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66 DESIGN007 MAGAZINE I MAY 2022 It's up to the PCB designer to request a stackup with impedance values and HDI rules if the board is higher up on the high-tech scale. None of us want to waste time. If the board is a common low-risk build, the build and rework risk is low if you have incomplete data about who will build the board. When we start using exotic materials and sequential laminations with varied copper weights and high layer counts, it would be foolish not to involve the PCB vendor on the design start. Dense designs require going right up to the edge of the rules without breaking them. To do that, you need to review the process and your expectations with the vendor to make sure they can build what you design. To do other- wise could put hundreds of hours of work at risk for rework, and no one wants that. Shaughnessy: What other missing information or data do you find yourself having to "design around"? Schattke: Where do we start with this question? I would say that the need to get something out the door ASAP drives many designs to start in a state of partial completion. e primary risk of starting PCB layout too early is that the optimal result may not be obtained. Common missing items include incomplete schematics, or incomplete sections of the schematics, and missing current information needed for calcu- lating trace widths. Other missing information includes voltage clearance rules from one class of nets to another, and missing nomenclature requirements like part numbers, assembly numbers, test marking, and any other special text. Board finish and copper weight are oen not provided, and the designer will decide what is best then. e environment the product will be used in is very important for the board design, and this is oen missing. Mechanical outline drawings are oen incom- plete and follow-up details have to be requested. Most oen missed are hole and connec- tor locations, dimensions, board thickness, NPTH/PTH, screw head clearances, height keep-outs, pin 1 locations, connector number- ing order. e quantity of the initial and life- time build. is will influence the decision to have ICT pads, or place only ICT pads on one side of the board. Which pins can be swapped? Oen, some simple schematic changes can greatly reduce board routing challenges. Shaughnessy: When you find yourself design- ing in a vacuum, what tricks have you devised as workarounds to get the job done? Schattke: If you are designing on this earth, you are not in a vacuum unless you let one be drawn upon you. You have to break out of your container with phone, email, or instant mes- sage, and talk to the stakeholders and find out what they want. Seldom are they so busy that they don't mind something being done wrong that they will have to live with later. I find it really valuable to go over each page of the schematics with the electrical engineer. When I do this, I want to understand the cir- cuit, what it does, what's critical about it (or not), what kind of current is involved, how much heat will certain parts need to dissipate, Carl Schattke

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