PCB and Heavy Copper: Design Considerations

What could be the first question you’d ask a PCB manufacturer about its capabilities? One would be the minimum line width.

However, only a few would be able to tell you that it all depends on the copper weight. In order to make the design process simple and ward away complications during the manufacturing phase, here is a brief demonstration of how and why the width of the line depends on copper’s mass.

Furthermore, you will find some almost-universal rules for trace width vs. copper weight, alongside other vital contemplations when using heavy copper.

Copper Weight and Trace Width

Throughout the etching process, copper traces are merely secluded from the top by a dry film or a tin plate. This means that as the copper is etched, the trace sides are also etched. It wouldn’t be wrong to say that the process of etching is isotropic.

As a result, the thick copper necessitates broader trace and space, and the traces are finished in a trapezoidal shape. Stated below are some guidelines for trace width vs. copper weight:

Preparatory Copper	Minimum Trace	Minimum Space
0.5oz	0.003″	0.003″
1oz	0.005″	0.005″
2oz	0.008″	0.008″
3oz	0.012″	0.012″
4oz	0.016″	0.016″

What would be the case if you need a tighter trace and space? In general, you can replicate the layer that needs heavy copper, and then divide the copper weight in halves. So, for instance, if you require 16 mil lines and 8 ounce copper, reproducing the layer and using 4oz copper would be the ideal choice.

Considerations of Heavy Copper

Basically, there are 2 added contemplations to heavy copper typically on the external layers. Foremost and fundamental is the soldermask.

When making use of liquid soldermask, several coats are needed to sufficiently guard the heavy copper traces. However, this issue is alleviated using 3D printed soldermask. Nonetheless, this technology is rarely being used and it is available in limited colors. 

Secondly, the surface mount pads often get jeopardized with heavy copper. While the Gerber file spells out the trace width from the bottom of the trace, the SMT takes place on the top. With heavy copper, chances are bright that the top of the SMT pad may turn out to be a number of mils thinner than planned, causing difficulties in placement and producing a debilitated solder joint.

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