Heavy copper PCBs
Copper is second only to Silver in the electrical and thermal conductivity of metals. With greater workability and a much lower cost than silver, copper is the material of choice for PCB fabrication. Copper clad dielectrics and bare copper foil are readily available in a variety of factory-standard thicknesses ranging from 0.25 [oz][ft] -2 to 4.0 [oz][ft] -2 . Thicker copper is available within a day from the warehouses of material suppliers like Insulectro, but aren’t necessarily stocked on-hand inside PCB fabricator storerooms. Once the density of copper exceeds 4 [oz][ft] -2 , fabricators and engineers tend to refer to these boards as heavy copper PCBs. There’s a simple reason -- these PCBs might look normal, but are at least four times denser than you’d expect of a standard PCB, and feel unusually heavy in your hands. That extra heft is as unexpected as picking up a fully loaded suitcase you anticipated being empty. The extra copper provides two benefits. ■ The larger cross-sectional area provides reduced resistance, which means less heat generation in the traces for direct (DC) and low-frequency alternating currents (AC).
You want to put how much current through your PCB?
■ The extra copper helps to conduct heat from higher- temperature to lower- temperature parts of your board. With a larger exposed surface area, it provides additional area for convective and radiative cooling. The goal of using heavy copper is to limit any high temperature excursions that shorten the life of your PCB. But it’s not all good news -- heavy copper has many drawbacks. ■ Increased minimum trace and air-space width to accommodate increased copper-etching cut- back and compensation. ■ Increased time in electroplating tanks to add copper traces and etching tanks to remove copper and create air-spaces. ■ Increased difficulty in PCB assembly and rework stages. ■ Increased weight
By Mark Hughes
How much copper is too much copper? Or should I design to the standards? Every time a client calls to ask me about using heavy copper on their next PCB design, the first question I always ask is “How did you determine your copper weight?” and nine times out of ten, they state “IPC-2221.” With that little nugget of information, I can usually talk them out of hiring me within five minutes, by sharing what I’m about to share with you. The best option you can use to determine your minimum copper trace width and thickness is a multiphysics or computational fluid dynamics simulation. But most electronics engineers lack the expertise, time, and most importantly, software licenses to fully simulate most designs.
Introduction A client once asked “What is wrong with my electronic speed controller design?” I quipped “it’s two of the three parts of the fire triangle.” Before you release the magic smoke from your next design, take a moment to learn about high current and heavy copper PCB design best practices that apply to everything from Automotive, to FPGA to Servers, to Drones! When I hear “High Currents,” My mind always drifts towards one thing: Heavy Copper. Let’s start our journey there!
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