PCB Trace - Everything You Need To Know
Every printed circuit board has a certain pattern or network of conductive layers made of copper — traces. These are of the outermost importance for as simply they are regarded as carriers of the electric signal.
Quick note: if a PCB starts to malfunction — it’s a copper tracing to blame, usually because of the corrosion or any other external deterioration. Knowing some trace calculations, formulas, and rules is the way to prevent electronics from malperforming.
What's PCB Trace?
Once a PCB laminate is coated with a thin layer of copper, it is to be etched. Simply the needed internal traces and external traces are protected with tin, and then a component is placed into the solution that removes unnecessary copper — etching.
What’s left is the precise network of copper traces leading from pad to pad. You can see an example of what traces look like in the image below.
Tracing network it thin. Yet, it is expected to be reliable enough to withstand heating up in the process because of carrying an electrical signal. The important consideration here is holes, pads, and vias. Their impact is often overlooked. Yet, their intersection with traces changes the length of the path for an electrical signal to pass.
Thickness of a PCB Trace
Trace thickness is one of several crucial aspects to consider. Inadequate, or more often, inconsistent thickness leads to such issues as power loss, voltage drop, electrical sparks, etc.
The reason is that an electrical signal of precisely controlled power can easily lead to trace temperature increase if it is too thin. Alternatively, the thin trace may not be capable of carrying an electrical signal.
Normally, the thickness of a PCB trace is measured rather in the volume of liquid copper applied to the board. The standard values range from 0.25 oz, which equals 0.3 mils thickness, to 5 oz, which equals 7 mils thickness.
The volumes of copper are chosen in the design process based on the size of a board and maximum current power.
PCB Trace Resistance-What Is It and How to Use
Another interesting aspect is trace resistance. Together with trace thickness, this parameter determines how prone the tracing would be to voltage drops and power losses.
Calculating PCB trace resistance should be as simple as the application of the ohms law. Of course, give if the variables are known. Take a good look at the formula in the image below.
The resistance of the final copper trace must be well-known to estimate the required power signal or vice versa. Likely, you do not have to calculate it yourself. Most programs for the development of PCB designs easily enable to calculate the trace resistance.
Calculating Trace Resistance
Let’s start by defining formula variables:
- L, W, and T represent the physical area of boards, i.e., height, width, and length.
- R is resistance and is defined as P x area.
- P is the resistivity of the given material, i.e., copper.
For example, given the heated to 25°C copper, its resistivity rate is 1724 (tables are available on the web). This variable is multiplied against the area, resulting in 6-10 ohms per square centimeter.
As mentioned, you do not have to apply the formula yourself. There are software solutions to perform the work for you. But, there is another interesting thing to add.
There is a direct dependence between trace width and trace resistance. In other words, increasing the area of the copper traces leaves more room for experimenting with ohm levels. Even without the calculation you may know what higher trace width results in higher resistance.
Issues with Trace Resistance
Returning back to the power loss that many electrical components experiences, this problem is exactly the case here. Depending on the types of traces used, small signal PCBs will have a minor impact on PCBs’ trace resistance. Yet, high-power PCBs are way more dependent on slight variation in resistance.
It means that uncompensated, unbalanced, inconsistent, and varying trace width and thickness easily result in trace fuse once power surges. It leaves instant and permanent damage.
A great way to deal with the issue is to adjust design parameters. Increasing trace area is the simplest way to increase the resistance of tracing in external and internal layers. This way, even the unexpected power surges will not have damage.
Besides, sensitive circuits of some electronic components have compensation subcircuits. It’s so to eliminate PCB’s trace resistance. Also, you have all the good reasons to use the trace width calculator.
How is PCB Trace Width Calculated?
So, traces of a circuit board are designed to handle maximum current before they fail. Any excess in electrical power leads to increased temperature and permanent damage.
The trace width is actually determined based on the applicable rise of temperature. What does it mean? The crucial aspect in trace width calculation is the difference between minimal and maximal operating temperature.
In practice, trace width determination is linked to ampere capacity. So you take your design specification and use a trace width calculator to determine the minimum trace width and maximum trace width
Trace Width Calculators: Types and Usage
What such software does is that they determine the overall path of copper tracing and correlate it to the needed electric power to manage this path to the end. Here are the types of PCB trace width calculators:
- IPC-2221 External Layers calculator. This software solution is based on graphs and measurements. It utilizes the I=kΔTbAc equation to find out the maximum trace capacity and optimal trace width, respectively. The con is that it does not consider PCB vias.
- IPC 2152 calculator. This one ground their calculations on another, mode advanced standard. It determines maximum trace capacity, especially of thinner traces, much better. Unlike IPC-2221, this calculator determines the unadjusted cross-sectional area first.
In the image below you may see the simplified logic for calculating trace max current amps and trace width.
Trace Width Calculators: Other Considerations
You have some approximations regarding the trace width. You also need to consider:
- The carrying capacity of a trace.
- The pitch and size of pads.
- The gaps between traces.
The spacing between traces is also carefully considered during the development of tracing, resistance, and current requirements. The propper spacing between traces, normally, is 6 to 30 mills.
At the end of the day, the trace width fully depends on the design specifications and the required amount of current. The minimum trace route width is always about 10mm/A. Which is suitable for 10oz/ft^2 of copper.
PCB Trace Current
There interesting point here is that PCBs use a direct current as a power source. It is much different compared to how wires with conductors perform. The electrons travel straight through the copper tracing to power up the battery of a device.
The maximum current that is supposed to be supplied to a PCB is of crucial importance here. As mentioned above, there is an obvious dependency between power and resistance. Still, the detailed nuances resemble only one aspect of the issue.
Calculating PCB Trace Current
In PCBs, copper is a conductor, and rubber is an insulator. The role of the latter is to prevent the generation of too much power and appearing of a short circuit. In contrast, to much insulation just does not allows for generating enough current.
Calculation of the current to supply to a PCB is done using the formula derived from Ohm’s law. You may familiarize yourself with it in the image below.
So, the resistance you had calculated in the steps above and the voltage can be measured by sufficient devices.
You should consider leaving some “space” resistance just in case of power surges, so your internal traces never deteriorate.
And some more useful calculations: 1 oz of copper normally can handle about 500mΩ per square inch. Now you know both how much copper you need to create a required trace thickness and how much current this tracking can possibly handle.
Most such calculations are naturally integrated into trace width calculators and other software. Always try to find an optimal balance between expenses on extra copper and the safety of the board.
FAQs
1. How to Estimate PCB Trace Impedance?
To properly calculate your trace impedance, you need to either use a Smith chart or run a computer simulation or use an online calculator. The goal here is to determine the reflection coefficient between source and load impedances. Additional software excels in finding both controlled impedance or trace parameters if the impedance is known.
2. How to Repair PCB Trace?
The easier method, assuming that the point of failure is known, is to reestablish a connection. Scrap soldering, remove failed part of the trace, apply a drop of liquid copper, smooth it so it is combined with the ends of the trace, and add some soldering on top of it, so it forms the joint.
3. How to Solder PCB Trace?
In case you have damaged soldering, you need to scrap a few millimeters of it, apply a small amount of photosensitive resin, and cure it with UV light. But, if you need to solder the entire tracing network, be sure to prepare the surface, preheat it, and only then apply solder and cure it.
Summary
In the end, there are enough aspects to pay attention to in order to receive a decent, reliable PCB design. It includes determining trace resistance, trace width, carrying capacity, and trace thickness.
You should not neglect using calculators, such as trace width calculator, to make the process way easier. Be sure to always consider additional nuances, such as possible power surges, vias and pads that are not calculated, etc.
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