What metal is used in PCBs?

Types of PCB metals

Several metals are commonly used in PCB manufacturing, each with its own advantages and disadvantages. The most popular PCB metals include:

1. Copper
2. Aluminum
3. Gold
4. Silver
5. Tin

Let’s examine each of these metals in more detail.

Copper

Copper is by far the most widely used metal in PCB manufacturing. It has several properties that make it an ideal choice for PCBs:

• High electrical conductivity
• Good thermal conductivity
• Relatively low cost compared to other conductive metals
• Easy to etch and plate

PCBs typically use copper foils laminated onto the insulating substrate. The thickness of the copper foil can vary depending on the application, with common thicknesses ranging from 0.5 oz/ft² to 2 oz/ft² (17 μm to 68 μm).

Copper traces are created by selectively etching away unwanted copper, leaving behind the desired circuit pattern. The etching process involves applying a photoresist layer, exposing it to UV light through a photomask, developing the photoresist, and then etching away the exposed copper using an etchant solution.

After etching, the remaining copper traces are often plated with another metal, such as gold, silver, or tin, to improve their solderability, corrosion resistance, or electrical contact properties.

Aluminum

While copper is the most common PCB metal, aluminum is sometimes used as an alternative due to its unique properties:

• Lightweight compared to copper
• Good thermal conductivity
• Lower cost than copper

However, aluminum has some drawbacks that limit its use in PCBs:

• Lower electrical conductivity than copper
• Prone to oxidation, which can degrade electrical connections
• Difficult to solder

Aluminum PCBs are often used in applications where weight is a critical factor, such as in aerospace and automotive industries. They can also be used as a substrate for high-power LED lighting applications, where the aluminum helps dissipate heat efficiently.

Gold

Gold is used in PCBs for its excellent corrosion resistance and electrical conductivity. However, due to its high cost, gold is seldom used for the entire PCB. Instead, it is typically used for plating connectors, contact fingers, and other critical areas where corrosion resistance and reliable electrical contact are essential.

Gold-plated PCBs are commonly found in high-reliability applications, such as medical devices, aerospace equipment, and military systems. The gold plating helps ensure long-term reliability and prevents oxidation of the underlying copper traces.

Silver

Silver is another metal used in PCBs, known for its high electrical conductivity – even higher than copper. Like gold, silver is usually used for plating rather than the entire PCB due to its high cost. Silver plating offers several benefits:

• Excellent electrical conductivity
• Good solderability
• Antimicrobial properties, preventing the growth of bacteria and fungi

Silver-plated PCBs are often used in high-frequency applications, such as RF and microwave circuits, where the higher conductivity of silver helps minimize signal losses.

Tin

Tin is a common plating material for PCBs, often used as a finish for copper traces. Tin plating offers several advantages:

• Good solderability
• Relatively low cost
• Protects the underlying copper from oxidation

However, tin plating can be prone to whiskering, where thin, hair-like metallic filaments grow from the surface of the tin. Tin whiskers can cause short circuits and other reliability issues, particularly in high-reliability applications. To mitigate this problem, tin alloys containing small amounts of other metals, such as lead or bismuth, are sometimes used.

Properties of PCB Metals

When selecting a metal for a PCB, several properties must be considered to ensure the board’s performance, reliability, and manufacturability. Some key properties of PCB metals include:

Electrical Conductivity

Electrical conductivity is a measure of a material’s ability to conduct electric current. In PCBs, higher conductivity means lower resistance, which translates to better signal integrity and less power loss. The electrical conductivity of common PCB metals, in order of decreasing conductivity, is:

1. Silver: 6.30 × 10⁷ S/m
2. Copper: 5.96 × 10⁷ S/m
3. Gold: 4.10 × 10⁷ S/m
4. Aluminum: 3.77 × 10⁷ S/m
5. Tin: 8.70 × 10⁶ S/m

Thermal Conductivity

Thermal conductivity is a measure of a material’s ability to conduct heat. In PCBs, good thermal conductivity helps dissipate heat generated by components, preventing overheating and thermal damage. The thermal conductivity of common PCB metals, in order of decreasing conductivity, is:

1. Silver: 429 W/(m·K)
2. Copper: 401 W/(m·K)
3. Gold: 318 W/(m·K)
4. Aluminum: 237 W/(m·K)
5. Tin: 67 W/(m·K)

Corrosion Resistance

Corrosion resistance is a material’s ability to withstand deterioration due to reactions with its environment. In PCBs, corrosion can degrade electrical connections and lead to failures. The corrosion resistance of common PCB metals, from best to worst, is:

