What material is most commonly used to form the traces on PCBs?

What are PCB traces?

PCB traces are the thin lines of copper that form the conductive pathways on a printed circuit board. Traces carry electrical signals and power between the various components mounted on the PCB. The trace pattern is carefully designed in PCB layout software to route all the necessary connections while avoiding interference between signals.

Characteristics of PCB Traces

Some of the key characteristics of PCB traces include:

Width – Trace width is determined based on the amount of current the trace needs to carry. Wider traces have lower resistance and can handle higher currents.
Thickness – Standard 1 oz copper results in 35 μm thick traces while heavy 2 oz copper doubles the thickness to 70 μm. Thicker traces lower resistance.
Spacing – The spacing between traces is important to avoid unintended short circuits and to minimize crosstalk between high-speed signals on adjacent traces. Typical trace spacing is around 0.006″ minimum.
Length – Shorter traces are generally preferred to minimize resistance and signal integrity issues. The trace lengths and routing are optimized in PCB layout.

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Copper – The Most Common PCB Trace Material

Copper is by far the most commonly used material for PCB traces. Nearly all rigid and flexible PCBs use copper traces to form the interconnects between electronic components.

Advantages of Copper for PCB Traces

Copper offers an excellent combination of properties that make it well-suited for use in PCBs:

High electrical conductivity
As an excellent electrical conductor, copper allows signals and power to pass through PCB traces with minimal resistive losses. This is critical for maintaining signal integrity and minimizing voltage drop in power distribution nets.
Good thermal conductivity
Copper is also a very good thermal conductor. This allows copper traces to help spread and dissipate heat generated by components on the PCB. High-power traces can aid in transferring heat to other copper areas like ground planes.
Reasonable cost
While not the cheapest metal, copper provides good value as a trace material. It is more affordable than other highly conductive metals like silver or gold.
Manufacturability
Copper etches cleanly and consistently, making it compatible with standard PCB fabrication processes. The copper thickness can be varied by plating to meet trace resistance and current-carrying requirements.
Reliability
Copper traces have a proven track record of reliability in PCB applications. Copper is resistant to corrosion and can withstand the high temperatures in PCB Soldering and reflow processes.

The PCB Copper Foil Specifications

The base copper foil used in PCB manufacturing is an important factor in the quality and performance of the final board. There are a few key parameters to consider.

Copper Weight

Copper foil weight refers to the thickness of the copper expressed in ounces per square foot. The common options are:

Copper Weight	Thickness (μm)	Typical Applications
0.5 oz/ft2	17.5	Very thin, fine-pitch traces
1 oz/ft2	35	Most common, standard density PCBs
2 oz/ft2	70	High-current and power applications
3 oz/ft2	105	Very high-current, e.g. battery packs

Thicker 2 oz and 3 oz copper helps minimize voltage drop and resistive heating in power distribution traces. 1 oz is the default for signal traces while 0.5 oz enables finer pitch traces for dense designs.

Copper Foil Type

There are two main types of copper foil used in PCBs:

Electrodeposited (ED) copper foil
ED copper is made by electroplating copper onto a rotating drum. It has a smooth shiny side (drum side) and a rough matte side with dendritic copper nodules that promote adhesion to the base laminate. ED copper is lower cost but has some impurities and physical defects.
Rolled annealed (RA) copper foil
RA copper is made by mechanically rolling and annealing copper into a foil. Both sides are smooth and RA copper has a finer grain structure than ED copper. RA copper is higher purity and has higher elongation at break but costs more than ED copper.

For most PCB applications, standard ED copper foil is used. RA copper may be used where higher reliability and ductility is required, such as in flexible PCBs.

Copper Surface Finish

After the PCB is fabricated, the exposed copper traces are typically coated with a surface finish. The main options are:

Hot air solder leveling (HASL) – solder is applied to the copper by passing the PCB over a molten solder bath followed by hot air leveling
Electroless nickel immersion gold (ENIG) – a thin layer of nickel is applied to the copper followed by a thin layer of gold
Immersion silver – a thin layer of silver is chemically deposited onto the copper
Immersion tin – similar to silver, a thin layer of tin is chemically deposited onto the copper

HASL is one of the most common and economical surface finishes and provides good solderability. ENIG offers better planarity and a diffusion barrier for higher reliability in more demanding applications. The immersion silver and tin finishes are lead-free and RoHS compliant options with good shelf-life.

The choice of copper surface finish depends on factors like cost, solderability requirements, shelf-life, fine-pitch compatibility, and the presence of sensitive components like BGAs.

Copper Pour and Power Planes

In addition to thin copper traces that route signals between components, larger copper areas are also commonly incorporated into PCB designs.

