Heavy copper circuits
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 PCB and extreme copper printed circuit boards.
What defines a heavy copper PCB?
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/ft 2 to 3oz/ft 2. A heavy copper circuit is manufactured with copper weights anywhere from 4oz/ft 2 to 20oz/ft 2. Copper weights higher than 20oz/ft 2, and up to 200oz/ft 2 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.
It’s probably no one really need fine line with heavy copper like 5 oz thick. If it is real, can we make it? Well, I did not specify the trace space. If no space issue and the definition of the trace width is to measuer its bottom side, then what will be the width of this 5 mil wide trace on its top side? For a etching factor of 2, the width will be zero. It means the 5 mil wide pattern will not able to created by standard etching process. So, how do I make this happen?
I invented a process to make thick copper by electroplating process. The purpose of the process is making heavy copper and thin copper on the same layer. Use the same way, we can make 5 oz copper trace with width of 5 mil. In fact, the more stringent criteria like 8 oz thick with 5 mil wide will probably OK. Since 8 oz copper is more than 11 mil thick, the trace aspect raio is bigger than 2!
So, how do I make it? We can start with 6 mil thick single side laminate with 1 oz copper clad. Cover the claded copper by photoresist or tape. Then, use laser to cut the circuits pattern from its no clading side with width of 5 mil (you can image that 4 mil or thinner is also possible). The laser cutting needs to cut into the copper foil on the opposite side so that the copper can be exposed. The claded copper is usually not damaged by laser if it is under control. The space between 2 traces can be 5 mil or higher, depend on how serious you have to concern on CAF issue. A desmear process is needed if you use CO2 laser. It can remove any resin residue on the bottom of the copper. Then, we can put it into the plating tank for copper plating. This is just like a pattern plating process used in PCB with very thick plating resisit. The plated copper will fill the laser cut groove, or circuit pattern, after enough plating time. In this case, the plated copper thickness is roughly 6 mil. We can use this plated laminate, after remove the photoresist, as an inner layer or outer layer core. After lamination with other circuits, the plated core can be etched from top side with copper foil clading. Together with the plated copper, the 1 oz copper foil plus 6 mil can have the total copper thickness over 5 oz.
Like the previuos article, the etching can make the board having thinner copper when this etching process does not on the plating area. So, the thick and thin copper can be done on the same layer. Since most of the heavy copper is buried into laminate, the this and thick copper is on the same level without worry its height difference. And, since the heavy copper buried into core, we don’t need to worry the peel strength of this thick trace even it has finer line.