The Nitty-Gritty of PCB Design: PCB Trace Width, Thickness, Spacing, and Length

pcb traces

PCB Traces are an essential part of every circuit board. Their goal is to connect any type of electrical signal from one junction to another. Traces are like a highway or path system of the PCB. In short, they are copper lines connecting everything on the board. 

When designing a PCB, there are five main questions you need to consider. 

  1. The purpose of the trace — Whether it will be a signal trace or power and ground trace.
  2. Amperage — Current that will run through the trace.
  3. Location — Whether it is a top, bottom, or inner layer.
  4. Thickness — What the thickness of the copper layer is.
  5. Length — How long your trace should be.

But before we jump straight into calculations and measurements, we should explain why it is so important to calculate everything correctly. If you skip this part, it will take you a lot of time, as well as resources, to fix everything. Moreover, it might even cause damage to the rest of the system or devices in the circuit. 

What Is Trace Width?

Let’s start with the trace width. Each trace is a copper foil that remains on the board after etching. The width of the trace is exactly what it sounds like — the width of the copper path. It is usually measured in mils, which is one-thousandth of an inch (0.0254 mm). 

If you miscalculate the width, the trace might burn. So, what determines trace width? The answer is current. However, the current is not the only deciding factor. You will also need to know the type of signal (power tracers are usually wider), temperature, thickness, and so on. 

Traces have three dimensions — width, thickness, and length. Each will play a role in the proper functionality of the PCB. Furthermore, it is not rare to see a PCB with various trace widths, depending on their purpose. 

For example, traces for general purposes, like TTL or transistor-transistor logic, require smaller traces than the ones designed for power delivery. While the best option seems to be picking the highest possible width, that’s not always doable. 

PCBs often need to have specific dimensions, and sometimes you’ll need to compromise to fit everything on it. That means you might not be able to have traces as wide as you’d like since you will also need to worry about spacing. But more on that later. 

Calculating Trace Width

The first thing you need to understand is that the copper trace will act as a resistor. The narrower and longer it is, the more resistance will it add to the equation. That means that if you plug, let’s say, a monitor to your PCB, the trace could introduce a voltage drop. 

Similarly, the high current could damage the trace, which is also something you want to avoid. So, the trace has to be just right. Of course, using a monitor in your circuit means that you know the electrical current that will run through the trace, which is the first step towards solving the problem. 

Calculating trace width is, in fact, quite simple. However, you will still need to know a few variables before you proceed. You will need to know the max current in the trace, thickness, and temperature (both ambient and temperature rise).

Once you have this info, you can use a formula I = k ΔT b A c. In this equation, k is 0.048, b is 0.44, and c is 0.725 based on the IPC standard. I is the current, ΔT is the temperature rise, and A is the cross-sectional area of the trace expressed in mils2.

This means that the only missing part of the equation is the area, and we will need to know the thickness of the trace before we can calculate the width. 

Trace Thickness

As we could see in the previous part, knowing the thickness of the printed circuit board trace is essential for calculating the width. That leads us to the next section — trace thickness. As we saw before, the trace area is essential for keeping components on the PCB safe. That means that you will need to consider the thickness of the trace too. 

Fortunately, trace thickness is mostly standardized, and it varies between 0.008 inches and 0.240 inches. That means that you can find nearly anything in-between — 0.0079 inches (or 0.2 mm), 0.016 inches (0.4 mm), 0.02 inches (0.5 mm), and so on. 

The thickness of the trace is picked by the designer of the PCB, and similarly to width, it can vary between traces on the same PCB. 

It is important to mention that the size of the inner copper layer will play an important role in the process. The user can pick half an ounce, one ounce, two once, inner layer copper foil for their board. 

For calculating the area of the trace, you can always check out the universal chart that offers precise estimates based on the IPC-2152. 

There is also an option of using trace width calculators based on the IPC-2221 or IPC-2152. They use simple formulas for calculating the area and width of traces in PCB. 

Trace Length

As previously mentioned, longer traces will have more resistance. That means that length has to be perfect as well. Traces can be quite long compared to other parameters, and they are usually measured in inches. 

Your goal is to make the PCB as small as possible. That also means shortening the length of each trace. Ideally, you will have short but wide traces, and the idea is to have the fastest signal propagation time. 

Length Matching

One of the important things for designing PCBs is length matching. The idea behind it is to match the lengths of two or more traces across the board. There are four ways to match traces on the PCB.

  1. Several single-ended traces with parallel routing
  2. Each end of a separate pair
  3. Several different pairs with parallel routing
  4. Single-ended or separate pairs with parallel routing with a clock signal

Printed circuit boards that carry a digital signal don’t need to be perfectly matched. The basic idea is to reduce the length of the trace and reduce the timing mismatch. Of course, even if you don’t know the allowed trace length mismatch for your system, that isn’t such a big problem. 

Most components and computer peripherals are standardized. All you need to do is check out signaling and interface standards, and you will easily find all routing specifications you need. 

Trace Spacing

The final aspect of tracing specifications we are going to discuss here is spacing. By this term, we mean the distance between two traces. Spacing is a minimum distance between the two different traces, and it is important for a variety of reasons. 

Most importantly, it will help you avoid flashovers or tracking between two (or more) electrical conductors. Flashover is a type of electric breakdown that may appear along the surface of the board or along the junction. 

There are two crucial terms we need to define that are important for spacing. The first one is clearance and the second one is creepage. Clearance is the shortest (minimum) distance between two traces, and it is usually an air distance. Creepage, on the other hand, is the distance between the traces, but along the surface of the PCB. 

Unfortunately, there is no single rule to avoid flashovers. The best course of action would be to follow industry standards for PCBs.

PCB Trace Resistance

Now that we’ve covered the basics parts of traces in PCBs, there is one final thing we should discuss — the resistance. We already mentioned the voltage drop before, and by calculating the resistance of the trace, we can determine the drop and potential power loss. 

To achieve this, all we need to do is use the values of our PCB in the equation: 

R = ρ * LT*W * (1 + α (t-25))

In this equation, R is the resistance we are trying to calculate, ρ is the resistivity of the copper, L is the length of the trace, T is the thickness, W is width, α is the temperature coefficient of the copper, and t is the temperature. 

This way, we can calculate the voltage drop and see whether it is within the boundaries of our system. 

Conclusion

Traces are one of the essential parts of printed circuit boards. They are used to connect any type of electrical signal across the PCB. We can think of traces as a network of wiring across the board. The most important parts of the traces are width, thickness, length, and spacing. 

Once you have calculated each of these factors, you can start with the design process of your PCB. For general info, you can always use one of the calculators that may give you a rough idea of how everything works. 

But the manufacturing process is a lot more complex, and there are many things you need to cover. Here, at MKTPCB, we have years of experience providing solutions for our clients. Why bother with PCB design software when we can help you every step of the way? 

All you need to do is contact us, and we’ll help you with everything, from design to mass production!

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