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24 DESIGN007 MAGAZINE I JANUARY 2021 Holden: With a block or a table, what are the reference layers for the particular signal there, if they've chosen to make it a layer that's not adjacent, which is much of the time? Ellis: Oh, that's a good point. Holden: A lot of times, they do. They've got to tell us it's not the adjacent layer. Dang: And whether the trace is coated or uncoated by solder mask. Ellis: That's a really good one, Happy, because a lot of times, we just hear, "We want 50, 100 or 120 ohms on all layers," and once we get into fine dielectrics, 120 ohms is really hard to hit because the lines can be so small that they may be impossible to etch. Holden: Nowadays, you have to tell people not just the solder mask, but are you going to put anything on top of solder mask, like thermal grease or conformal coating, because if you've got micro strips, the fields will go up into those materials. You can't assume that it's air. Ellis: That's interesting. Our simulators only cover for coated microstrip, which would be solder mask. I don't think we have simulators that include conformal coat. Dang: We don't. Shaughnessy: Julie, how much of what you do involves making life easier for the assembly guys? Ellis: Since I have an assembly background, I always include that, too, because another issue is the dimensions of the bare individual printed circuit board versus how it's going to convey down a surface mount assembly line. It usually needs a minimum 5 mm, or 0.2", of clearance with no components on both of the long sides of the part. No assembler is going to want to be hand loading 2" x 2" small parts or putting it into a fixture to run it down the volume production assembly line. So with my customers, a lot of times if it's a smaller part, we try to plan the multiple-up array with rails that is optimized on our fabrication panel and delivered to the assembler. The other thing with my regular custom- ers is they also know the standard panel size is normally 18" x 24", and we need an aver- age of 0.7" to 1" all the way around for our tooling. So just on average, we're looking at a 16" x 22" fabrication panel. Sometimes a really big power board will come in, but they've got a little bit of room to play with dimensions, so we will squeeze down to the last 0.01". We may adjust one side, shorten one side and adjust the other dimension, to achieve two-up, instead of only one, on a fabrication panel, where we've squeezed every inch that we can. For volume production, especially for big boards, material optimization is the lowest hanging fruit that can save the most money by planning ahead. The best circuit board designers select components that aren't so small they drive advanced fabrication requirements, take into account design rules for both the fabricator and assembler, work with both on the array with rails to achieve best material utilization with the least amount of assembly loading time, and provide accessible test points for the final assembly. Shaughnessy: Do you have any advice for a designer or engineer having issues with stackup? Ellis: Find a trusted fabricator and work with them, initially, to develop the stackup with the design rules for any project that will set the circuit board design precedence for a complicated, high-reliability, or production volume level of product. If it's a really simple four-layer stackup that doesn't have any con- trolled impedances or special requirements like high aspect ratio, then engineers can pretty much use their own software to create it. But if there are any higher requirements, or controlled impedances, please work with your fabricator.

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