Heavy copper usually means the copper foil thickness of PCB higher than 3 ounce (4 mil or 100 micron). It mostly used for high power (electric current) electronics like power supply or some power circuit in automotive industry. It can be design in inner layer or outer layer. In PCB production, it is more difficult than traditional circuits with copper foil less than 2 ounce.
Etching of pattern is one of the major issue need to overcome for heavy copper PCB. When the copper thickness become thicker, the etching process time will be longer. When the etching solution remove the copper vertically, it will also cause side etching simutaneously. Finally, the pattern will have a big “foot” which have much smaller width on the top than on the bottom. It always reduce the copper volume used to transfer current. To meet the design criteria, PCB fabricator needs to make trace width compensation first so that the line width can pass the specification. It means a wider trace space is also important. When the copper thickness higher than 5 ounce, the problem become more difficult. The thicker the copper foil, the wider the design trace/space width is a common perception.
The second process to be concerned is the lamination. To fill the space being etched off, it needs a lot of resin to fill in. Usually the resin need to come from the prepreg. So the PCB fabricator is always use multiple high resin content prepreg in heavy copper construction. However, it will cause many problems.
The third issue need to be concerned is the drilling. When the design has many thick copper, the drilling parameter need to adjust to more similar to drill a thick copper plate. The drill bit wear and debris removal need to be process carefully.
The forth problem is on the solder mask process. It’s difficult to apply enough solder mask to cover thick copper pattern and base material with severe height difference. Usually, manufacturer need to fill more solder mask to fill in the space between the trace. It’s common to apply multiple printing. The first printing fill most of the pattern gap and second printing cover enough thick of solder mask on the trace pattern. But it still have some risk of voiding. The thick solder mask is also more difficult to exposure and develop. If the exposure energy is too weak, then some undercut problem may happen.
One issue that power supply designer looked is the high potential test (Hi-Pot Test). To get enough insulation to resist high voltage testing, the material, multilayer stack-up, inner layer cleaness, etching and design are all important. Sometimes the drilling, routing and plating also play an important role to get good electrical insulation.
When the copper thickness go even higher like 10 ounce or higher, the manufacturing process needs to make some change. Manufacturer can apply some resin on the trace gap first to prevent too much resin filling or the risk of voiding. This is also the key to manufacture multiple thick of copper pattern on one layer.
Applications which require running a high power we offer heavy and extreme copper tracing, plated through holes (PTH), surface pads and ground planes. By electroplating heavier copper to your pcb you are insuring high reliability and efficient power distribution. In fact, we have found that heavy copper tracing can even act as its own heat exchanger dissipating as much as 20% of overall temperature. Extreme copper thickness can be plated as high as 30oz.