Issue link: https://iconnect007.uberflip.com/i/1500943
JUNE 2023 I DESIGN007 MAGAZINE 33 spaces. If the proto shop says they can produce within an impedance tolerance of ±2%, many times we find that same strict value stuck in the fabrication notes even though a more lenient value would perform sufficiently. Some designers think that a proto supplier's CNC drilling equipment is so accurate that annular ring sizing can be drastically reduced without concerns for breakout conditions. ey fail to understand that drilling tolerances are only part of the system PCB manufactur- ing operation and attributes which must come together in order to perform a concentric plated through-hole. ere are also printing, etching, and plating variables. ere are even material stretching, expansion and contraction factors which the supplier must allow room for. In the oen-misunderstood world of PCB prototyping, an amazing feat of fabrication success is deliverable, even when the design layout is red-lining on the upper limits of pro- totype supplier capabilities. But what many PCB designers do not under- stand is that in order to furnish the extraordi- nary, "impossible" design capabilities men- tioned in their advertising, the proto shop will have to build ridiculous amounts of extra parts to yield enough to deliver the small order. Now, it may be easier for some to realize why they are being charged $500 each for that quick-turn run of 10 tiny PCBs: $4,500 of the order went to pay for time and materials for the other 100 parts on the manufacturing panel which did not meet spec and had to be scrapped. e cost was high because the yield was so low. In fact, for a few years now, CAD tool compa- nies have picked up on this trend and have been working with PCB manufacturing companies to capture their capabilities—their speed lim- its, so to speak—in hopes of PCB shops devel- oping automated design "cruise controls" for layout in the same fashion as towns, counties and states post their speed limits. e outcomes of these endeavors have been hopeful at best. ey have been able to publish some design constraints and even automati- cally provide a quote aer running a quick, automated DFM review based upon their own unique manufacturing constraints. "Just set it and forget it" is the DRC concept being sold here. is business strategy makes a lot of sense to quick-turn prototype suppliers and CAD tool companies. ey are trying to solve the problem of their tools being used to create unmanufacturable PCBs. But guiding next-generation PCB designers to use rules uniquely achievable only by these isolated resource service arrangements seems to miss the point when a design must eventually scale to production. e real world of volume manufacturing requires more relaxed DRC set- tings for volume, cost constraints, and design material allowances. If our volume manufactur- ing stakeholders had a nickel every time they heard a customer complain, "Well, the pro- totype shop could do it," they could all retire. Unfortunately, by way of our onshore, custom prototyping capabilities, many PCB designers are groomed to expect their offshore stake- holders to machine to exact numeric values and meet exact material and stackup recipes.