Weight is still used as a determinant for copper thickness. Why?

Sometimes my columns tie to issues or stackups that appear in my inbox each week. I’m occasionally asked why 0.6 mils (15µm) is often used for the thickness of 0.5-oz. copper, rather than 0.7 mils (18µm), and similarly why 1.2 mils (30µm) is often used for 1-oz. copper instead of 1.4 mils (36µm). If you’re curious about the details, or if none of these numbers seems familiar, here’s a quick primer. The thickness parameter “t” in FIGURE 1 shows the thickness we’re interested in here.

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