Printed Circuit Board Construction - Understanding Signal Integrity - page 45

5.4.4 What Copper Types Are Used to Form Traces and What Are Their
Characteristics?
The copper foil used on circuit boards is created by either an electrodeposition or
rolling process. The end result from either process is very pure copper foil, but the
foil's mechanical and high-frequency electrical characteristics are quite different.
Electrodeposited (ED) foils are formed on a drum rotating in a solution con-
taining copper [3]. This leaves one side of the foil smooth and the other side rough.
The rough side of these foils attach to the laminate. This foil is the most commonly
used in commercial circuit boards. Double treat foils roughen both sides. Reverse
treat foils roughen the normally smooth side of the foil. With these foils the smooth
side attaches to the laminate. These configurations are illustrated in Figure 5.6.
In contrast, rolled (also called wrought) foils are created by squeezing cop-
per sheets through closely spaced rollers. This hardens the copper and crushes the
copper crystals, making the copper smooth on both sides. In fact, rolled foils are
smoother and denser than ED foils. For this reason the bulk resistivity of rolled
foils is lower than that of ED foils [17], as is apparent in Table 5.5. In later chapters
this information is used to determine the DC resistance and AC loss of traces.
Although the majority of ridged circuit boards for the commercial market use
ED foils, rolled foils are often used in flex circuitry [10] and in those situations
where there is a large temperature swing (such as space and avionics applications)
[17]. As shown in Chapter 8, high-frequency loss depends in part on the smooth-
ness of the copper surface. For this reason, rolled foils are often used in RF work.

5.4.5 What Is Surface Roughness and What Are Typical Values?
To promote adhesion, the copper foil is mechanically roughened and various ad-
hesion, barrier, and stabilization layers are added [19]. These steps are necessary

Copper
trace

Laminate

Reverse
treat
Figure 5.6 Roughness of various copper foils, exaggerated for clarity. Typical foils are rougher on
the laminate side, while double treat is rough on both sides. Reverse treat attaches on the smooth
side, and rolled copper is smooth on both sides.

Typical Double

treat

Rolled

Table 5.5 Resistivity in μΩ -cm for
Copper Traces [18]
Weight
Ounces ( μ m) ED Rolled

1/2 1.82 1.78

1 1.78 1.74

2 1.78 1.74