Heavy Copper in FPC

This Gold Phoenix PCB Knowledge Center article explains heavy copper in flexible printed circuits (FPC), including copper weight standards, impact on flexibility, design considerations, drawbacks, and best practices.

Creating a high-performance flexible printed circuit board depends on selecting the appropriate copper weight. In high-temperature, high-power, and long-lasting applications, the copper weight becomes even more important.

In flexible PCBs, copper weight is a critical factor that influences both the electrical performance and the mechanical characteristics of the circuit. Understanding how copper weight impacts the functionality of a flexible printed circuit board, the impact that copper weight has on flexibility, design considerations, and drawbacks of heavier copper weights is critical for anyone developing a new high-performance flexible PCB.

Introduction

With modern flexible printed circuit boards, a copper weight of 2 oz per square foot (oz/ft²) or higher is considered “heavy copper”. Typical rigid PCBs have a copper weight of around 1 oz/ft². Thicker copper layers can enhance the current-carrying capability of a circuit, but they also come with significant challenges when it comes to manufacturability, flexibility, and overall cost.

Impact Of Heavy Copper

Perhaps the biggest impact of heavy copper usage within a flexible PCB is on the board’s overall flexibility. Higher copper weights result in less flexibility because the copper itself is thicker and more difficult to bend. The added rigidity also comes from the additional adhesives used for lamination and coverlay (insulative layers).

Example of a flexible printed circuit board with heavy copper weights.

Reduced flexibility can affect the functionality of a flexible PCB, so there is often an effort to balance flexibility and capability to carry a current.

One option is to split the copper weight across multiple layers instead of concentrating the entire weight in a single layer. Doing this can support overall performance while also helping with manufacturability and cost reduction.

Line Widths And Spacing

Heavy copper weights also influence the minimum line widths and spacing in the circuit design. Thicker copper runs require wider lines in the flexible PCB to accommodate the conductor, plus additional space for manufacturing.

This increase in line widths and spacing can limit the circuit’s density, making it challenging to achieve high-density interconnects (HDI) in designs that also require heavy copper.

When working with copper weights of 3 oz. and above, consider additional processes such as plating or laminating copper sheets together.

For instance, a 3-oz. copper weight typically requires plating from 2 oz. copper up to 3 oz., or special ordering of material if the quantities are large enough. Beyond 3 oz., manufacturers may need to laminate copper sheets together, which further complicates the manufacturing process and increases costs.

Drawbacks

Using heavy copper in flexible printed circuit boards comes with some noteworthy drawbacks. The increased copper weight leads to higher material costs and more complex manufacturing processes.

In addition, the reduced flexibility and the necessity for specialized manufacturing techniques can limit the number of manufacturers capable of producing heavy copper flexible printed circuit boards. This can make it more difficult to find a manufacturer capable of building heavy copper flexible circuit boards.

Another challenge is the availability of coverlay materials. As copper weights increase, standard coverlay materials may no longer be suitable. Instead, designers will need to rely on flexible liquid photoimageable (LPI) solder masks. This limitation can affect the overall design and durability of the flexible PCB.

The use of heavy copper can also result in longer lead times, especially for specialty copper weights that may not be readily available. This extended lead time can impact project timelines, making it essential for designers to plan accordingly.

Finally, the reduced flexibility associated with heavy copper can limit the number of layers in a flexible PCB design. This limitation is particularly important in complex circuits where multiple layers are required to achieve the desired functionality.

Why Use it?

Heavy copper weights are indispensable in certain applications. Heavier copper has superior current-carrying capacity and thermal conductivity. These properties make heavy copper flexible PCBs ideal for power electronics, military applications, certain medical devices, automation systems, and electric vehicles.

In power applications, the need to handle high current loads without significant voltage drops or overheating is critical. Heavy copper provides the necessary cross-sectional area to carry large currents efficiently while minimizing the overall heat that’s generated.

In applications where thermal management is a concern, like power supplies or motor drives, the enhanced thermal conductivity of heavy copper can help dissipate heat more effectively. This can significantly improve the reliability and lifespan of the circuit.

Best Practices For Implementing

When designing with heavy copper, it’s crucial to consider both the electrical and mechanical requirements of the application. As previously mentioned, splitting the copper weight across multiple layers can help balance performance with manufacturability. Working closely with manufacturers to choose an appropriate copper weight can also help. This can help balance cost, functionality, and turnaround times.

In applications where flexibility is critical, designers should evaluate whether heavy copper is necessary for all areas of the circuit or if it can be localized to specific regions. This approach can maintain flexibility where needed while still benefiting from the enhanced current-carrying capacity of heavy copper.

Summary

Heavy copper weights play a vital role in the performance of flexible printed circuit boards, particularly in applications requiring high current-carrying capacity and thermal management. However, their use comes with significant challenges, including reduced flexibility, increased manufacturing complexity, and higher costs.

By understanding these factors and using best practices in design and manufacturing, engineers can effectively leverage heavy copper in their flexible PCB designs, ensuring that the final product meets both electrical and mechanical requirements.