Various power electronics products are being designed every day for a range of applications. Increasingly, these projects are taking advantage of a growing trend in the printed circuit board industry: heavy copper and extreme copper printed circuit boards.
What defines a heavy copper circuit? Most commercially available PCBs are manufactured for low-voltage/low-power applications, with copper traces/planes made up of copper weights ranging from 1/2oz/ft2 to 3oz/ft2. A heavy copper circuit is manufactured with copper weights anywhere from 4oz/ft2 to 20oz/ft2. Copper weights higher than 20oz/ft2, and up to 200oz/ft2 are also possible and are referred to as extreme copper. For our discussion here, we will focus primarily on heavy copper. The increased copper weight, combined with a suitable substrate and thicker plating in the through holes, can transform the once unreliable, weak circuit board into a durable and reliable wiring platform.
Sample featuring 2oz, 10oz, 20oz, and 30oz copper features on the same layer.
The construction of a heavy copper circuit endows a board with benefits such as:
- Increased endurance against thermal strains
- Increased current-carrying capacity
- Increased mechanical strength at connector sites and in PTH holes
- Use of exotic materials to their full potential (i.e., high temperature) without circuit failure
- Reduced product size by incorporating multiple copper weights on the same layer of circuitry (see image above)
- Heavy copper-plated vias carry higher current through the board and help to transfer heat to an external heat sink.
- Onboard heat sinks directly plated on to the board surface using up to 120oz copper planes
- Onboard high-power-density planar transformers
Although the disadvantages are few, it's important to understand the heavy copper circuit's basic construction to fully appreciate its capabilities and potential applications.
Heavy copper circuit construction
Standard printed circuit boards, whether double-sided or multilayer, are manufactured using a combination of copper etching and plating processes. Circuit layers start as thin sheets of copper foil (generally 0.5oz/ft2 to 2oz/ft2) that are etched to remove unwanted copper and plated to add copper thickness to planes, traces, pads, and plated through holes. All of the circuit layers are laminated into a complete package using an epoxy-based substrate, such as FR4 or polyimide.
Boards incorporating heavy copper circuits are produced in exactly the same way, albeit with specialized etching and plating techniques, such as high-speed/step plating and differential etching. Historically, heavy copper features were formed entirely by etching thick copper-clad laminated board material, causing uneven trace sidewalls and unacceptable undercutting. Advances in plating technology have allowed heavy copper features to be formed with a combination of plating and etching, resulting in straight sidewalls and negligible undercut.
Plating of a heavy copper circuit enables the board fabricator to increase the amount of copper thickness in plated holes and via sidewalls. It's now possible to mix heavy copper with standard features on a single board. We refer to this as PowerLink. Advantages include reduced layer count, low-impedance power distribution, smaller footprints, and potential cost savings. Normally, high-current/high-power circuits and their control circuits were produced separately on separate boards. Heavy copper plating makes it possible to integrate high-current circuits and control circuits to realize a highly dense yet simple board structure.
The heavy copper features can be seamlessly connected to standard circuits. Heavy copper and standard features can be placed with minimal restriction, provided the designer and fabricator discuss manufacturing tolerances and abilities prior to final design (see the image on the next page).