What metal are the tracks of a PCB made from?

Introduction to PCB tracks

Printed circuit boards (PCBs) are the backbone of modern electronics. These boards contain the electrical connections and components that make devices function. One crucial part of a PCB is the network of thin lines etched or printed onto the board, known as tracks or traces. PCB tracks provide conductive pathways for electricity and signals to travel between different components on the board.

The metal used for PCB tracks is a key factor in the performance, reliability, and cost of the final circuit board. In this article, we’ll take an in-depth look at the common metals used for PCB tracks, their properties, and considerations for choosing the right metal for a given application.

Copper: The Most Common Metal for PCB Tracks

Properties of Copper

Copper is by far the most widely used metal for creating PCB tracks. It has several properties that make it an excellent choice:

1. High electrical conductivity
2. Good thermal conductivity
3. Relatively low cost compared to other conductive metals
4. Easy to etch and plate onto PCB substrates
5. Resistant to corrosion when properly treated

The high electrical conductivity of copper allows it to efficiently carry current with minimal losses. Copper’s thermal conductivity also helps dissipate heat generated by components, which is important for maintaining a stable operating temperature.

Copper Thickness and Weight

PCB copper is typically specified by weight per unit area, measured in ounces per square foot (oz/ft²). Common weights range from 0.5 oz/ft² for thin, low-power traces up to 3 oz/ft² or more for high-current applications. The table below shows the approximate thickness of copper layers for different weights:

Copper Weight (oz/ft²)	Thickness (μm)	Thickness (mils)
0.5	17.5	0.7
1	35	1.4
2	70	2.8
3	105	4.2

Higher copper weights provide lower resistance and higher current-carrying capacity but also increase the cost and make fine-pitch traces more challenging to manufacture.

Copper Plating and Surface Finishes

After etching the base copper layer to form tracks, additional copper is often electroplated onto the traces to increase their thickness and improve conductivity. This process is known as copper plating.

To protect the copper from oxidation and enhance solderability, various surface finishes can be applied over the plated copper:

• Hot Air Solder Leveling (HASL): A tin-lead alloy is applied and leveled with hot air.
• Electroless Nickel Immersion Gold (ENIG): A thin layer of gold is deposited over a nickel barrier layer.
• Immersion Silver: A thin layer of silver is chemically deposited onto the copper.
• Immersion Tin: A thin layer of tin is chemically deposited onto the copper.
• Organic Solderability Preservative (OSP): An organic compound is applied to protect the copper.

The choice of surface finish depends on factors such as solderability requirements, shelf life, cost, and compatibility with the assembly process.

Aluminum: A Lightweight Alternative for PCB Tracks

Properties and Applications of Aluminum PCBs

While copper dominates the PCB industry, aluminum is sometimes used as an alternative material for creating PCB tracks. Aluminum offers some unique advantages:

1. Lightweight compared to copper
2. Excellent thermal conductivity
3. Good electrical conductivity, though lower than copper
4. Cost-effective for certain applications

Aluminum PCBs are often used in applications where weight savings are critical, such as aerospace and automotive systems. The high thermal conductivity of aluminum also makes it useful for LED lighting PCBs and other heat-sensitive applications.

Challenges and Limitations of Aluminum PCBs

Despite its benefits, aluminum has some drawbacks that limit its widespread use in PCBs:

1. Lower electrical conductivity than copper, requiring thicker traces for equivalent performance
2. More difficult to solder due to the formation of an oxide layer
3. Prone to thermal expansion mismatches with other PCB materials
4. Limited availability and higher setup costs compared to copper PCBs

To mitigate soldering issues, aluminum PCBs often have a thin copper layer plated onto the pads and through-holes. This allows for easier soldering while retaining the benefits of aluminum traces.

Other Metals Used in PCBs

Gold for High-Reliability Applications

Gold is sometimes used for PCB tracks in high-reliability applications, such as aerospace, military, and medical devices. Gold offers several advantages:

1. Excellent corrosion resistance
2. High electrical conductivity
3. Ease of wire bonding
4. Biocompatibility for medical applications

However, gold is significantly more expensive than copper and is typically used only for critical signal traces or in harsh environments where corrosion is a concern.

Silver for High-Frequency and RF Applications

Silver has the highest electrical conductivity of all metals, making it an attractive option for high-frequency and RF applications where signal integrity is paramount. Silver-plated copper traces can provide better high-frequency performance than bare copper.

However, silver is more expensive than copper and can tarnish over time if not properly protected. Silver migration can also occur in high-humidity environments, leading to short circuits.

Selecting the Right Metal for Your PCB Tracks

When choosing the metal for your PCB tracks, consider the following factors:

1. Electrical requirements: Current-carrying capacity, conductivity, and signal integrity needs.
2. Thermal requirements: Heat dissipation and thermal management.
3. Environmental factors: Corrosion resistance, temperature range, and humidity.
4. Manufacturing considerations: Etching and plating processes, solderability, and assembly compatibility.
5. Cost and availability: Material costs, setup fees, and lead times.

For most applications, copper remains the go-to choice due to its excellent balance of electrical and thermal properties, cost-effectiveness, and widespread availability. However, aluminum, gold, and silver may be viable alternatives for specific applications with unique requirements.

Frequently Asked Questions (FAQ)

1. What is the most common metal used for PCB tracks?

Copper is by far the most widely used metal for PCB tracks due to its high electrical and thermal conductivity, relatively low cost, and ease of manufacturing.

2. What are the advantages of using aluminum for PCB tracks?

Aluminum offers weight savings, excellent thermal conductivity, and cost-effectiveness for certain applications, such as aerospace, automotive, and LED lighting.

3. Why is gold sometimes used for PCB tracks?

Gold is used in high-reliability applications for its excellent corrosion resistance, high electrical conductivity, ease of wire bonding, and biocompatibility.

4. What are the benefits of using silver for PCB tracks?

Silver has the highest electrical conductivity of all metals, making it suitable for high-frequency and RF applications where signal integrity is critical.

5. What factors should be considered when selecting the metal for PCB tracks?

When choosing the metal for PCB tracks, consider electrical requirements, thermal requirements, environmental factors, manufacturing considerations, cost, and availability.

Conclusion

PCB tracks are the unsung heroes of modern electronics, providing the critical connections that enable devices to function. The choice of metal for these tracks is a crucial decision that impacts the performance, reliability, and cost of the final product. Copper remains the dominant material for its excellent properties and versatility, but aluminum, gold, and silver offer unique advantages for specific applications.

By understanding the properties and trade-offs of each metal, engineers and designers can make informed decisions when selecting the right material for their PCB tracks. As technology continues to advance, the demands on PCB materials will only increase, driving innovation in materials science and manufacturing processes to keep pace with the ever-evolving world of electronics.