Heavy Copper PCB Plating And Trace Thickness
Most boards are built with one ounce of copper on each layer, which is about 35 microns after plating. Boards that carry motor current, battery current or the output of a power supply need more, and the term heavy copper covers the range from two ounces to ten ounces or more per layer. The extra metal changes the plating line, the etch line and the layout rules, and it has to be designed for from the beginning rather than added at the end.
This article explains what heavy copper means in practice, how the thickness is produced, why the etch becomes difficult, what the layout must respect, and how the result is inspected.
What Heavy Copper Means
Copper thickness is quoted in ounces per square foot, which is the weight of a layer one foot square. One ounce corresponds to about 35 microns, two ounces to 70 microns, and four ounces to about 140 microns. The figure is quoted before plating, so a board ordered as two ounce copper with a 25 micron plating on the outer layers ends up thicker than the nominal value on those layers.
The reason to use heavy copper is current capacity and heat spreading, and the two are connected. A wide trace of thick copper has a low resistance, so it dissipates less heat for the same current, and it also conducts heat away from the component that generated it. Copper that is used for its thermal mass is often far wider than the current alone would require, and that width is a mechanical and a layout decision rather than an electrical one.
How The Thickness Is Built Up
Two routes are used. The first starts with thick laminate copper and plates only as much as the hole walls need, which keeps the plating line short but asks the etch line to remove a lot of metal. The second starts with thin copper, patterns it, and then plates up the traces in a second plating step, using a mask or a resist to keep the copper where it is wanted. The second route is slower and more expensive, but it produces a more uniform result and allows the fine features to be etched before the thick copper is added.
Where a heavy copper layer has to carry fine features as well, the two are sometimes separated: the fine geometry goes on a layer with normal copper, and the heavy current goes on a layer that carries only wide traces and planes. The stackup then has to be arranged so that the heavy layers do not distort the controlled impedance of the signal layers, which is a problem in press cycle as well as in field solving.
Current Density And Plating Time
Plating thickness is proportional to current density multiplied by time, so the line controls either the current or the time and monitors the other. For heavy copper, the current density is kept lower than for normal plating, because a high current density produces a rough, nodular deposit with poor ductility and a risk of burning at the edges of the panel. The lower the current density, the longer the panel stays in the bath.
Time in the bath is not free. The plating resist is exposed to the chemistry for hours, the bath is loaded with the panel and its own anode area, and the thickness varies between the edge of the panel and the centre because the current distribution is never uniform. That variation is the reason the specification for heavy copper usually quotes a wide tolerance and a minimum value, and the reason test coupons are placed at several positions.
Etching Thick Copper
Etching removes metal from the top and from the sides at the same time, so a thick layer loses more width for the same etch time than a thin one. The result is a trapezoidal cross section, wider at the bottom than at the top, and a trace that is nominally 500 microns wide may lose 100 microns or more in the process. Compensation is applied in the artwork, and the amount is established from measured traces rather than from a rule of thumb.
The etch also struggles with fine gaps. A gap that is narrow and deep cannot be cleared by the spray, because the fresh chemistry cannot reach the bottom and the dissolved copper cannot leave, so the etch stalls and leaves a web of copper between the traces. That limit, rather than the artwork resolution, sets the minimum spacing that a heavy copper layer can achieve, and the design has to respect it.
Layout Rules For Heavy Copper
The minimum trace width and spacing on a heavy layer are set by the etch capability, and they are far larger than the values on a signal layer. It is common for a four ounce layer to be limited to widths and gaps of 0.3 millimetres or more, and for the annular ring around a hole to be larger as well, since the plated copper has to reach around the barrel. The current carrying capacity of the resulting trace is calculated with the same tools used for ordinary layers, as described under trace width and current calculation.
Thermal reliefs, plane splits and the way the copper is distributed across the board all matter more on a heavy layer, because the copper affects the press cycle and the dimensional stability of the panel. The rules for splitting planes and for the shape of the copper are given under power plane splitting rules and copper flooding, mesh or solid.
Inspection And Acceptance
Copper thickness is measured on a coupon by weight, by a microsection or by X-ray fluorescence on the finished board. A section is the more informative method on heavy copper, because it shows the shape of the trace as well as its thickness and reveals undercut at the base of the sidewalls, which a surface measurement cannot see.
Acceptance is usually written as a minimum thickness with a generous upper value, together with a minimum trace width after etch and a maximum undercut. Where the copper is carrying current rather than providing a thermal path, the trace resistance is sometimes measured directly on a test pattern, which is the most direct evidence that the layer will do what the design intended.
Mechanical And Thermal Consequences
Thick copper is not only an electrical material. It raises the stiffness of the panel, changes the expansion behaviour of the layer it occupies, and makes the laminate harder to press flat. A board with two heavy layers on one side and none on the other will bow, so heavy copper is normally balanced around the centre of the stackup, and the balance is checked before the stackup is released rather than after the first lot warps in the oven.
The same copper that spreads heat also stores it. A heavy plane under a power device improves the spreading of the heat but lengthens the time the assembly takes to cool, which shifts the reflow profile and can affect the grain structure of the solder. Where the thermal behaviour and the electrical behaviour pull in different directions, the layout that satisfies both is usually one that keeps the heavy copper close to the component and connects it to the opposite side with a field of thermal vias.
FAQ
How heavy can copper be before the process changes? Above about two ounces the etch compensation becomes significant and the minimum spacing grows; above four ounces the plating and etch lines are usually reviewed for the job before a quotation is given.
Does heavy copper remove the need for thermal vias? No. Copper spreads heat laterally, while vias carry it through the board to a surface that can reject it, and a heavy layer under a hot component is more useful when it is connected through the board as well.
Why is the copper thicker than the nominal value on the outer layers? Because the nominal figure describes the laminate foil, and the plating that fills the holes also adds metal to the traces on both outer surfaces.
