Track Width & Insulation
TRACK WIDTH & INSULATION
The trace width changes due to predictable losses during the etching process. The diagram below shows a cross sectional view of the inner and outer layers trace after etching. During the etching process, the etchant removes copper downward and laterally. The tin etch resist in the case of outer layers and the dry film etch resist in inner layers, establishes the original line width, but cannot avoid eventual undercut of this boundary. For outer layer, by virtue of the additional electroplated copper, the effective ratio of vertical versus lateral etch is approximately 1:1. For inner layers the etch ratio is approximately 2:1. This leads to trace profiles as shown in the diagrams shown below.
Copper clad weight is the most important factor in controlling trace width. Using 17µ copper clad reduces the total copper thickness etched and thereby reduces the lateral etching.
The etching process does not cause a significant change in the base line width (foot of line). The top of the line is reduced however. This is significant for electrical performance characteristics, such as impedance, since it reduces the cross sectional area and the effective (average) width of the line.
| Outer layers: Minimum nominal track width: | ||
| Minimum: Price optimum: |
0.100 mm (17 µ basic Cu.) 0.125 mm (17 µ basic Cu.) | |
| Minimum nominal insulation distance: | ||
| Minimum: Price optimum: |
0.100 mm (17 µ basic Cu.) 0.125 mm (17 µ basic Cu.) | |
| Inner layers: Minimum nominal track width: | ||
| Minimum std.: Price optimum: |
0.100 mm (35 µ basic Cu.) 0.125 mm (35 µ basic Cu.) | |
| Minimum nominal insulation distance: | ||
| Minimum std.: Price optimum: |
0.100 mm (35 µ basic Cu.) 0.125 mm (35 µ basic Cu.) | |