1. Gold
2. Silver
3. Tin
4. Copper
5. Aluminum

Cost

The cost of PCB metals is an important consideration, as it directly impacts the overall cost of the PCB. The cost of common PCB metals, from most expensive to least expensive, is:

1. Gold
2. Silver
3. Copper
4. Tin
5. Aluminum

PCB Metal Selection

Choosing the right metal for a PCB depends on the specific requirements of the application. Factors to consider include:

• Electrical performance requirements (e.g., signal integrity, power handling)
• Environmental conditions (e.g., temperature, humidity, corrosive environments)
• Reliability requirements (e.g., expected product lifespan, failure rates)
• Manufacturing constraints (e.g., etching process, plating compatibility)
• Cost constraints

In most cases, copper is the primary metal used for PCB traces due to its good balance of electrical conductivity, thermal conductivity, and cost. Other metals, such as gold, silver, and tin, are used for plating in specific areas where their unique properties are required.

For high-frequency applications, such as RF and microwave circuits, silver plating may be preferred due to its higher conductivity. In high-reliability applications, such as medical devices and aerospace equipment, gold plating is often used for its excellent corrosion resistance.

Aluminum PCBs are less common but can be a good choice for applications where weight is a critical factor, such as in aerospace and automotive industries. However, the lower electrical conductivity and solderability of aluminum must be taken into account.

PCB Metal Recycling

As the demand for electronics continues to grow, so does the importance of recycling PCBs to recover valuable metals and reduce environmental impact. PCBs contain a mix of metals, including copper, gold, silver, and tin, which can be extracted and reused in new products.

The PCB recycling process typically involves the following steps:

1. Collection and sorting of PCBs
2. Shredding and grinding of PCBs into small pieces
3. Separation of metals and non-metals using various techniques, such as magnetic separation, electrostatic separation, and density separation
4. Purification of the recovered metals
5. Reuse of the purified metals in new products

Recycling PCB metals offers several benefits:

• Conservation of finite metal resources
• Reduction of environmental impact from mining and processing of virgin metals
• Economic benefits from the sale of recovered metals
• Compliance with environmental regulations and waste management standards

Frequently Asked Questions (FAQ)

1. What is the most commonly used metal in PCBs?

Copper is the most widely used metal in PCB manufacturing due to its high electrical conductivity, good thermal conductivity, and relatively low cost.

2. Why are some PCBs plated with gold?

Gold plating is used in PCBs for its excellent corrosion resistance and reliable electrical contact properties. Gold-plated PCBs are often used in high-reliability applications, such as medical devices, aerospace equipment, and military systems.

3. What are the advantages of using aluminum in PCBs?

Aluminum PCBs offer several advantages, including light weight, good thermal conductivity, and lower cost compared to copper. They are often used in applications where weight is a critical factor, such as in aerospace and automotive industries.

4. Why is silver sometimes used for plating PCBs?

Silver plating is sometimes used in PCBs for its high electrical conductivity, which is even higher than copper. Silver-plated PCBs are often used in high-frequency applications, such as RF and microwave circuits, where the higher conductivity helps minimize signal losses.

5. What are the benefits of recycling PCB metals?

Recycling PCB metals offers several benefits, including conservation of finite metal resources, reduction of environmental impact from mining and processing of virgin metals, economic benefits from the sale of recovered metals, and compliance with environmental regulations and waste management standards.

Metal	Electrical Conductivity (S/m)	Thermal Conductivity (W/(m·K))	Relative Cost
Copper	5.96 × 10⁷	401	Medium
Aluminum	3.77 × 10⁷	237	Low
Gold	4.10 × 10⁷	318	High
Silver	6.30 × 10⁷	429	High
Tin	8.70 × 10⁶	67	Low

In conclusion, the choice of metal in PCBs plays a crucial role in determining the board’s electrical performance, durability, and manufacturability. Copper remains the most widely used PCB metal due to its excellent balance of properties and cost-effectiveness. Other metals, such as gold, silver, and tin, are used for selective plating to enhance specific attributes like corrosion resistance or electrical conductivity. Aluminum PCBs offer unique advantages in weight-sensitive applications but come with trade-offs in electrical performance and solderability.

As the electronics industry continues to evolve, innovations in PCB metal technologies and recycling processes will be essential to meeting the growing demand for high-performance, reliable, and environmentally sustainable electronic products.