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I know what you’re thinking. He can’t possibly write an entire article dedicated to the use of copper in PCBs. I say, hold my beer.
Copper is the primary metal for standard PCBs. You can find these boards in pretty much every type of electronic device on the planet. Standard PCB capabilities depend on what materials are used and how they are constructed, but copper is the go-to.
Copper works very well for the conduction of electrical current in any environment. In fact, it has the highest electrical conductivity rating of all non-precious metals, making it highly effective for performance and cost. For a vast majority of PCB designs, copper is the conductor of choice. Copper conducts the signal and power for just about every electronic item in your life.
Way to go, copper! You are doing a great job. Keep it up.
There are however challenges associated with its use. Though incredibly conductive, copper is a relatively soft metal and susceptible to corrosion. Insufficient copper-to-edge clearances can result, potentially causing exposed copper, shorts, or corrosion. Resist can flake off of very narrow traces. During etching, long slivers can wander around in the bath. These loose cannons of conductivity can affix themselves to a board keeping unwanted copper from being etched away, creating unwanted circuits and failed boards.
So how do you get copper to function in a nice orderly manner the way your design calls for it? First, keep an eye out for potential problems during the design phase.
To properly control resistance and temperature rise, use a PCB trace width calculator to determine how thick and wide traces need to be. If your board is large enough, wider traces increase reliability. Skinny traces can get hot and release all of your magic smoke. Once it gets out, it’s really hard to put back in.
In design, you can reduce the likelihood of shorts, damaged traces, and exposed copper by keeping adequate distance between traces and mounting holes. Make sure to leave room for screw heads, washers, and other sorts of fasteners. Allow as much annular ring as possible for vias and plated holes. Within reason, more is always better. Vias need a bare minimum pad 0.010” larger than the drill. Other plated holes need 0.016” larger than the drill size. You can design to reduce the chance of slivers as well; keep widths and spacing above 0.006 inches.
For PCB manufacturing, copper is integrated into the board in three different ways. The raw laminate material (dielectric) is copper clad, meaning there is a consistent thickness of copper adhered to both sides of the dielectric material. This copper has either a plating mask or an etching mask applied that defines all circuitry and pads for that layer. Eventually all the copper that is between and surrounding the desired circuitry and pads will be chemically etched away, leaving only the needed copper.
For the external and other plated layers —where there are plated holes— the second application of copper is needed and added after the holes are drilled. This copper is a very thin layer of catalytically deposited copper used primarily to metalize the plated through holes to the point that they will conduct current for electroplating. This deposition of copper is a critical step to ensure that all the holes and vias will successfully plate with copper.
Once these holes are metalized and the plating mask is applied, it is time to electroplate all the circuitry, pads and through holes with copper. On a standard PCB, the electroplate process increases the thickness of the base copper foil by approximately 1.0 mil of copper and deposit a total thickness of 0.8 mils of copper into the holes for typical class 2 designs. This gives the circuitry the current-carrying capacity needed and makes the plated through holes robust enough to withstand thermal expansion during operation and maintain electrical connection to all of the necessary internal layers.
Whew! I’m out of breath now, pass me back that beer!
We lean on copper for a lot of reasons. Along with great conductivity, it is very tolerant of heat – aka “thermal stress.” This is increasingly important as more boards use exotic materials like ceramic, teflon, or next gen laminates that are often part of devices that need to function to their full potential in high temperatures.
Last, but not least, we should recognize copper’s other big selling points. It’s plentiful and relatively inexpensive. Over the past year at Sunstone, we have dedicated a fair amount to assessing the cost of quality. With supply chains in so much flux with respect to cost and availability, copper’s reliability and usability make standard PCB use that much more compelling for designers, engineers, and entrepreneurs.
Copper is pretty cool.
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