Copper Weight Selection for PCB Design

Copper weight is the thickness of the foil used on a layer, expressed in ounces per square foot. One ounce is about 35 micrometres, and the number decides how much current a trace can carry, how well heat spreads, how finely the copper can be patterned and how much the finished board costs. Choosing it is a balance between electrical need and fabrication capability.

What the Weight Actually Means

The figure describes the starting foil, not the finished thickness. Plating adds copper to the surface and into the holes, so a finished outer layer is always thicker than its nominal weight, and the finished inner layer thickness depends on the lamination process. When a specification quotes a weight, it is quoting the input material.

Weight also determines the mechanical behaviour of the laminate. Heavier copper makes the board stiffer and changes the thermal expansion in the plane of the layer, which affects how the assembly behaves during reflow.

Current Capacity

Current capacity comes from the cross sectional area of the trace, which is the width multiplied by the thickness after plating. Doubling the weight roughly halves the resistance of a trace of the same width, or allows the width to be halved for the same resistance. The temperature rise above ambient is the limiting factor rather than the current itself.

The usual calculations assume a permissible temperature rise and a trace in still air, but the real board has neighbouring traces that also heat and a solder mask that changes the emissivity. Our article on high current capacity explains how to work with those effects for a heavy load.

Thermal Spreading

Heavy copper spreads heat laterally, which is useful under a power device and helpful for keeping a hot component from warming its neighbours. A two ounce plane under a power stage can reduce the temperature rise more than any change to the component selection.

The benefit works in both directions. A large copper area also makes soldering harder, because it draws heat away from the joint during reflow. Thermal reliefs and a profile with a longer soak compensate, as described in our article on copper balance.

Etching and Feature Resolution

Thicker copper is harder to etch. The etchant removes material laterally as well as vertically, so a heavy layer produces wider traces than designed and narrower gaps than intended. The minimum achievable trace width and spacing both increase with weight.

The practical limits are roughly 100 micrometres at one ounce, 150 micrometres at two ounces and considerably more beyond that. Where a design needs fine features and heavy copper, the answer is usually a mixed stackup: a heavy inner layer for power and a light outer layer for routing.

Stackup and Impedance Effects

Changing the weight of a layer changes its thickness and therefore the impedance of the traces referenced to it. A controlled impedance design that is requoted at a different weight will not meet the same tolerance without adjustments to the dielectric thickness or the trace width.

Layer thickness also affects how much resin has to fill the space between copper features during lamination. Heavy copper with tight spacing leaves deep valleys that the prepreg must fill, and insufficient fill leads to voids and delamination. The stackup has to account for that, and the fabrication notes should say so explicitly.

Cost and Availability

Heavier foil is more expensive and less widely stocked, and the fabrication steps take longer because etching and lamination are both more demanding. Lead time increases, and the number of suppliers able to build the design shrinks with each step up in weight.

Beyond about four ounces, the process usually changes to thick copper plating or to the sequential build of multiple layers, and the cost curve steepens. That is the point at which the design should be reconsidered rather than forced through the standard process.

Selecting by Function

A practical approach is to assign weight by function. A signal layer rarely needs more than one ounce, a ground plane benefits from one or two, and a power layer that carries tens of amps may need four or more. Mixed weight stackups are common and cost less than making every layer heavy.

Where a heavy layer is needed only in a small area, a copper bar or a busbar on the assembly may be cheaper than heavy copper throughout. The comparison should include the assembly cost, because a soldered busbar adds a process step.

Confirming the Choice

Confirmation comes from measurement. A thermal image of a prototype under load shows whether the copper is spreading heat as intended, and the finished copper thickness should be verified on a coupon, since plating varies across a panel. The methods are described in our note on plating thickness.

Where the design is marginal, the cheaper answer is often a wider trace or a second layer in parallel rather than heavier foil. That decision is best made with a calculation and a measurement rather than with a rule of thumb carried over from a previous project.

Process Control and Verification

On a design of this kind, two ounce copper is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, two ounce copper is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

FAQ

Is two ounce copper needed for a power supply? Often one ounce is sufficient if the traces are wide. Two ounces helps where space is tight or where the copper also has to spread heat.

Can heavy copper be used with fine pitch components? The outer layer usually has to stay light for fine features. Heavy copper belongs on inner layers or on areas away from fine pitch routing.

Does heavier copper change impedance? Yes, because it changes the geometry. Controlled impedance traces have to be recalculated whenever the weight or dielectric thickness changes.