Printed circuit boards (PCBs) are essential components in most modern electronic devices. They provide the physical structure and electrical connections between components. With the ongoing miniaturization and complexity of electronics, there is a growing need for more advanced PCBs. One such technology is copper core PCBs.

Copper core PCBs have a solid copper layer at the center instead of the typical insulating FR-4 fiberglass. This provides superior thermal conductivity and allows heat to spread more efficiently. This is advantageous for high power devices. However, copper core PCBs also introduce unique challenges during manufacturing and recycling. Specifically, separating the copper core from the rest of the PCB after use is difficult.

This article will examine the thermoelectric separation methods used to recover the valuable copper in copper core PCBs.

Overview of Copper Core PCBs

Copper core PCBs were first developed in the 1980s for power electronics applications. The need for better thermal performance as electronics become more compact has made copper core PCBs more common.

Construction

A copper core PCB consists of:

• Solid copper inner layer – This serves as the central heat spreader. Thicknesses range from 0.8mm to 3mm.
• Dielectric insulation layers – This electrically isolates the copper core from the other layers. Typical materials used are polyimide and epoxy composites.
• Copper foil layers – This forms the conductive tracks and pads for components. There are typically 2 to 20 copper layers.
• Protective coatings – This includes solder mask and silkscreen layers. It protects the copper from oxidation and markings for components.

Applications

The excellent thermal conductivity makes copper core PCBs well suited for:

• High power electronics – IGBTs, power amplifiers, CPU/GPUs, etc.
• RF microwave circuits – High frequency results in localized heating.
• LED lighting – High luminescence causes heat concentration.
• Electric vehicle electronics – Battery packs generate significant heat.

Challenges

While copper core PCBs have substantial advantages, they also pose manufacturing and recycling challenges including:

• Difficulty drilling through the thick copper layer.
• More expensive materials and processes.
• Separation of the copper core during recycling.

This last challenge is the focus of this article. Recovering the valuable copper for reuse helps make copper core PCB recycling economical.

Thermoelectric Separation Methods

There are two main approaches used to separate the copper core from the rest of the PCB: pyrolysis and electrolysis. Both utilize heat to help breakdown the dielectric layers bonding the copper core to the PCB.

Pyrolysis

Pyrolysis involves heating the PCBs to high temperatures without oxygen to thermally decompose the organic components. As the dielectric layers break down, the copper core can be mechanically separated.

The process steps are:

1. Shredding – The PCBs are shredded into small fragments to expose more surface area.
2. Pyrolysis – Fragments are heated to 500-800°C in an oxygen-free environment. Organics like epoxy resin decompose while metals remain intact.
3. Separation – Cooled material is separated using density differences. Magnetic separation can help recover ferrous metals.
4. Refining – The recovered copper cores undergo final cleaning and refining to pure copper.

Pyrolysis advantage:

• Simultaneously recovers several metals.
• Minimal chemical waste.

Pyrolysis disadvantages:

• High energy consumption for heating.
• Emission control is complex.
• Other metals can contaminate the copper core.

Electrolysis

Electrolysis uses an electrical current to dissolve the dielectric layers to free the copper core.

The process steps are:

1. Shredding – PCBs are shredded to expose the copper layers.
2. Heating – Fragments are heated to approximately 500°C to crack the dielectric.
3. Electrolysis – Fragments are submerged in an electrolyte bath. Applying a current dissolves dielectric while copper remains intact.
4. Separation – Density separation is used to recover liberated copper cores.
5. Refining – Final cleaning and refining to pure copper.

Electrolysis advantages:

• More controlled process results in higher purity copper recovery.
• Lower temperatures reduce energy consumption.

Electrolysis disadvantages:

• Multiple steps increases complexity.
• Generates metal-laden chemical waste.

Factors Affecting Separation Efficiency

Several parameters influence the effectiveness of the thermoelectric separation processes:

PCB Composition

• More copper content improves recovery efficiency.
• High-temp resins (e.g. polyimide) require higher temperatures.
• Presence of other metals affects purity.

