Mark Thompson's Biggest Problems With PCB Designs

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What are the top problems I see with PCB design? From where I sit now on the assembly side, one of my biggest concerns related to PCB design is the lack of uniform part markings on the Gerber or ODB++ data, specifically the way customers reference diodes. We would prefer either an “A” depicting the anode side or a “C” or “K” for the cathode side. Many customers simply use either a line or a dot, which requires us to contact them to clarify which side is the cathode and which side is the anode.

If a line is used to denote the cathode side, many times the silkscreen clip using the mask clearance by the fabricator wipes out most of the line, making it difficult or impossible to determine the location of the cathode side. Using a dot doesn’t really help; does a dot depict an anode or cathode? Besides, dots are typically used to denote Pin 1 designators. Please use an “A” for the anode side and a “C” or “K” for the cathode side.

Controlling Controlled Impedances
Another area that concerns me is controlled impedances. Don’t make assumptions about controlled impedances. Your goal as a PCB designer is to simulate impedance to within 10% of your goal. The fabricator should be able to take it the rest of the way since the fabricator knows its press parameters, press values, and effective Dk. Trust your chosen fabricator and use the numbers they provide you.

If you simply go online and check material PDFs for the purpose of establishing impedances yourself be aware that many of the generic spec sheets are for very thick dielectrics (.014" core and sometimes even .028" core) the Dk associated with the thicker dielectrics are higher which, if used for trace widths .005" and below will create a mismatch and the fabricator will come back with a line size that your space may not support.

Also, consider the speed. If the literature is showing 4.5 Dk at 1 Mhz, and you know this product will be running at higher speeds, such as 5,10 or even 20 Ghz, understand the Dk will be driven way down. You may be looking at something closer to 3.8 Dk at 5 Ghz and the mismatch will affect the impedance.

Mark Thompson is an engineering manager with Out of the Box Manufacturing and a longtime CAM expert.

This article appeared in the June 2023 issue of Design007 Magazine.