Copper weight VS Line/Space

Copper Weight and Trace Width

During etching, copper traces are only protected from the top by either a dry film or a tin plate, meaning that as copper is etched away, the sides of the trace are also etched (in other words, etching is an isotropic process). There are two main results of the isotropic etching – first, thick copper requires wider trace and space, and second, traces finish with a trapezoidal shape.

What if you need tighter trace and space? Typically, you can duplicate the layer account that requires heavy copper, then cut the copper weight in half. So if you need 2layer and 4 ounce copper, duplicating the layer account to 4layer and using 2oz copper is the best alternative.

For starting copper weights of 5oz or greater, we also recommend doubling a layer as opposed to using thicker copper. The cost and processing difficulty of the thicker copper means that adding layers is cheaper than using the thick copper. In other words, a 2-layer, 6oz copper board is usually more expensive than a 4-layer, 3oz copper board.

Minimum line width is one of the first questions asked of a PCB manufacturer capabilities, but the answer rarely includes “depending on copper weight.” To simplify your design process and avoid problems at the time of manufacturing, we will demonstrate how and why line width depends on copper weight. This brief overview gives some near-universal rules for trace width versus copper weight, along with other important considerations when using heavy copper.

copper width and thickness vs current

Heavy Copper Considerations

On outer layers, there are two additional considerations to heavy copper. First and simplest is soldermask – when using liquid soldermask, multiple coats are required to adequately protect heavy copper traces. This problem is mitigated by 3D printed soldermask, but that technology is not available at every facility or in every color.

Second, surface mount pads can be compromised with heavy copper. Your Gerber file specifies the trace width at the base of the trace, but SMT happens on the top of the trace. With heavy copper, the top of the SMT pad may be several mils thinner than designed, leading to more difficult placement and potentially a weaker solder joint.