Unlock lower trace and space minimums by using lower copper weights

What’s the problem? Often times, electrical engineers design boards with thin traces and narrow spacing between traces for the sake of reducing the size of the board. If these fall below a certain threshold, the board yields become low and, consequently, the boards become expensive to manufacture and/or might not be delivered on time because they have to be re-made again and again.

Why is it a problem? If the traces are a certain thinness, the chemical etching part of the manufacturing process risks eating away a section of the trace entirely, breaking continuity. If the traces are too close together, there might be a short. This could be due to either not etching away all of the copper between the traces or the plating process depositing copper that bridges the traces.

How can you fix it? Basic solution: run our design rule checks to validate that you have no traces or spaces smaller than our minimums.

Advanced solution: make it possible to use smaller trace and space minimums by modifying your stackup such that the layers with fine features have a lower copper weight (the weight — in ounces — of a given thickness of copper applied to a one square foot surface). This table describes the relationship:

You might wonder why this is. An early step of the PCB fabrication process is the application of photoresist to parts of the board that you want to leave copper on (e.g., traces, pads). When the board is run through the etch phase, copper not covered with resist is removed. The resist isn’t perfect at protecting the copper under it, though. The longer you leave the board in the etching solution, the more the solution eats away the copper under the edges of the resist. This implies that if you leave the board in etch for a longer time, you can eat away fine features entirely (e.g., create opens in thin traces). Conversely, if you leave the board in etch for a shorter time, the integrity of the features under the resist is better maintained. To clarify, these images depict a cross section of a board as it goes through the relevant steps of the fabrication process:

Copper clad board (green = dielectric e.g., FR4, red = copper)

Photoresist (blue) added to protect traces and pads from chemical etching process

A board left in etch for a short time — notice that copper under the resist is preserved

A board left in etch for a long time — notice that the copper under the resist has begun to be removed. If this goes on for too long, the copper under the resist will be completely eaten away.

If you value fine features (i.e., thin traces or narrow spacing), then, the question becomes how to minimize the amount of time your board spends in etch. The answer is lowering the copper weight (the lower the copper weight, the thinner the layer of copper that must be removed; the thinner the layer of copper that must be removed, the less time it takes to etch away).

Why would anyone use thick copper weights, then? To better conduct both heat and electricity. For example, because many LEDs are quite sensitive to heat, higher copper weights are used. Many motor control applications use high copper weights, because the they require sourcing a ton of current for the motor.

So, what should you use then? Some guidelines: “middle of the road” is 5 mil traces with 1 oz copper. Use 0.5oz copper for those crazy-dense BGAs, and 2oz copper with 7 mil traces for the LED designs. Improve yield and save a little money with 6 mil traces and 1 oz copper.

Thanks to Lorenzo Ramirez and Tom Anderson for their contributions to this article.

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