Ounce (OZ) is a unit of weight. Why is it used to express thickness in PCB?

The first thing to note is that the ounce (OZ) itself is a unit of weight. The conversion formula between ounces and grams (g) is: 1OZ ≈28.35g.

In the PCB industry, 1OZ means the thickness of copper weighing 1OZ evenly spread over an area of 1 square foot (FT2). It expresses the average thickness of copper foil in terms of weight per unit area. Expressed by the formula, 1OZ=28.35g/FT2 (FT2 is square feet, 1 square foot=0.09290304 square meters).

First of all, we know that the density constant of copper and the related unit conversion formula are as follows:

  • The density of copper is ρ=8.9g/cm3
  • 1 centimeter (cm) = 10 millimeters (mm)
  • 1 millimeter (mm) = 1000 microns (um)
  • 1mil≈25.4um
  • 1FT2≈929.0304cm2
  • 1mil≈25.4um

According to the mass calculation formula m = ρ × V (volume) = ρ × S (area) × t (thickness), we know that the weight of the copper foil divided by the density and area of the copper is the thickness of the copper foil.

We also know from the previous article that 1OZ=t×929.0304cm2×8.9g/ cm3=28.35g

Therefore, t=28.35÷929.0304÷8.9cm≈0.0034287cm


≈34.287÷25.4mil≈1.35mil. It can be seen that the thickness of 1OZ copper foil is about 35um or 1.35mil.

  • Copper thickness 1 OZ (0.035mm)
  • Copper thickness 1.5 OZ (0.05mm)
  • Copper thickness 2 OZ (0.07mm)

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