Introduction to Heavy Copper PCBs

Heavy copper printed circuit boards (PCBs) are specialized PCBs that use thicker copper traces than standard PCBs. While a typical PCB has copper traces in the 0.5 oz to 2 oz range (17 to 70 µm thick), heavy copper PCBs have copper weights of 3 oz or greater, with some going up to 20 oz (560 µm thick).

The thicker copper allows heavy copper PCBs to handle much higher currents than standard PCBs. This makes them ideal for power electronics applications such as:

• High-current power supplies
• Inverters and converters
• Motor drives
• Welding equipment
• Electric vehicle charging stations
• LED lighting
• Aerospace and military electronics

In addition to their high current handling capability, heavy copper PCBs also provide improved thermal management. The extra copper helps conduct heat away from components more efficiently.

Advantages of Heavy Copper PCBs

There are several key benefits that heavy copper PCBs provide compared to standard thickness boards:

Higher Current Carrying Capacity

The main advantage of heavy copper PCBs is their ability to handle much higher currents. The cross-sectional area of the traces increases linearly with copper thickness. This allows higher currents to flow without causing excessive resistive heating.

For example, here is a table comparing the typical current carrying capacity for traces of different widths and copper weights:

Trace Width (mm)	0.5 oz Cu (0.5A)	1 oz Cu (1A)	2 oz Cu (2A)	3 oz Cu (3A)	4 oz Cu (4A)
0.2	0.4	0.7	1.0	1.2	1.4
0.4	0.7	1.0	1.4	1.7	2.0
0.8	1.1	1.5	2.2	2.6	3.0
1.6	1.6	2.2	3.1	3.8	4.3

As can be seen, moving to heavier copper weights allows substantially more current flow for the same trace width. A 3 oz copper trace can handle 3 times the current of a 1 oz trace. This allows the PCB Designer to use fewer and/or narrower traces while still meeting current requirements.

Improved Thermal Management

In addition to increasing current capacity, the extra copper in heavy copper PCBs also helps to spread and dissipate heat. This is because copper is an excellent thermal conductor.

For a given temperature rise, a heavy copper PCB can handle more power than a standard board. Alternatively, for the same power level, a heavy copper board will run cooler. This can help improve reliability, especially for applications where the PCB is enclosed and air flow is limited.

The improved Thermal Conductivity of heavy copper is especially beneficial for PCBs with large components like MOSFETs, IGBTs, and rectifier diodes. These parts can generate significant amounts of waste heat. Using heavy copper PCBs helps get the heat out of the component and into the board where it can be dissipated.

Longer Trace Lengths

The higher current capacity of heavy copper also allows traces to be extended over longer distances without suffering from resistive losses or voltage drops. This can be useful in larger form factor PCBs where components may be spread out.

For example, a 1 oz copper trace carrying 1A over a 500 mm length would suffer a voltage drop of about 500 mV. The same trace in 3 oz copper would have a voltage drop of only 167 mV.

Of course, long traces can also have other issues like higher inductance and capacitance. So the ability to extend heavy copper traces is still limited in high frequency designs. But for low frequency/high current applications, it can be very beneficial.

Greater Mechanical Strength

Another advantage of heavy copper PCBs is their greater mechanical strength and rigidity. This is due to the extra thickness of the copper layers.

Standard 0.5 oz and 1 oz copper PCBs can be quite flimsy, especially in larger form factors. This can cause issues during assembly, as the board may bend or flex excessively.

The added stiffness of heavy copper helps prevent these issues. The boards stay flatter and more stable. This can improve yields and reduce defects during manufacturing.

The extra mechanical strength also makes heavy copper PCBs more resistant to vibration and physical stress. This can improve long-term reliability in harsh environments like vehicles and industrial equipment.

Disadvantages of Heavy Copper PCBs

While heavy copper PCBs offer many benefits, there are also some drawbacks to consider:

Higher Cost

The most obvious disadvantage of heavy copper PCBs is their higher cost compared to standard boards. There are several factors that contribute to this:

• Thicker copper foils are more expensive than thinner ones
• More copper is used overall, increasing material costs
• Plating and etching heavy copper takes longer and consumes more chemicals
• Drilling, routing, and other machining operations are more difficult and time-consuming
• Yields may be lower due to the greater demands on manufacturing processes

In general, moving to heavy copper will increase PCB costs by 20-50% or more. The exact increase will depend on the specific copper weight and other design factors.

However, it’s important to consider the total system cost rather than just the bare PCB cost. In many cases, using heavy copper can reduce the number of components, heatsinks, and other hardware required. This can help offset the higher PCB cost.

And in some applications, the improved performance and reliability of heavy copper may be worth the added expense. It all depends on the specific requirements and economics of the end product.

