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42 DESIGN007 MAGAZINE I JANUARY 2021 ing soon to help offset issues with these small parts. (Very exciting stuff! More on this later.) Number of pins: After you have the number of components, you'll need to know how many total pins you need to connect. This, too, can be put into the component attributes and extracted easily from a database. Signal vs. power pins: You can subtract the number of power pins from the total pins to get the signal pins. When calculating the number of layers below, you'll just use the signal pins. Return planes: Don't forget the return planes. All signals must have a return somewhere; the best practice is to place your return directly under the signal. Board space: This is the size and shape of your board. Take out any keep-outs, edge relief, and areas around mounting holes. Include both surface areas. Avoid having any small "penin- sulas" on the board edges if you can. These should not be counted as usable space. If the board is too large or too small you can have placement issues. Too large and you'll probably waste a lot of space. Too small and the boards become difficult to route and place parts. Anything under 1.5" in any direction will require some attention. Also, odd board shapes can increase layer count due to limited routing space. The size and shape of the board can influence the type of layer structure that will work easiest. Trace and space: This is key. Your routing takes up room. Getting those connections made in the fewest number of layers really depends on your trace width and space width. You'll need to use an average of the largest and smallest trace sizes and the spaces between them that you're going to use. Or better yet, utilize a root mean square (RMS) of the differ- ent sizes you'll use. (Don't forget the spaces.) Via area: How this affects your layers is going to depend on HDI. If you need to have blind and buried vias, you must add those separately depending on what you choose to use. (Just blind or blind and buried.) If you use through vias, then you will need to add them into the space used on all layers. Use the diameter of the [via pad] x [number of pins]. This may be a bit more than what you will need, but it will help allow for the area for test points. Cost: This can limit the number and style of layers you can use. If cost is an issue, you've got to be creative and find ways to reduce your layer and via structures. Just a hint: HDI designs will usually not be in the cheap $10- $50 per board category unless you're ordering them in the 10K+ ranges. Figure 2: Example of a stackup for a simple PCB.

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