Heavy Copper PCBs: Thickness, Process and Design Rules
A heavy copper board is not simply a board with more copper on it. The weight of the conductor changes how the copper is etched, how the holes are drilled, how the layers are pressed and how the impedance is controlled, and the design rules that apply to ordinary one-ounce foil stop being the right ones.
This article explains where heavy copper is used, what it changes in the process, and which design rules have to be adjusted before the data is released. The adjustments are not optional, and a design that ignores them is either rejected or quietly altered.
What Counts as Heavy Copper
Ordinary boards are built with copper weighing one or two ounces per square foot, which corresponds to a thickness of about thirty-five or seventy microns. Heavy copper generally means three ounces and above, and the constructions used for power electronics often run from four to ten ounces, with some applications going higher.
The significant consequence of the weight is that the copper is no longer a thin film. At several ounces, the thickness of the conductor becomes comparable with the width of a fine trace, and that ratio is what makes the process difficult: an etchant that has to remove a thick layer attacks the sides of the trace as well as its top, and the cross section that results is trapezoidal rather than rectangular.
Why Heavy Copper Is Used
The first reason is current. A thicker conductor has a larger cross section, and the resistance per unit length falls accordingly, so a heavy copper board can carry currents that would heat an ordinary board beyond its rating. The same property allows narrower traces to carry the same current, which is useful where the layout is constrained.
The second reason is thermal management. Copper conducts heat well, and a heavy layer spreads the heat generated by a power device over a large area, which lowers the temperature at the junction and reduces the demands on the heatsink. That is why motor drives, inverters, battery management systems, power supplies and electric vehicle controllers are the usual users of the technology.
What Changes in Fabrication
Etching is the first operation to change. Because the etchant works from the side as well as from above, the minimum gap that can be produced grows with copper thickness, and the achievable trace width grows with it. The etching is also slower, and the process window is narrower, so the design has to leave more room than the calculation of current alone would suggest.
Plating is the second. Thick copper in the holes has to be built up evenly, and a hole that is plated unevenly has a thinner wall on one side. Drilling is affected as well, because copper is harder on the tool than laminate and the wear rate rises, which changes the cost of the drill programme and the condition of the hole wall. Lamination is the third: more resin has to fill the spaces between heavy traces, so the bonding sheets have to be chosen for the geometry rather than for the total thickness alone. The general causes of plating defects are more likely to appear on this kind of work.
Adjusted Minimum Features
The minimum trace width and the minimum gap both increase with copper weight, and the values are set by the etching process rather than by the design intent. A layout that uses the same spacing as an ordinary board will not be produced as drawn, and the fabricator will either adjust the data or ask for a change.
The designer should also allow for the trapezoidal cross section. A trace drawn at a given width will be narrower at the top of the copper and wider at the base, so the current-carrying area is less than the nominal width suggests. The calculation of trace width from current has to use the actual cross section rather than the drawn one, and the temperature rise used in the calculation has to be the one the product allows.
Impedance Control on Heavy Copper
Impedance is set by the geometry of the trace and its distance from the reference plane, and heavy copper changes both terms. The conductor is thicker, which reduces the resistance and alters the inductance, and the dielectric above and below it is different because more resin is required to fill around the traces.
The consequence is that the numbers calculated for an ordinary stack do not transfer. Where a heavy copper board also carries controlled impedance nets, the stack-up and the trace geometry have to be calculated together and confirmed by the fabricator, and the tolerance has to account for the wider variation in etched width that the thicker copper produces.
Thermal Design and Current Capacity
Heavy copper improves current capacity and heat spreading at the same time, and the two benefits are used together in power designs. A device mounted on a thick copper area has a short thermal path into a large mass of metal, and the heat is then removed by the board itself rather than only by a heatsink.
Where the heat has to travel between layers, an array of thermal vias is used, and those vias also have to be filled if a pad is to be soldered over them. The combination of heavy copper, filled vias and a thermal pad is a common construction in power electronics, and the requirements are described in the notes on thermal design for placement and reliefs.
DFM, Lead Time and Cost
A heavy copper design should be checked against the specific capability of the shop that will build it, because the limits vary more between suppliers on this kind of work than on ordinary boards. The minimum trace and space, the minimum hole size, the achievable copper thickness in the hole and the maximum copper weight per layer are all shop-specific figures.
The lead time is longer for the same reason, and the number of process steps rises with the copper weight. More copper means slower etching, more drilling, more careful lamination and, often, a second plating stage, and the prototype that takes a week on ordinary foil can take considerably longer here. Planning the build into the schedule rather than assuming the standard lead time is part of the preparation for a multilayer prototype.
FAQ
Can any trace width be used with heavy copper? No. The achievable width and gap are set by the etching process and grow with the copper weight, so the design has to respect the capability of the shop rather than a nominal rule.
Is heavy copper a substitute for a heatsink? It spreads heat and lowers the temperature rise, and it rarely removes the need for a heatsink on a high-power device. It reduces the size of the heatsink that is required, and in many power designs that reduction is what makes the mechanical envelope possible.
Does heavy copper affect the board thickness? It contributes to it. The finished thickness is the sum of the layers and the copper, so a stack with several heavy layers is thicker unless the dielectric is reduced, which then affects the impedance.
