Issue link: https://iconnect007.uberflip.com/i/1500943
32 DESIGN007 MAGAZINE I JUNE 2023 feature can stray from perfection and still be form, fit, and functionally acceptable. For example, in a PCB outline, the absolute CAD data may reflect a horizontal (X direction) value of 6.00937". But there is not a machine, material, or process in our universe which can form a glass-epoxy laminate board to such a degree of accuracy. When a PCB designer's data shows a coordinate of 6.00937" and a run of parts are cut measuring 6.006", 6.012", 6.013" and 6.014", must they all be scrapped? Who says, and by what criteria? Manufac- turing stakeholders must be given a range of dimensional acceptability because the yield of their processes vary. Setting Your DRC Within Legal Limits Drive down a country road and you're bound to see a speed limit sign. How do you process this and adjust your driving speed? Whatever your process, you are considering a driving constraint and making the first attempt to hit the target condition for a rate of speed. If the posted limit is 60 mph, most drivers will adjust their car's rate of speed right up to the limit. Why? Is it human nature to set things right to the edge? Do we think—erroneously—that by setting a complex system to a particular value the result will be exactly 60 mph? It will not. "Exact" is subjective to the context within which the subject must inter-relate with other parts of the system. is is especially true when it comes to the complex systems applied in vol- ume production. e manufacturing system which produced the car's speedometer and cruise control mod- ule is subject to a "stackup" of manufacturing process tolerances. As the modules are assem- bled, they must be calibrated for accuracy due to the differing manufacturing variations. To compound the challenge of system accuracy, these modules are then installed into automo- bile systems which have another set of system variables the manufacturer cannot foresee. e variables of heavy, off-road tire diameter, road grades, and differing temperatures will chal- lenge a cruise control system's ability to achieve any kind of consistent perfection. According to the NPR program "Car Talk," cruise control systems can vary from the set value by a fac- tor of ±10%. So, even if you set the cruise con- trol at 60 mph, the variables in play could send the car whizzing by a county sheriff at 66 mph. You may soon be having a discussion with the local gendarme about accounting for tolerance when setting the cruise control. Should we all set our cruise controls to 54.5 mph to allow for a system tolerance of ±10%? DRC Settings Must Allow for Manufacturing System Tolerances Generally speaking, PCB designers manu- ally adjust their CAD tool's DRC settings like a driver sets a car's cruise control. Many inex- perienced designers set DRC values to the "reduced manufacturability" Class 3 limits sup- plied by local prototype suppliers. If a hot-shot prototype supplier can provide .003" finished traces and spaces, designers are somehow inclined to set the DRC for .003" traces and Figure 2: Board edge mouse bite.