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38 DESIGN007 MAGAZINE I NOVEMBER 2024 by placing the components on the board using some additional PCB layout rules of thumb: • Keep analog and digital areas of circuitry separate from each other. Even though you've done this in your floor plan, be careful when it comes to the actual placing of the parts. Placement is an iterative pro- cess with changes constantly being made, and you could easily slip a part out of its preferred position. • Maintain separation between noisy power supply components and the digital cir- cuitry they are supplying. • Work with your mechanical team to place larger memory and processor components to help dissipate heat evenly throughout the board. • Place analog and RF components accord- ing to the signal paths designated in the schematic. A resistor that is inline between a source and a load should be placed between those pins as much as possible. • Analog and power supply circuitry should have their components placed for the most optimum routing. e traces used in this circuitry oen need to be short, direct, and wider than other traces to help reduce their inductances, resistance, and noise. • PCB parts also need to be placed to best facilitate their assembly to the board. Design for manufacturability (DFM) rules are oen treated as rules of thumb, but they are critical to the success of the design. ese rules should be confirmed with the manufacturer before starting PCB layout to ensure that the board can be built as designed. Placement Considerations for Routing Placement drives routing, but it's equally true that routing has an impact on how the parts should be placed. For this reason, com- ponents also need to be placed with the follow- ing routing considerations in mind. Here are some of those rules of thumb: • Give yourself enough room for routing large data memory buses. Remember to allow for escape routing as well. • Place parts so that sensitive digital signals do not get routed through analog circuitry or analog signals through digital circuitry. • Make sure that there is enough room for sensitive signals such as differential pairs, which will need extra spacing to other routing. • Do not let your component placement force you to route traces across voids or splits in the reference ground plane. is will ruin the signal return paths and create noise on the board. e good news is that many of these place- ment and routing rules of thumb can be set up in your CAD system as official design rules and constraints. We'll examine that next. Converting Rules of Thumb Into Design Rules and Constraints While simpler designs can sometimes be completed without a lot of automation, most PCB layouts today have too many require- ments to keep track of them all manually. For- tunately, the rules of thumb that we discussed previously for component placement and rout- ing can usually be set up as design rules and constraints in your CAD system. One example, the Allegro X PCB Editor, has a comprehen- sive design rule and constraint system built into it for this very purpose. e constraint manager provides an easy input tool for designers to legalize their rules of thumb. Starting with default values, the spreadsheet-style application allows you to set one value that will be propagated throughout, or manually copy and paste entries as desired. e constraint manager, which is accessible in both the schematic and the layout, also allows the creation of classes that groups of nets or components can be added to. Trace widths and spaces, along with other constraints, can be added to individual nets or to the net classes.