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28 DESIGN007 MAGAZINE I SEPTEMBER 2020 expertise is invaluable to produce a high-per- formance substrate. However, they are not sig- nal integrity gurus; most know very little about high-speed digital design. Why leave critical impedance selection decisions to them when the product designer has far more knowledge about the requirements of controlled imped- ance? The people who do not understand impedance control like goal-seeking because they can push a button and get a quick answer, albeit the wrong configuration in most cases. Let's take a look at the impedance variables and how they interact. 1. Trace Width There are three main considerations for trace width selection: a. Minimum manufacturable width, which is generally 4 mils. 3 mils can be achieved, but yields drop, and costs increase. b. Minimum spacing between BGA balls; in most cases, 4 mils will get you through. c. Current handling, which is not an issue for digital design. For power distribution, the trace may need to be thick to accom- modate the required current. Trace width increases as impedance decreases (Figure 1). Start with the highest impedance technology first, which is typically 50-ohm sin- gle-ended, 100-ohm differential set at 4 mils. 2. Trace (Copper) Thickness The main considerations for trace copper thickness selection (Figure 2) include: a. Copper on either side of a core material must be the same thickness. b. Copper plating (typically 1 mil) is added to outer microstrip copper foil hence traces due to the through-hole barrel plating. c. Current handling capability at low frequencies. High-frequency current flows in the outer skin of the copper. Typical values are 1 oz (1.4 mils) and ½ oz (0.7 mils). 1.4 mil is a good first choice unless you have a high layer count stackup. 3. Trace Clearance (Separation) Trace clearance determines the differential impedance: a. Differential signals that are closely coupled will operate mainly in the dif- ferential mode with some common-mode radiation from imbalances in the signals. b. If the two traces are separated enough to prevent coupling then both act as single- ended signals. A 100-ohm differential pair becomes two individual 50-ohm single-ended signals. Begin with 4-mil clearance and expand this out to where the impedance levels off (10–12 Figure 1: Impedance vs. trace width (iCD Stackup Planner). Figure 2: Impedance vs. copper thickness.