When designing a PCB, reliability starts with stackup. When planning copper thickness and trace width engineers must consider current capacity and thermal rise of every signal. Too thin, and your board may overheat, have a high voltage drop or lift off the board entirely. With devices becoming smaller and voltages continuing to increase it’s more important than ever to understand PCB design rules.

What Is PCB Copper Thickness?

Put simply, PCB copper thickness is how tall the copper column is on any layer of a PCB. PCB copper thickness is typically defined in terms of “copper weight” ($oz/ft^2$).

How Much is 1 oz Copper?

The reason manufacturers define copper thickness in $oz/ft^2$ is that this is typically how fabricators buy copper foil. Think of it like grocery store meat labels. Instead of defining a steak as “¾” thick, butchers will sell by the pound.

Certain manufacturers will start with different base copper thicknesses, called the finished copper thickness. Since both Electroless copper and Electrolytic copper are added during PCB manufacturing your designer must be aware that the “1 oz PCB” may actually be 0.85 or 1.15 oz after finishing.

What Is Trace Width?

The PCB trace width is how wide the copper trace is. Unlike copper thickness, this is the primary dimension that a designer can control to increase current while limiting a specified temperature rise.

Current versus Temperature Rise:

Current flows through a trace like any other resistor. As current increases, the temperature of the trace will also increase. For a set voltage drop, we can use Ohm’s law ($P = I^2R$) to see why increasing current means more heat.

If we want to limit the value of R (resistance), we must increase the cross sectional area of the copper. Quite simply, wider copper = less resistivity. Since most PCB designers will set their stackup with a fixed height for copper thickness, the designer is usually free to increase the width of a trace to whatever is needed.

Internal vs External PCB Traces:

However, there are differences between internal and external layers when it comes to thermal dissipation:

• Internal Layers: Since internal layers are “sandwiched” by an insulating dielectric material (FR4), they do not cool as well as an external layer.
• External Layers: External traces benefit from convection and radiation into the surrounding air.

A design rule of thumb is that an internal trace must be about twice as wide as an external trace to handle the same amount of current. Combined, copper thickness and trace width define the total cross sectional area of a copper trace.

$$Area = Width_{[mil]} \times Thickness_{[mil]}$$

Design Tradeoff

Unfortunately, not every PCB has the luxury of wide traces. Maybe your routing is limited by large reference designators or you’re designing for fine-pitch components. Ball Grid Arrays require very small spacing between pads, which often limits your routing options.

What do you do if you can’t make your trace wide enough? You make it tall. Designing for heavy copper PCBs is one way to save horizontal space by using the vertical height.

When you increase copper thickness, you can route narrower traces while still maintaining the same current and thermal rise. The tradeoff is that heavy copper (2 oz, 3 oz) cannot be routed as closely together since the chemistries used to etch the copper will likely short those traces together.

To Calculate Width for PCB Trace Widths

The good news is that this isn’t a guessing game. Like most things in engineering, we have empirically derived formulas and industry standard tables to tell us what size each trace needs to be.

The Theory

First, we need to know three things:

1. Target Current (A): The maximum current you expect to flow through the trace at any time.
2. Allowed Temp. Rise (°C): How much hotter can you let the trace get compared to surroundings. (Typically 10°C or 20°C)
3. Copper Thickness oz (or mils)

If we have an assigned or pre-selected copper thickness we can calculate the required width.

Can We Use a Standard?

Yes! This is where IPC-2221 comes in handy. If you don’t know about IPC-2221 you’re likely using it right now. The graphs below are taken from IPC-2221 and plot Trace Width vs Current for a variety of temperature rises and copper thickness options.

IPC Copper Thickness and Trace Width Standards

• IPC-2221: Standard calculations.
• IPC-2152: Use for High Power / Accuracy.

The IPC-2221 standard has been updated several times since it was first drafted as MIL-STD-275. The latest version, IPC-2221B, offers more detailed guidelines and calculations for designers to follow.

And IPC-2152 is the latest, high-fidelity standard for current-carrying capacity in PCB design. It takes more variables into account, such as thermal conductivity of your substrate material.

Applications Requiring Higher Currents

Don’t let the term “Power PCB” fool you. Many electronics require large amounts of current to function, even if they are not powering a motor. Here are a few common examples:

• HMS and BMS: Battery system board require frequent high current connections to the battery cells.
• Power regulation: Large switching regulators can put out 10A or more on a single leg.
• Motor Controllers: DC and AC motor drivers typically require large input currents.
• UPS: Uninterruptable power systems need to supply large loads when the input power fails.

Heavy Copper PCBs

Heavier copper PCBs aren’t just for increasing current capacity. The additional copper also helps to pull heat away from high-power components (think: MOSFETs), dissipating heat across the entire board.

Signal Integrity PCB Design

The flip-side of heavy current traces are signal traces. Signal traces aren’t concerned with how much current they can carry, but rather how they affect signal. When designing for signal integrity, we care about controlling PCB trace resistance and ensuring that voltage drop doesn’t cause our components to fail.

What About High-Speed Signals?

Ok, high-speed signals do care about current, but indirectly. High speed signals are generally controlled by the PCB trace width and target impedance. Oftentimes high-speed signals are designed to 50 ohms single-ended or 100 ohms differential. To properly calculate these widths, your fab will need to control copper thickness to a much higher tolerance.

Increasing copper thickness will decrease the impedance of your trace (ouch), and varying copper thickness can cause serious signal integrity issues like EMI and signal reflection.

Mistakes To Avoid

• Forgetting Internal vs External: Placing wires that are only wide enough for a signal trace on a power rail.
• Not Doubling Width on Internal Layers: Not accounting for the fact that internal layers are slower to heat dissipate than external layers.
• SPECING 3 OZ WHEN 1 OZ WOULD WORK: Increasing copper thickness will increase cost and make very fine-pitch components impossible to etch.
• No Thinking About Manufacturing Tolerances: Remember most PCB manufacturers etch away copper to create your traces. This means that the finished width of your traces will be smaller than what you designed.
• Never Checking the PCB Power Plane Sizes: After sizing every trace on your board, be sure to check the power plane sizes match up as well.

Frequently Asked Questions

1. What thickness is considered 1 oz copper on PCB?

After plating, 1 oz copper is around 35 microns thick.

2. Is more copper better?

Not necessarily. More copper equals more current and reduced thermal rise, but comes at the cost of price and manufacturing constraints. Heavier copper PCBs are more difficult to etch fine-pitch components.

3. Does copper thickness matter for signals?

Just like power circuits, signal circuits also have to worry about voltage drop across their traces. For low voltage logic signals (1.2V, 1.8V) even a small drop can cause the circuit to not work correctly. Increasing copper thickness or trace width can help reduce this.

4. What standard is used for trace width?

IPC-2221 is still considered the industry standard for trace width calculation. However, for higher fidelity designs and high power applications, IPC-2152 is the better standard to use.

5. Could my manufacturer change copper thickness?

Typically this is something that is changed at the panel level. Most manufacturers won’t be able to provide you with 2 different copper thicknesses on the same internal layer. Be sure to specify your finished copper thickness in your fab notes or stackup PDF.