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DECEMBER 2024 I DESIGN007 MAGAZINE 17 be prepared for bow and twist (physics) at assembly, as an uneven stackup will create CTE stresses during the assembly process. You may need to factor in a stiffener, special fixtures, and handling at assembly as in the case of the boards getting hot during normal function. Layer thickness: The thickness of each signal layer is a crucial factor. Again, this is not quite right. Instead, it should be: "When specific impedance is required for the signals, the layer thickness between the sig- nal and its return path becomes an important factor." Be sure to communicate all impedance needs to your fabricator in your documentation. Layer thicknesses also affect aspect ratios of blind and buried vias. Material: The material of the layers affects the dielectric constant and loss tangent. Yes, it does. ere are three parts to the material that create changes: resin, reinforcement, and metal (mostly copper). ere are many great programs for engineers and designers used to calculate these factors. Some are from the material suppliers themselves. at said, don't be too specific with your fabricator. ere are a couple of things to remember about specifying materials for your board: • Is this material that your fabricator stocks or will they need to special order it? • Have they successfully used this type of material before? • Many material specifications change slightly from batch to batch. • Will the material influence the assembly of the board? If so, in what way? Drill size and aspect ratio: The selection of drill size and aspect ratio affects the layer count, routing density, and mechanical stabil- ity of the board. is is not quite right. Aspect ratio is the drill size divided by the thickness the drill has to travel: Aspect ratio: 0.060"/0.012" = 5:1 Layer count doesn't come into play unless it makes the board thicker. Drill sizes can affect routing density, as you must go around each of the vias. Mechanical stability of the board material is affected by placing the drilled holes too close to each other. is cuts through the reinforce- ment of the material. Signal return efficiency is compromised by creating slots in the planes due to the density of the vias. Number of layers: The total number of layers required for a given design depends on the complexity of the design. Let's redefine "complexity" to "density." e following will get you an estimate of the num- ber of layers you'll need for the design: Part placement density: • Find the total area needed to place the com- ponents (CP). • Use the "courtyard area" for each compo- nent, not the body size or footprint size. • How much "useable" board area do you have on both surfaces of the board. (AB) • Remove all mounting areas, keep-out areas, and unusable tab areas. • Divide AB by CP. [AB/CP] Routing density: • How many total signal pins do you have to connect? (SP) • How many power and ground pins do you have to connect? (PP) • What size vias will you use for these? (V) • What is the smallest trace needed to carry the currents in the circuit? Calculating aspect ratio.