What are Heavy Copper PCBs?

PCBs carry current through the copper tracks etched on them. The current carrying capacity of a copper track can be enhanced by increasing the width of the copper tracks, but this is not always feasible. Another way to increase the current-carrying capacity without changing the width of the copper traces is to use ‘Heavy Copper PCBs’ that carry more current and improve efficiency with better heat management.

Although, there are no defined rules that specify the amount of copper to use when fabricating heavy copper PCBs. The PCB industry follows a rule, which says, three or more ounces of copper on any PCB layer classifies as a ‘Heavy Copper PCB’. We have seen instances where there are up to 8 oz used for the inner layers of a PCB and up to 40 oz used on the outer layers of the PCB.

Characteristics of Heavy Copper PCBs

• Simple Strong Structure: Heavy copper PCBs can carry high-currents, and have a higher density as they reduce the overall layer count. They also keep the structure strong and less complex.
• Low Failure Rates: These PCBs generate more heat, which is subsidized by copper, resulting in reduced failure rates. Copper has high heat-conducting ability, so it is good at conducting the heat away from the sensitive components.

Design Consideration for Heavy Copper PCBs

First of all, the designers decide the current values they need a track to carry. They can then design the PCB and the copper tracks by carefully selecting the copper thickness and width. The amount of heat a heavy copper PCB generates is closely related to the amount of current it carries. So the operation temperature can also be estimated.

When current passes through the copper track, its resistance causes it to be hot. The main idea is to transit this heat into the environment through conduction and convection. Here, a designer’s job is to calculate the amount of current a track will carry before the temperature reaches around 30°C. It is worth noticing that the inner PCB layers can bear only about 50% of the current as compared to the outer layers. The Glass transition temperatures (Tg) of the copper and substrate material is also important, as a high current may cause cracks and layer separation.

Board surface thickness can be increased through additive plating, say electroplating. It reduces the resistance of copper traces by improving the heat conduction. Electroplating is also used by fabricators for PTHs to reduce their thermal resistance. Resistance reduction helps to dissipate heat efficiently through conduction, convection, and radiation phenomenon.

Heavy Copper Fabrication Methods

• Blue Bar Method: This method is used to fabricate automotive PCBs. It involves embedding thick copper bars into PCBs for carrying the extra current. Since high-power circuits require only a few traces to carry more current. This method saves both material and reduces the PCB weight. For multi-layer PCBs, fabricators have to pay extra attention to the copper fill level between inner layers to avoid future delamination.
• Embedded Copper Method: In this method, the heavy copper PCB is fabricated with a flat surface with heavy copper embedded within the prepreg resin. The thickness of the heavy copper to be used depends upon the thickness of the resin.

Manufacturing Limitations of Heavy Copper PCBs: Fabrication of thick copper PCBs has some limitations:

• Difficult to etch, which increases production time and the cost
• More copper is required to be removed during etching.
• Heavy copper traces make the board surface uneven, which makes solder mask application difficult.

Applications of Heavy Copper PCBs

Heavy Copper PCBs find applications in the military (weapon control systems, power supply for RADAR systems, windings of high-power plant transformers, battery chargers & monitoring systems), power amplifier modules, DC-to-DC power converters, heat dissipation products, automotive power distribution junction boxes, high power distribution boards, and others that require high currents.