Reduced Routing Density

Another issue with heavy copper PCBs is reduced routing density. The thicker copper traces take up more space on the board, leaving less room for routing.

This is especially problematic for fine-pitch components and dense BGAs. The clearances between pads may not be sufficient for heavy copper traces. In some cases, it may be necessary to increase the component pitch or use via-in-pad designs.

Reduced routing density can also make it more difficult to escape traces from dense component clusters. More layers may be needed to route all the signals. This further increases cost and manufacturing complexity.

One way to mitigate this issue is to use heavy copper only where needed for high current traces. The rest of the design can use standard copper weights. This hybrid approach optimizes the tradeoff between current capacity and routing density.

Increased Minimum Hole Size

The thicker copper layers in heavy copper PCBs also require larger drilled holes. This is because the holes need to be plated with a sufficient thickness of copper to meet the plating aspect ratio.

For a given copper weight, the minimum finished hole size can be calculated as:

Minimum hole size = 8 * (copper thickness in µm) / (plating aspect ratio)

The plating aspect ratio depends on the skill of the PCB shop and their specific processes. But a typical value is around 8:1.

So for a 3 oz (105 µm) copper board, the minimum hole size would be:

Minimum hole size = 8 * 105 µm / 8 = 105 µm = 0.105 mm

This is significantly larger than the minimum 0.2-0.3mm holes possible with standard copper weights. It can create problems for routing dense components like 0201 resistors and capacitors. Even 0402 components may be difficult to use with heavy copper.

Once again, this issue can be mitigated by using heavy copper only where necessary and standard copper for the rest of the design. But it’s still something that needs to be carefully considered during the PCB Layout process.

Choosing the Right Copper Weight

With heavy copper PCBs, one of the key design decisions is selecting the appropriate copper weight. Here are some factors to consider:

Current Requirements

The most important factor in choosing a copper weight is the current requirements of the design. Higher currents will require thicker copper traces to avoid excessive heating and voltage drop.

As a general rule of thumb, you can use the following table to estimate the required copper weight based on the maximum current:

Max Current (A)	Copper Weight (oz)
1-5	0.5-1
5-10	1-2
10-20	2-3
20-50	3-4
50-100	4-5
100-200	5-10
200-400	10-20

Of course, the actual copper weight required will also depend on the trace width, ambient temperature, allowable temperature rise, and other design factors. These should be carefully calculated using tools like Saturn PCB Toolkit or PCB Trace Width Calculator.

Cost Constraints

As mentioned earlier, heavy copper PCBs are significantly more expensive than standard boards. So the copper weight selection needs to balance current requirements against cost constraints.

In some cases, it may be more cost-effective to use a lower copper weight and wider traces, rather than going to a heavier copper. This is especially true for designs with a lot of routing congestion.

The hybrid approach of using heavy copper only for the high current traces can also help reduce costs. The power and ground planes can use heavy copper while the signal layers use standard weights.

Manufacturing Capabilities

Another important consideration is the manufacturing capabilities of your PCB fabricator. Not all shops are equipped to handle very heavy copper weights.

Most PCB manufacturers can handle copper weights up to 4 oz without any issues. Some can go up to 10 oz or even 20 oz. But this requires more specialized equipment and processes.

It’s important to check with your PCB fabricator early in the design process to see what copper weights they can handle. You should also get quotes for different copper weights to see how it affects pricing.

Some PCB shops may also have restrictions on minimum trace/space widths, minimum hole sizes, and other design rules for heavy copper. Be sure to get this information upfront and design accordingly.

IPC-2152 Standards

Finally, it’s important to make sure your heavy copper PCB design meets relevant industry standards and guidelines. The most important of these is IPC-2152, the standard for determining current carrying capacity of printed board conductors.

IPC-2152 provides formulas and tables for calculating the maximum current for a given trace width, copper thickness, temperature rise, and other factors. It also specifies acceptable methods for testing and verifying the current capacity of PCB traces.

By designing to IPC-2152, you can ensure that your heavy copper PCB will perform reliably and meet industry accepted standards. This can be important for getting your product certified and accepted by customers.

Designing Heavy Copper PCBs

Designing heavy copper PCBs requires some additional considerations beyond standard PCB design. Here are some tips and best practices:

Use Wide Traces

To take full advantage of the current carrying capacity of heavy copper, it’s important to use sufficiently wide traces. A good rule of thumb is to use trace widths of at least 0.010″ (0.25mm) per amp of current.

So for a trace carrying 10A, you would want to use at least a 0.100″ (2.5mm) wide trace. Of course, this is just a starting point. The actual trace width should be calculated based on the specific copper weight, allowable temperature rise, and other factors.

In general, it’s better to err on the side of wider traces when using heavy copper. This helps ensure good current capacity and thermal dissipation. It also provides some margin for manufacturing tolerances.

