The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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64 Just about everything pertaining to copper in a PCB design is controllable in the design rules portion of a PCB layout tool. The pre-set design rule values or "default settings" are set by a layout tool provider who knows nothing about the new layout a designer may be working on. There can be hundreds, if not thousands, of control variants in the tool's default settings which can cause catastrophic manufacturing conflicts if not re-set to match the requirements of a new project. Once the connectivity of the layout is synchronized with the schematic, it is essential for a designer to surmise which general design rule constraints will best control the new design and reset them for the required result. For instance, if not re-set, a pre-set copper plane clearance value of .005 [0.127mm] will yield an un-manufacturable etching condition if a 2oz base copper thickness requirement has been setup in the stack-up. When the thickness of copper used in a stack-up is increased, the copper clearances must be increased accordingly. Unfortunately, PCB software does not check for this condition, but it will adjust for the condition. The PCB design engineer must control the copper clearance setting to ensure there will be enough artwork clearance for the supplier to etch enough copper clearance. Another negative result of failing to reset default values might involve noisy clock lines. If a clock line is not identified as an aggressor and is treated with the same 5mil default spacing constraint rules as other lines, there is a risk of the line being packed too closely to other lines during routing. To prevent this, a designer can specify special spacing constraints for aggressive or sensitive lines and add them to a design "class." A design constraint class may be assigned many unique design attributes, including wider spacing constraints. If the class spacing is set to 20mils, adding a clock line into the class will impose the new spacing of 20mils onto the line and the design rules checking process will audit for the condition. In general, there are five basic areas which will need to be configured to match the design you are working on. Four of the five category settings directly affect manufacturability: copper plane clearance, part outline clearance, drill (hole) clearance, legend (markings) clearance, and trace length clearance. The last setting category affects performance: trace length and clearance. It will likely take some time for a new designer to learn how to control all the design rule settings. However, concentrating on these five categories first should help. Click to Enlarge

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