In the electronic designing process, a PCB trace is a network of wiring, copper, insulation, and fuses found in a circuit board. Inconsistencies in PCB traces cause most electronic faults. Besides, trace resistance is the most common problem technicians encounter in the PCB industry. This article discusses everything you need to know about PCB traces – from what a PCB is to calculating PCB trace width.
What is a PCB Trace?
The role of a PCB is to conduct signals and data from one point to another. To achieve this, the signals must travel on a path. PCB traces act as the transmission paths for these signals and data. They link various PCB components, such as the IC, capacitors, and resistors. Therefore, without traces, a PCB cannot function properly.
A PCB trace. (Source)
The transmitted signals consist of electric current; thus, materials for making traces must be good conductors of electricity and reasonably stable. The common material for making PCB traces is copper foil, but there are other options to consider. For instance, you can use aluminum or gold.
PCB Trace Resistance
Almost everything on the Earth displays resistance and other parasitic properties. PCB trace resistance is an important factor that must be established and scrutinized during the design phase. While copper is the most suitable material for making traces, it displays diverse compositions and properties. The circuit board trace resistance can cause various design and implementation problems. As the PCB becomes more complex, the problems also increase.
How to Calculate the PCB Trace Resistance
Finding the circuit board trace resistance is as easy as applying the ohms principle in establishing a current voltage. Most design assemblies and working environments have incorporated trace calculators, facilitating the calculation of the PCB trace resistance. These calculators apply a standard formula to find the trace resistance as shown below:
R = (ρ * L / (T * W)) * (1 + α * (Tamb – 25 °C))
R stands for the resistance;
L is the trace length
T is the trace thickness
W is the trace width.
Tamb is the ambient temperature.
Besides, the equation has two constants – ρ and α, which represent the resistivity and resistivity temperature coefficient, in that order. Does the formula seem intimidating to you? Do not worry. Just enter the values and leave the PCB trace calculator to do the computation for you!
Power Loss and Interference
The major cause of trace resistance is power loss. Small boards leave a negligible impact on trace resistance, while large boards, like power circuits, greatly impact trace resistance. Power loss eventually increases the temperature and minimizes the transmission capacity of the board.
In power circuits, the high trace resistance can act as a fuse in case of an increase in power and damage to the trace. In high-speed digital circuits, trace resistance and parasitic capacitance can cause oscillation effects and interfere with EM issues in the circuitry. Long circuit board tracks can also cause an increase in reissuance.
The most effective method of solving the trace resistance issue is increasing the trace area. You can achieve this by configuring design tools to layout traces with adequate width. Besides, you can design your power circuits to utilize other techniques, like solder bridges, to minimize power loss and raise the current capacity. In high-speed PCBs, you can design matched lines to eliminate the interference caused by oscillation.
Analyzing the PCB Trace Resistance Equation
The trace width is basically the width of the copper foil, measured in mils – 0.0254 mm. Miscalculating the width might burn the trace. So, what defines the trace width? The trace width is determined by various factors like current, signal type, temperature, thickness, and others.
A PCB trace comprises width, thickness, and length. All these elements affect the functioning of the circuit board. Besides, it is normal to design PCBs with distinct widths based on their intended applications. For instance, traces designed for general applications, such as TTL demand smaller widths than those designed for power transmission. Though the best option seems to be selecting the biggest possible measurement, this is not always true.
Sometimes, circuit boards need to contain specific measurements; therefore, you can compromise on general principles to meet design requirements. This implies creating traces as wide as you would desire because of spacing.
How to Calculate the Trace Width
It is advisable to use a PCB trace width calculator to find the trace width. You simply need to enter the design specifications into the calculator, such as the tolerable current in amperage, the length of the copper foil, and the change in temperature caused by trail resistance. After entering these specifications, the device will carry out its computation and provide the least width you need to meet the design specifications you entered.
The computed width will facilitate the smooth transmission of current without damaging the board. The trace width of the inner layers may be bigger than that of the outer layers because they are more likely to generate more heat. The external layers produce less heat because of convection effects.
It is not advisable to just establish the general PCB trace or apply a simple principle to find the trace resistance. You should calculate both the circuit board traces and trace width. Doing so will assist you in minimizing the PCB trace resistance factor.
The PCB Trace Width Formula
The best way of finding the PCB trace width is by applying the trace width equation. The formula will enable you to find the tolerable current conducted through a particular trace. Below is the IPC 2221 trace width formula:
I is the current, and it is always constant. ΔT is the temperature change, and A is the cross-sectional area. You can restructure the above equation to have the trace width on the left-hand side as shown below:
Area [mils^2] = (Current [Amps]/ (k*(Temp_Rise [deg. C]) ^ 0.44)) ^ (1/0.725)
However, to create the right width, you must consider your trace thickness.
Width [mils] = Area [mils^2]/ (Thickness [oz]*1.378[mils/oz])
You can only apply the above PCB trace width equation from amperes ranging from 0-35 because it allows temperature change from 100C to 1000C. Additionally, the formula accommodates a trace width of just 400 mils.
Disclaimer: Most circuit board service providers apply the PCB trace formula as the electronic industry standard and assume it to be right. Nevertheless, the formula may not be ideal for each design requirement. Thus, it is advisable to specify your trace width in the Gerber file and seek advice from the manufacturer on whether the specified width is ideal for your application.
As mentioned earlier, trace thickness is one of the main determinants of calculating trace width. You must consider the trace area if you want to keep your PCB components safe. The trace area narrows down to the trace thickness.
The trace thickness is a standard value, ranging from 0.008 to 0.24 inches. Therefore, your trace thickness should lie in between this range. Remember, it is the designer who selects the trace thickness, and just like the trace width, it varies from one trace to another on the same circuit board.
Additionally, it is essential to note that the size of the internal layer plays a significant role in determining the trace thickness. You can use half, one, or two ounces for your internal layers. To calculate the trace area, you can rely on the universal chart for accurate figures from the IPC 2152 standard. Another option is applying a trace width calculator supported by the IPC 2221 AND IPC 2152 standards.
As mentioned earlier, longer traces create a high resistance, implying that length is also an important PCB trace resistance consideration. You should aim to design a small board by minimizing the length of the traces. Though you will end up with short and wide traces, your PCB will have the quickest signal transmission time.
Trace spacing refers to the distance between two traces. Like the trace width and length, it is also an important trace resistance consideration. Importantly, trace spacing will enable you to prevent flashovers between two conductors. A flashover is a current breakdown that appears along the PCB surface or junction.
PCBs form the backbones of almost every electronic device. This article has emphasized the importance of establishing the correct trace width to prevent your PCB from damages. Finding the right width will also allow you to establish the current tolerance level of your board.
MKTPCB is among the leading PCB manufacturing and assembling companies from China. Our engineers will guide you throughout the entire PCB manufacturing process and ensure you choose the right trace width that fits your design requirements. Kindly contact us today to discuss your PCB trace requirements.