Process Temperature

• Temperatures around 500°C provides the best balance.
• Too low fails to sufficiently decompose organics.
• Too high risks oxidizing metals.

Physical Size

• Shredding into small, thin fragments improves heat transfer and exposure.
• But too small increases losses during separation.
• Optimal fragment size is 2-10mm.

Duration

• Sufficient time required for full decomposition or electrolysis.
• But excess time wastes energy.
• Electrolysis requires close monitoring and controls.

Economics

• High material and energy costs can outweigh copper value.
• Requires high volume throughput to balance costs.

Comparison of Pyrolysis and Electrolysis Methods

Pyrolysis and electrolysis take different approaches to thermoelectric separation with distinct tradeoffs:

Parameter	Pyrolysis	Electrolysis
Process	Thermal decomposition	Electrochemical dissolution
Temperature	500-800°C	~500°C
Equipment	Furnace, shredder	Electrolytic bath, shredder
Speed	Slow due to heating time	Faster dissolution
Energy	High for heating	Lower
Complexity	Relatively simple	Multi-step process
Copper Purity	Lower due to mixing	Higher
Waste Produced	Minimal	Metal-laden chemical waste
Scalability	Limited by furnace size	Easy to scale up
Capital Costs	Lower	Higher for processing equipment
Operating Costs	Higher energy costs	Cost of chemicals

Considering these factors, electrolysis is generally the preferred method for copper core PCB recycling. The ability to produce higher purity copper with relatively low temperatures compensates for the greater complexity.

Future Developments

There are several promising directions that may improve thermoelectric separation of copper core PCBs:

• Alternative dielectric materials that decompose at lower temperatures.
• Use of microwaves or ultrasonics to accelerate heating.
• Integrated processes combining thermal and electrochemical steps.
• New electrically driven separation techniques.
• Oxygen-free pyrolysis to prevent oxidation.
• Automated systems to cut labor costs and dangers.
• Improved recycling economics through economies of scale.

Research continues to make thermoelectric recycling more efficient and economically viable. This will support the sustainability of electronics manufacturing.

Conclusion

Copper core PCBs provide enhanced thermal performance crucial for many electronics applications. However, their construction poses challenges for recycling and material recovery after use. Thermoelectric processes utilizing heat assist in separating the copper core by degrading the binding dielectric layers.

Pyrolysis decomposes the organic fractions while electrolysis dissolves them electrochemically. Both methods have their merits depending on factors like desired purity and cost. Ongoing improvements to separation efficiency and economics will help make copper core PCB recycling more accessible and sustainable.

Frequently Asked Questions

What are the main advantages of copper core PCBs?

The key advantages of copper core PCBs are:

• Excellent thermal conductivity that spreads heat efficiently from components
• Allows higher power densities crucial in compact devices
• Prevents overheating failures in electronics
• Widely used in power electronics, RF/microwave, LEDs, EVs, etc.

Why is it difficult to recycle copper core PCBs?

Recycling copper core PCBs is challenging because the valuable copper core is bonded to the board by tough dielectric layers. Separating the copper requires breaking down these materials which takes considerable energy and specialized processing.

What temperature is needed for thermoelectric separation?

Thermoelectric separation processes operate at around 500-800°C. This temperature range is high enough to decompose the organic dielectric layers but low enough to avoid oxidizing the metals. 500°C provides the best balance for most methods.

What are the main differences between pyrolysis and electrolysis?

The main differences are:

• Pyrolysis uses heat alone to decompose organics while electrolysis uses electrochemical dissolution.
• Pyrolysis requires high temperatures while electrolysis can work at lower temperatures.
• Pyrolysis recovers several metals simultaneously while electrolysis focuses on copper.
• Electrolysis produces higher copper purity but generates chemical waste.

What recent improvements have been made in thermoelectric separation?

Recent improvements include new dielectric materials that decompose at lower temperatures, microwave and ultrasonic heating to accelerate processing, integrated thermal-electrochemical systems, and automated shredding and separation equipment to lower costs.