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40 DESIGN007 MAGAZINE I MAY 2020 Choosing Materials Knowing what you need is always a good place to start. The choices seem endless. High Tg or low Tg? Halogen-free? High speed? Controlled impedance? Is insertion loss criti- cal? Rough or smooth copper? Reliability and susceptibility to thermal cycling and thermal stress? Expected lifetime of the product? Will the PCB go in a short-lived consumer product, a satellite or undersea cable, or automotive with harsh environments and be highly-price sensitive but with a lifespan midway between mobile phone and aerospace applications? Will the PCB be laser drilled? Sequential laminated or HDI and subject to repeated press cycles? Once designed, will the board be produced in low quantities by one fabricator, or will it ramp up and be sourced from a volume fabricator in another country? Maybe the OEM will handle procurement directly or—as often happens— the procurement will be outsourced to a spe- cialist broker. What happens to the carefully crafted materials specification as the specifica- tion wings its way across the supply chain? Cost is almost always there to intervene, and getting the PCB specification correct for the specific application—neither over- or un- der-specified—is important if you wish to mi- nimise the cost for a specific application. Conversations with technical buyers often raise the following question: "When I send out a PCB specification for quotation or purchase PCBs from multiple suppliers, why is the con- struction of the stackup different from the same stack specification?" In more than one case, a technical buyer expressed frustration that the same PCB repeatedly purchased from the same supplier was realised with different material stackups. The question is, "Does this matter?" And the answer is sometimes yes, sometimes no, and sometimes maybe; however, the more the procurement team knows of the ultimate application, the better they are able to specify the PCB to an appropriate level. Looking at two extremes of PCB specification, here are some examples of the requirements, from simple to very demanding, that can be asked of a design. At the lowest end, the PCB could simply be a carrier for low-power LEDs for an indicator in machine status lights; the PCB may be wired into the machine, and the PCB fitted with screws to a backplate. Thick- ness isn't critical—neither is power handling— and the environmental stress on the board could be pretty benign, too. Likewise, the vol- ume produced may be low, so, really, all that matters is that the board be the correct X and Y dimensions, have holes in the correct place, and the correct data imaged on the appropriate layer. It might need to be lead-free and halogen- free to comply with environmental regulations and minimal specifications required. At the highest end, a large aerospace PCB will need to be built by a fabricator with all Figure 2: Mandated materials (Report: Polar Speedstack).

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