Optimize Routing

Heavy copper PCBs often have much simpler routing than standard boards. This is because the high current traces tend to be short and direct, with few branches or vias.

When routing heavy copper, it’s important to focus on minimizing resistance and inductance. This means keeping traces as short and straight as possible. Avoid unnecessary turns or jogs.

For high frequency designs, it’s also important to consider the skin effect and proximity effect. These can cause the effective resistance of a trace to increase at high frequencies. Using wider traces can help mitigate this effect.

In some cases, it may also be beneficial to use multiple parallel traces instead of a single wide trace. This can help reduce inductance and improve current sharing.

Use Copper Pours

In addition to traces, heavy copper PCBs often make extensive use of copper pours or planes. These are large areas of copper that are used to distribute power or ground.

Copper pours provide several benefits for heavy copper designs:

• They help distribute current more evenly, reducing hot spots
• They provide a low impedance path for current, reducing voltage drop
• They help spread and dissipate heat more efficiently
• They can act as an EMI shield, reducing noise and crosstalk

When using copper pours, it’s important to make sure they are properly connected to the relevant traces and vias. There should be sufficient thermal relief and clearance around holes and pads.

It’s also a good idea to use thicker copper for the pours than the traces. This helps ensure good current carrying capacity and thermal dissipation. A common approach is to use 2-3 oz copper for the pours and 1-2 oz for the traces.

Consider Thermal Management

As mentioned earlier, one of the key benefits of heavy copper PCBs is improved thermal management. However, this still requires careful design and analysis.

For high power designs, it’s important to do a thorough thermal simulation of the PCB. This will help identify hot spots and ensure that temperatures stay within acceptable limits.

There are many software tools available for PCB thermal analysis, such as Ansys Icepak, Mentor Graphics FloTHERM, and Autodesk Fusion 360. These tools can simulate the heat flow and temperature distribution in the PCB based on the layout, copper weights, and power dissipation of components.

In some cases, it may be necessary to use additional thermal management techniques in addition to heavy copper. These can include:

• Thermal vias to transfer heat to inner layers or the back side of the board
• Heatsinks or heat spreaders attached to high power components
• Forced air cooling with fans or blowers
• Liquid cooling with cold plates or heat pipes

The specific thermal management approach will depend on the power levels, operating environment, and reliability requirements of the design.

Test and Verify

Finally, it’s important to thoroughly test and verify the performance of heavy copper PCBs before putting them into production. This includes both electrical and thermal testing.

For electrical testing, it’s important to verify that the PCB meets all relevant specifications for resistance, inductance, and current capacity. This can be done with a combination of manual probing, flying probe testing, and automated optical inspection (AOI).

Thermal testing is also critical for heavy copper PCBs. This can involve using thermocouples, infrared cameras, or other temperature sensors to measure the actual temperature rise and distribution in the board under load.

It’s a good idea to test the PCB under a variety of operating conditions, including worst-case scenarios like maximum load and ambient temperature. This helps ensure that the design has sufficient margin and reliability.

Any issues or discrepancies found during testing should be carefully analyzed and corrected before moving to production. It may be necessary to modify the layout, adjust the copper weights, or add additional thermal management features.

FAQ

What is the difference between heavy copper and standard PCBs?

The main difference is the thickness of the copper layers. Heavy copper PCBs have copper weights of 3 oz or greater, while standard PCBs typically use 0.5-2 oz copper. This allows heavy copper PCBs to handle much higher currents and power levels.

How much does heavy copper add to the cost of a PCB?

The cost increase for heavy copper can vary widely depending on the specific design and manufacturer. In general, using 3-4 oz copper will add 20-50% to the cost of a standard PCB. Very heavy copper weights (10 oz or more) can double the cost or more. It’s important to get quotes from multiple manufacturers and consider the total system cost, not just the PCB cost.

What are some common applications for heavy copper PCBs?

Heavy copper PCBs are commonly used in power electronics applications such as motor drives, inverters, power supplies, battery management systems, and renewable energy systems. They are also used in high reliability applications such as aerospace, military, and medical devices.

What are some design considerations for heavy copper PCBs?

When designing heavy copper PCBs, it’s important to use sufficiently wide traces to take advantage of the current capacity. Copper pours and planes should also be used to distribute current and heat. Thermal management is critical, and may require the use of thermal vias, heatsinks, or other techniques. Testing and verification are also important to ensure that the PCB meets all performance and reliability requirements.

Are there any limitations to using heavy copper PCBs?

The main limitations of heavy copper PCBs are increased cost, reduced routing density, and increased minimum feature sizes. Heavy copper can make it more difficult to route dense components like BGAs and 0201 passives. It also requires larger drilled holes and vias. These limitations need to be carefully considered during the design process.

Conclusion

Heavy copper PCBs offer