further. We are guided by the drilling characteristics of the most-sensitive materials. Unlike the duroids, Rog- ers 4000 materials have drill- ing characteristics that are relatively close to FR-4. Some defects may result because the 4000 materials must be drilled more slowly and gen- erate more heat. There is a tendency for the intercon- nects to smear a little bit in the holes—so-called nailheading— though that's typically not cause for rejecting a board. Slowing down the feed rate for a given material in a stack runs the risk of causing some defects in the region of the hole where that material is located. Polyimides and the ceramic-filled materials have slower in-feed rates and higher spindle speeds because they are hard, and therefore less material is removed per revolution of the drill. The combination of feed rate and drill speed is sometimes referred to as chip load. A harder material necessitates a lower chip load; that is, a lower feed rate and a higher spindle speed. When you have a combi- nation of materials, you have to adjust the drill- ing parameters to meet the requirements for the hardest material, or distortion inside the hole can result and that can interfere with plating the hole. But when you have two materials with very different drilling parameters, the settings are a compromise. For manufacturers, there's yet another con- sideration besides chip load and that's drill hit count; the sharper the drill, the fewer the issues that will be encountered. For FR-4 the drill hit count typically is around 800, but for a hard material, drills have to be changed after 400 or so hits, and that affects project cost. Still another consideration internal to man- ufacturers is which of three entry materials will be used on top of the board stack for drill- ing support. There's a coated aluminum material that's best for drilling small holes; there's an aluminum material with a paper core that's used for most other drilling needs; and there's phenolic mate- rial, which provides the most surface support and would usually be used when a soft material, such as a Rogers duroid, is the top layer of a board. The phenolic material is the worst of the three entry materials for drilling accu- racy; it is hard and drills can skate when they start. If the drill diameters involved are not less than 10 mils, accuracy is not compromised by the phenolic material. If the drill diameters are much smaller, the coat- ed aluminum material must be used or the drills will snap. However, in most cases, the amount of burring, the debris left in small holes as a re- sult of using the coated aluminum material,l is negligible. There is considerable work among laminate suppliers to provide alternatives to polyimide and PTFE materials whose manufacturing char- acteristics are closer to those of FR-4. Switching Gears Let me turn to a different design consider- ation that influences manufacturing and cost as well as electrical and mechanical performance: surface finish. For example, if HASL (hot-air sol- der leveling) is selected, the PCB design must not include any fine-pitch components because a HASL surface finish will be to too uneven to design for manufacturing When you have a combination of materials, you have to adjust the drilling parameters to meet the requirements for the hardest material, or distortion inside the hole can result and that can interfere with plating the hole. " " CONSuLT WITH YOuR FABRICATOR—ESPECIALLY FOR HYBRID DESIGNS continues

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