What is PCB Copper and Why is it Important?

Printed Circuit Board (PCB) copper refers to the conductive copper layers laminated onto the non-conductive substrate material to form the circuit board. The copper acts as the “wires” that electrically connect the various components mounted on the PCB. The thickness of the copper layer is typically measured in ounces per square foot (oz/ft²), with common thicknesses ranging from 0.5 oz/ft² to 3 oz/ft². However, for certain high-power applications, much thicker copper layers known as “heavy copper” ranging from 4 oz/ft² up to 20 oz/ft² may be used.

PCB Copper Thickness is a critical design parameter that impacts many aspects of the board’s performance, manufacturability, and cost. The primary reasons copper thickness is important include:
1. Current carrying capacity – Thicker copper can handle higher currents without overheating
2. Thermal management – More copper improves heat spreading and dissipation
3. Mechanical strength – Thicker copper makes the board more robust
4. Impedance control – Copper weight affects characteristic impedance of traces
5. Manufacturing process – Heavier copper requires adjustments to etching and plating processes

In most PCB designs, the copper weight specified is the minimum required to meet the electrical and thermal requirements of the circuitry. This is because thicker copper increases material costs and requires more sophisticated manufacturing processes. However, in applications involving high currents, high frequencies, or extreme environments, using heavy copper can provide significant performance benefits that justify the added costs.

Applications and Benefits of Heavy Copper PCBs

Heavy copper PCBs are used in a wide range of industries and applications where high power handling and reliability are critical. Some common applications include:

Industry	Applications
Automotive	Electric vehicle power inverters, battery management systems, motor controllers
Aerospace & Defense	Radar systems, avionic control units, military power supplies
Industrial	High-current motor drives, welding equipment, induction heating
Medical	X-ray machines, MRI scanners, defibrillators
Power Electronics	Solar inverters, wind turbine converters, EV charging stations
Telecommunications	Base station power amplifiers, microwave radios, satellite transceivers

The key benefits of using heavy copper in these applications include:

Improved Current Carrying Capacity

The current carrying capacity of a PCB trace is directly proportional to its cross-sectional area. Doubling the copper thickness doubles the current capacity, while also reducing resistive losses and voltage drop. This allows heavy copper PCBs to handle much higher power levels than standard boards.

For example, a 10 oz/ft² copper trace that is 200 mils wide can carry over 50 amps of current, compared to only 12 amps for a 2 oz/ft² trace of the same width. This increased capacity allows designers to use fewer or narrower traces, freeing up board space.

Enhanced Thermal Management

Copper is an excellent thermal conductor, so thicker copper layers provide a low-resistance path for heat to flow away from hot components and spread evenly across the board. This reduces peak temperatures and thermal gradients, improving reliability.

In power electronics applications, heavy copper PCBs are often used as heatsinks, with components mounted directly to the thick copper areas. The thermal conductivity of 10 oz/ft² copper is nearly 3 times higher than 2 oz/ft² copper, resulting in much lower thermal resistance.

Improved Mechanical Strength

Thick copper layers make PCBs much more rigid and resistant to flexing and vibration. This is especially important in automotive and aerospace applications where boards may be subjected to high G-forces and extreme temperature cycles.

A 10-layer board with 6 oz/ft² copper inner layers is about 50% thicker and 3 times stiffer than the same stackup using 1 oz/ft² copper. This extra strength helps prevent trace cracking and pad cratering failures.

Better Impedance Control

In high-frequency RF and microwave PCBs, the characteristic impedance of traces is determined by their width, spacing, and height above the reference plane. Using thicker copper allows traces to be narrower while maintaining the same impedance, which reduces conductor losses.

For example, a 50-ohm microstrip trace on 10 mil thick dielectric would need to be 30 mils wide with 1 oz/ft² copper, but only 20 mils wide with 2 oz/ft² copper. The narrower trace has 25% less surface area and thus lower skin effect losses.

Heavy Copper PCB Manufacturing Process Considerations

Making PCBs with heavy copper layers requires some special considerations and adjustments to standard manufacturing processes. The main challenges include:

Etching

Etching is the process of removing unwanted copper to define the circuitry pattern. Thicker copper takes longer to etch and is more prone to undercutting, where the sides of traces get eaten away. Etching heavy copper requires:
– Slower conveyor speeds and longer dwell times
– More concentrated etchant chemistry
– Tighter process control to maintain line width and prevent opens/shorts

Drilling

Drilling holes in heavy copper boards puts more wear and tear on drill bits, and the thicker material is more likely to cause burrs or other defects. Drilling heavy copper may require:
– More frequent drill bit changes
– Slower feed rates and speeds
– Specialized drill bit geometries or coatings
– Additional deburring steps

Plating

The plating process deposits additional copper into drilled holes to create conductive vias. Plating heavy copper takes longer and consumes more chemistry. It may require:
– Additional plating steps or thicker plating
– Pulse plating or other special techniques to ensure even coverage
– Larger plating tanks and more powerful rectifiers
– Extra rinsing and drying to remove chemical residues

Despite these challenges, PCB fabricators have developed reliable processes for manufacturing heavy copper boards. Many shops specialize in this area and have invested in equipment and expertise to handle the unique requirements. However, designers should expect that heavy copper boards may have slightly longer lead times and higher costs than standard boards due to the extra processing steps involved.

