High Current Path Planning on a PCB: Routing Strategy

When a board has to deliver tens of amperes, the usual assumption that a trace is just a connection stops being adequate. A high current path has resistance, it produces heat, and the heat changes the resistance, so the design has to be planned as a thermal and electrical system rather than routed as a net. The planning decisions are made early, because they determine how much board area the path consumes and whether the copper is thick enough.

Start From the Path, Not the Net

The first step is to define the whole path rather than the individual nets. A power path begins at the source, passes through a connector, crosses the board, reaches the switching or regulating stage, continues to the load, and returns through a ground path that is just as important as the positive conductor. Each of those segments has its own current and its own cross-section, and the path should be drawn as one continuous route so that the narrowest point is visible before the routing begins. A path that is designed as a set of connected nets tends to acquire pinch points at the boundaries between them, and those pinch points set the real capability of the board.

Length, Thickness and Width

Resistance depends on the resistivity of copper, the length of the path and its cross-sectional area, and only the last two are under the designer”s control. Copper thickness is specified in ounces, with one ounce corresponding to about thirty-five micrometres, so a board ordered with two ounce copper has twice the cross-section of a standard board for the same trace width. The engineering description of capability uses three quantities together: copper thickness, permitted temperature rise and conductor width. A useful reference point is that a 2.5 mm wide conductor on one ounce copper carries about 4.5 A with a ten degree Celsius rise. Two consequences follow. The first is that the permitted rise is a choice, and increasing it allows more current at the cost of a hotter board. The second is that the relationship is not linear: as the conductor widens, the current gained per millimetre falls, so tables or curves should be used rather than a simple scaling. Working from those numbers, a 100 A path is realistic with four ounce copper, a conductor around 15 mm wide, the same conductor on both sides of the board, and additional heat sinking to hold the rise down.

Using Layers in Parallel

Duplicating the conductor on several layers is usually more practical than widening it further, because it uses the vertical space the stackup already provides. Two identical conductors joined at both ends halve the resistance, and inner layers can be added where the stackup allows. The joints at each end must be real connections rather than symbolic ones, which means a sufficient number of vias sized for the current and arranged so that the current spreads rather than crowding into a single barrel. Those vias also transfer heat from the conductors into the inner planes, and their number should be chosen from the current and from the heat that has to leave. Where the path narrows to pass a component or a via field, the narrowest section governs, and the general calculation for width against current is described in this article on trace width and current calculation.

Moving the Current Off the Board

At some current level it becomes cheaper and more reliable to stop using copper foil as the conductor. Terminal posts, surface mount studs and PCB terminal blocks rated for the current allow the connection to be made with a cable and a crimped lug, which removes the heat from the board and makes the joint serviceable. A custom bus bar goes further, and it is the standard industrial solution for very high currents: a formed copper bar bolted to the board or the chassis has a large cross-section, is cooled on all sides, and provides a defined contact area at each end. Its dimensions rather than the board”s copper then set the limit. A bus bar is also easier to inspect, because the joint can be measured and re-torqued rather than examined by cross-section.

Heavy Copper Stackups

Between a conventional board and a bolted bus bar there is a third option: a stackup built specifically to carry current. One arrangement uses thin signal layers on the outside and a very thick copper layer in the middle, with the thick layer dedicated to power distribution. A board of this type can carry well over a hundred amperes in a small volume, because the conductor is a solid sheet rather than a widened trace. The arrangement is not available from every fabricator, because etching and laminating very thick copper requires different process capability, and the design rules that accompany it are more restrictive. Where the current requirement is high and the volume is tight, it is worth asking whether such a stackup is available before committing to a planar solution, and the general options for layer arrangements are described in this article on layer stackup from one to eight layers.

Connections and Joints

The joints decide whether the planned capacity is achieved. Every connector in the path has a current rating and a contact resistance, and a connector chosen for signal use will fail long before the copper does. Solder joints in heavy copper need more heat to form, because the copper conducts heat away from the joint, so the assembly process must be adjusted to the board. Where a bolted joint is used, the contact area, the surface finish and the torque determine the resistance, and all three should be specified. Thermal relief, which is normally helpful for soldering, is the wrong choice on a high current pad, because it reduces the copper cross-section exactly where the current density is highest. The same reasoning applies to the sense connections of a current measurement: they should be taken from defined points on the conductor so that the measured value reflects the current rather than the joint.

Verification

A high current path should be measured rather than assumed. Thermal imaging at full load shows whether the heat is where the calculation predicted and whether any joint runs hotter than its surroundings. A four-wire measurement or a voltage drop at a known current gives the actual resistance of the path to compare with the calculated value, and it should be taken at the maximum expected ambient temperature, because a joint that is acceptable at room temperature may not be inside a warm enclosure. Where the load is pulsed, the transient behaviour should be checked as well, since the thermal response over milliseconds differs from the steady state. The materials that must tolerate this heat without changing their properties are described in this article on copper plating defects and prevention.

FAQ

How much copper is needed for a 100 A path? Roughly four ounce copper, a conductor about 15 mm wide, the same conductor on both sides, and additional cooling. Below that, terminals or a bus bar are the more practical answer.

Why is a very wide trace wasteful? Because the gain in current capacity per millimetre falls as the conductor widens, since the outer regions are less effective at removing heat.

Why avoid thermal relief on a power pad? Because it reduces the copper cross-section at the pad, raising the resistance where the current density is highest and restricting the path for heat to leave the joint.