You are not the only one to look at a PCB design and realize that you may not know whether your copper traces will be able to carry the current that you are about to pour into them. One of such essential issues of PCB design that can break or make your project is getting trace width right. Too thin and you can easily overheat, or even burn the traces. Too broad and you are both wasting real estate on the board and you are also risking a rise in manufacturing costs.
Understanding the Basics of PCB Trace Width
It can be useful to know what is really going on in those narrow copper lanes on your board before you start computing it. As fluctuation passes through a copper trace, it has an opposition. This resistance results in two major effects, heat generation and voltage drop. Resistance varies depending on a number of factors such as trace width, thickness, length and even the copper resistivity itself.
The relationship is fairly intuitive when you think about it. A broad trace has a greater cross-sectional area that the electrons can flow through resulting in reduced resistance. Reduced resistance means reduced heat generation and reduced voltage drop. It is like a broader highway has the capacity to carry a larger traffic without congestion as compared to the narrow country road.
The majority of PCB designers use conventional copper weights, which are generally in ounces per square foot. One-ounce copper is the most typical, and this is one that would be about 1.4 mils or 35 micrometers. Heavy copper-weights of two to three ounces are obtainable in that case when heavy current to be carried, but they add to the cost and complexity of production.
Current Carrying Capacity and Temperature Rise
The first thing you have to determine is the amount of current that you will run on your trace. Though this is where it gets interesting, the allowable trace width does not only depend on the current itself, but also the amount of temperature increase that you would accept.
The industry standards, especially the IPC-2221, gives the guidelines on the trace width as per the current and permissible temperature rise. Another popular design goal is one of ten degrees Celsius temperature increase over ambient, but again this may depend on your application. In the case of consumer electronics whose work requires them to be in controlled climatic conditions, ten degrees could be just dandy. In case of automotive or industrial application that will be subjected to extreme temperatures at times, you may wish to design to a significantly lower temperature rise.
The issue of temperature rise is particularly, very critical when your traces are passing close to heat sensitive components. Although your trace alone may be good to the heat, the thermal behaviour of surrounding components may lead to performance problems or an early death.
This can be made much easier with a PCB Trace Width Calculator. These tools enable you to enter your present needs, desired temperature increase, copper mass, and trace length and obtain instant width suggestions. You do not have to go and labor through the complicated formulas manually to get the best result but rather you can go through the various design situations in a very short time to get the best solution.
Accounting for Voltage Drop
Although the increase of temperature frequently takes center stage in trace-width discourse, voltage drop is also a significant issue, particularly in power distribution networks and low-voltage digital circuits. With a current passing through a resistive trace, there is a voltage which is dependent on the current and the resist.
In the case of power delivery, a high voltage drop may imply that your circuitry is not getting sufficient voltage to operate. Ground bouncing and signal integrity problems may also occur in high-speed digital designs where voltage drop in ground planes or return paths may cause problems. A drop of a few hundred millivolts is enough to put sensitive components out of operating limits.
Practical Design Considerations
The PCB design in real world is associated with trade offs and practical constraints. Board area is typically a luxury, particularly in small products. You may work out that you require a 200-mil-wide trace to give the best performance, but just do not have such real estate.
It is here where solutions that are creative come in. Is it possible to trace the route on more than one layer and interconnect them using vias to increase the cross-sectional area effectively? Is it possible to use heavier copper weight? Is it possible to redesign this circuit to lower the current requirements? In some cases, it is not that the trace width needs to be changed but rather the general strategy of designing.
When designing complicated boards that contain signals of varying types, employing Impedance Calculator is perfect for ensuring high-speed signals retain proper impedance and dealing with width requirements of power traces simultaneously. It is more like an art in printed circuit boards design to take care of these, along with the many material and other constraints across a multi-layer stackup.
The other consideration is manufacturing capability. Nobody ever has an issue with traces smaller than 6 mils in standard manufacturing processes, but finer geometries are possible with advanced processes. Calculating a Wire Size, A Wire Size Calculator can be used to compute the correct dimensions when you only have a voltage drop as your constraint, using the same principles of electricity that apply to PCB traces.
External Wiring and System Integration.
Your PCB is not in the vacuum. The majority of boards must have connector to the external world, and connector may utilize wire harnesses. Your PCB traces should be appropriately scaled so do the power and signal-carrying wires that go to your board.
When it comes to the external wiring, working closely with a wire harness manufacturer is of utmost importance to ensure that the careful routing and layout you did on your PCB is mirrored on your exterior wiring. There is no “point” in designing all those accurate sized PCB traces if they end in a bottleneck of under-sized wires for delivery of power.
Making the Final Decision
Choosing the appropriate trace width is all a matter of knowing what is important and what is not. First, be sure to specify your requirements: maximum current, allowable temperature rise, maximum voltage drop, available board space. Find out what is possible using those available calculators and design software, but remember that engineering judgment is also necessary.
In case, the best bet is to be on the safe side. The traces should be made a little bit larger than the minimum calculated to allow room in manufacturing, spikes in current and ageing effects. This little additional board space is normally a good payback against field failures or redesigns.