Learning Current Density: Don’t Pop the Balloon
One area of PCB design that probably does not get the attention that it deserves is current density. Current density in a PCB trace is much like the pressure in a balloon. You want to squeeze as much air into the balloon as you can, but if you put too much: boom! The same is true of PCB traces many times. We want the traces to be as small as possible yet carry the most current possible, and if the trace carries too much: boom! For PCBs, that boom may not only mean the trace has exploded, but circuit board components may be destroyed as well.
Prior to designing or laying out a printed circuit board, it is routine to perform some type of circuit analysis to ensure that the networks of the system have the proper operational parameters. We want to make sure that the copper foil, copper thickness, voltage drop, current density distribution, and impedances meet our objectives. Once these are verified, we proceed to the PCB design confident that the system will perform as intended. However, there are additional considerations once we transition to the PCB design. One of the most important of these is the current density capabilities of our network traces. Traces without enough current can cause misinterpreted signals and those with too much can cause severe consequences including circuit board failure. By knowing the current densities of the traces we can ensure that the current is between these extremes. Let’s examine PCB current density and look at ways to ensure our circuit boards perform as designed.
What is Current Density?
In short, current density is the current flow per unit area through a material. For PCBs that material is generally copper traces on layers or metal plated vias. The formula for calculating the current density is
J = I / A
where
J denotes the current density in amperes/meters squared (A/m2)
I is the current in amperes (A)
A is the cross-sectional area (m2)
Current density quantifies how much current or electric charge is flowing through the trace or via. This is an important quantity because each material type has a limiting amount of current carry capacity. This is denoted by its ampacity, which is the maximum amount of current that the conductor can carry before deterioration of the conductor takes place. The movement of current through the conductor generates heat. This heat is dissipated into the surrounding area and generally denoted as a power loss proportional to the resistance of the conductor. This relationship is described by the following formula.
Ploss = I2R
where
Ploss is the real power loss through heat dissipation in watts (W)
I2 is the square of the current flowing through the conductor in amperes squared (A2)
R is the resistance of the conductor in ohms (Ω)
Calculating current density will allow you to better understand the power capacities for your PCB, and to design around potential power failures. With the alternatives being potentially destructive shortages and smoking, being able to check your current density will assuredly save you both time and money.
Know How Much Current Density and Avoid the Boom
Having a current density that is too high by allowing too much current to flow through an area could lead to a catastrophe. Although, it is possible to manually calculate these parameters, this would be quite a daunting task for all but the most basic simplistic PCBs. There are a number of considerations that make this especially challenging. For example, fanouts where current flow is distributed among a number of lines, trace corners, and induced current from nearby conductors or devices will have an immense number of variables and hard-to-calculate areas. The more you leave your calculations and design open to errors, the more errors that will happen.
To avoid this we need to check voltages and currents for all traces prior to having the board manufactured. Through analyzers and simulation systems within your PCB design software, you’ll be able to run tests on your current flow distribution so as to better understand where potential points of catastrophe could occur, and to re-design these areas to address the potential problems before the board ever reaches manufacturing.
By knowing the limitations of your boards power distribution, you will be able to avoid having your boards go up in flames in the future. Instead of coughing up your hard-work and your revenue as your PCB goes up in smoke, make sure to have a basic understanding of how current density will play a role in your PCB design.
To assist you in ensuring that your PCB design does not have current density and current flow problems that could lead to performance issues or even board failures, Altium includes a Power Distribution Network Analyzer (PDNA) with its Altium Designer package. This tool allows you to analyze the status of your system’s voltages and current flows during the PCB design process.
For more information on how to alleviate current density issues from your PCB design, contact an Altium PCB design expert.