Design Tips for Heavy Copper PCBs

When designing PCBs that will use heavy copper, there are several best practices that can help ensure manufacturability and reliability:

1. Use a symmetrical stackup – Heavy copper layers should be balanced on both sides of the board to prevent warping. For example, a 4-layer board might have 4 oz/ft² copper on the outer layers and 2 oz/ft² on the inner layers.

2. Add tear-drops to pads – The junction between a pad and trace is a weak point prone to cracking under stress. Adding a teardrop shape to the pad reinforces this junction and strengthens the connection, especially important for heavy copper.

3. Specify selective plating if possible – In some designs, only certain areas of the board may need heavy copper. Selective plating allows the fab shop to add extra copper only where needed, saving cost and processing time. The designer must create copper “dams” on the outer layers to contain the plating.

4. Allow for etch factor – Because heavy copper is more prone to undercutting during etching, traces will end up narrower than designed. The fabricator can advise on how much etch factor to compensate for based on their specific process, but 1-2 mils per oz/ft² is a typical rule of thumb.

5. Follow fab drawing requirements – Include detailed fabrication notes on your drawings to communicate any special requirements related to heavy copper. Specify the finished copper weights, plating thicknesses, acceptable etch factors, and any other critical parameters. Work closely with the fab shop to ensure they can meet your specifications.

Heavy Copper PCB Design in EDA Tools

The specific steps to design a heavy copper PCB will vary depending on the EDA (electronic design automation) tool being used, but the general process is:

1. Define the layer stackup, specifying the copper weight for each layer
2. Create the board outline and place components
3. Route traces, polygons, and power planes, taking into account the current carrying and impedance requirements
4. Add teardrops, thermal reliefs, and other copper features to pads and vias
5. Generate Gerber files and fab drawings, including any special notes related to heavy copper

Most modern EDA tools have built-in support for defining custom stackups with heavy copper layers. For example, in Altium Designer, this is done in the Layer Stack Manager:

In the above example, the top and bottom layers are specified as 4 oz/ft² copper (1.4 mils thick), while the inner layers are 1 oz/ft² (0.36 mils thick). The tool automatically calculates the total stackup thickness and displays a cross-sectional view.

When routing traces, the designer can specify the width and copper weight using the PCB Rules and Constraints Editor:

Here, a routing rule is created for a power net class, specifying a minimum trace width of 100 mils and a copper weight of 4 oz/ft². The tool will then ensure that all traces assigned to this net class meet these constraints.

Finally, when generating manufacturing outputs, the Gerber files will include the specified copper weights for each layer, and the fab drawing can be annotated with any special instructions related to heavy copper:

In this example, the fab drawing notes specify the required copper weights for each layer and call out the selective plating areas.

Frequently Asked Questions (FAQ)

1. What is considered “heavy copper” in PCBs?

Heavy copper typically refers to copper weights of 4 oz/ft² or greater. Standard PCBs usually use 0.5-2 oz/ft² copper, so 4 oz/ft² and up is considered heavy.

2. Can I mix heavy and standard copper weights on the same board?

Yes, it is possible to use different copper weights on different layers, or even on different areas of the same layer (selective plating). However, the stackup should still be symmetrical to prevent warping.

3. Are there any disadvantages to using heavy copper in PCBs?

The main disadvantages are increased material cost and longer processing time, as heavy copper requires more etching, drilling, and plating. The final boards may also be thicker and heavier overall.

4. What are some common applications for heavy copper PCBs?

Heavy copper is commonly used in high power applications such as motor drives, inverters, and power supplies. It is also used in high-frequency RF and microwave boards for impedance control.

5. How does heavy copper affect PCB assembly?

Heavy copper boards may require special handling during assembly, as the extra weight can put more stress on components during reflow soldering. The thicker copper also acts as a heatsink, so more time or higher temperatures may be needed to achieve proper solder joint formation.

Conclusion

Using heavy copper in PCB fabrication offers many benefits for high-power and high-reliability applications. The increased current carrying capacity, thermal conductivity, and mechanical strength of heavy copper allow PCBs to handle greater electrical and environmental stresses.

However, designing and manufacturing heavy copper boards requires careful consideration of the unique processing challenges and costs involved. Designers must work closely with their fabrication partners to ensure that the boards meet all performance and quality requirements.

As PCB technology continues to advance, the use of heavy copper is becoming more common in a wider range of industries and applications. By understanding the benefits and tradeoffs of this material, designers can make informed decisions about when and how to incorporate heavy copper into their products.