Copper Pour

Many PCB designs make use of copper pour, also known as copper fill. After routing all the traces, the unused areas of the outer layers are filled with copper, usually connected to ground. Some of the advantages of copper pour are:

Reduces EMI by minimizing the loop area between signals and the reference ground plane
Improves ESD protection by providing a low-impedance path to ground
Helps with heat spreading and dissipation
Provides shielding for sensitive signals
Minimizes etchant consumption during PCB fabrication
Improves mechanical strength of the PCB

Copper pour is created in the PCB layout software by defining the pour areas and connecting them to a net, usually ground. The PCB maker will remove all the unwanted copper but leave the copper fill pattern intact.

Power and Ground Planes

On multi-layer PCBs, one or more entire inner layers are often dedicated as uninterrupted copper planes for power distribution. A solid copper plane offers several benefits over just routing power traces:

Lower DC resistance for more efficient power distribution
Lower AC impedance provides better power integrity for ICs
Greater current-carrying capacity without excessive voltage drop
Spreads and sinks heat generated by active components on the PCB
Acts as a reference plane for controlled impedance signals on adjacent layers

4-layer PCBs typically have one internal ground plane and one power plane, while 6-layer and higher PCBs may have multiple power planes for different supply voltages. The use of copper power and ground planes is a key aspect of good PCB design for reliable high-speed and mixed-signal circuits.

Alternative PCB trace materials

While copper is the dominant choice for PCB traces, there are some alternative materials that may be used in certain specialized applications.

Silver Traces

Silver is sometimes used as a trace material where very high conductivity is needed. Silver has even lower resistivity than copper. However, silver is significantly more expensive and its use is limited to niche applications that can justify the high cost.

Silver is also used as the conductive material in some specialized types of PCBs:

Printed silver ink traces may be used for very low-cost, disposable electronic circuits on paper or plastic films.
Screen-printed silver traces are used in applications like membrane switches, where flexibility and thinness are important.

Silver migration can be a concern, so silver traces are often immersion plated with gold or another metal for protection.

Carbon Nano-material Traces

Conductive carbon nano-materials like graphene and carbon nanotubes have been an active area of research for potential use in PCBs. These materials are of interest because they combine high electrical and thermal conductivity with mechanical flexibility.

Some of the key challenges with carbon nano-material traces are:

High material costs
Difficult to scale up the manufacturing to large panels
Lower conductivity than copper
Reliability concerns

So far carbon nano-material traces have been limited to research and some niche applications. Advances in materials and manufacturing technology may enable wider adoption in the future, particularly for flexible and wearable electronics.

Frequently Asked Questions

What are PCB traces made of?

PCB traces are most commonly made of copper. Nearly all rigid and flexible PCBs use copper foil that is laminated onto the insulating substrate and then patterned and etched to form the interconnecting traces.

Why is copper used for PCB traces?

Copper is an excellent electrical and thermal conductor while still being relatively affordable compared to other metals like silver and gold. Copper’s material properties and manufacturability make it well-suited for creating the conductive trace patterns in PCBs.

What are the different copper weights used in PCBs?

Common copper foil weights used in PCBs include:
– 0.5 oz (17.5 μm thick) – for very fine traces and dense designs
– 1 oz (35 μm thick) – most common, used for typical signal traces
– 2 oz (70 μm thick) – for high-current traces and better heat spreading
– 3 oz (105 μm) – for very high-current applications like battery packs

What are some alternatives to copper traces?

The main alternatives to copper traces are:
– Silver traces – have even lower resistivity but are more expensive, may be used where the highest conductivity is needed
– Printed silver ink – very low cost for disposable electronics on paper/plastic
– Carbon nano-material traces – offer high conductivity and flexibility but are still an emerging technology with high costs and manufacturing challenges

What is copper pour and why is it used?

Copper pour is a technique where unused areas of the outer PCB Layers are filled with copper, usually connected to ground. This offers several benefits:
– Reduces EMI
– Improves ESD protection
– Helps with heat spreading
– Provides shielding for sensitive signals
– Minimizes etchant use in manufacturing
– Improves mechanical strength of the PCB

Conclusion

Copper is the clear choice for PCB traces in the vast majority of applications thanks to its excellent combination of high conductivity, good manufacturability, reasonable cost, and proven reliability. The copper foil weight and surface finish can be optimized based on the specific technical and economic requirements of the end application.

Techniques like copper pour on outer layers and dedicated power and ground planes on inner layers of the PCB further leverage copper’s properties to improve signal integrity, power delivery, heat spreading, and mechanical robustness.

Alternative materials like silver and carbon nano-materials may grow in use for some specialized PCB applications in the future. But for now, copper remains the dominant trace material that underpins the complex multilayer PCBs at the heart of our electronic devices and systems.