What is the wire bonding on the PCB?

What is Wire Bonding?

Wire bonding is a critical technique in semiconductor manufacturing for connecting integrated circuits (ICs) to electrical appliances and between PCBs. It stands out for its cost-effectiveness and durability in making interconnections. Utilizing welding methods and various techniques, wire bonding involves precisely attaching metal wires to chips, requiring high technological expertise to ensure accurate and durable connections. The evolution of wire bonding has addressed the limitations of older solder bonding methods, enhancing the reliability and performance of semiconductor systems.

What are the applications?

Mobile Phones

In mobile phone manufacturing, wire bonding, particularly for battery packs, offers advantages over traditional spot welding. This technique, utilizing ultrasonic energy, enables more reliable and less damaging connections between battery cells, enhancing the battery’s lifespan and performance. It addresses the limitations of spot welding, such as potential short circuits and adverse effects on the battery management system (BMS) due to high temperatures and loose connections, making wire bonding a preferred choice for assembling battery packs in the industry.

Automotive

In electric vehicles, wire bonding is used in battery management systems to connect individual battery cells, ensuring efficient power distribution and management.
For electrical vehicle battery pack, connecting cylindrical cells to a busbar in batteries involves various techniques like wire bonding, laser welding, and spot welding. Wire bonding, using ultrasonic energy, forms a strong atomic bond by removing the oxide layer on aluminum, enhancing the connection’s reliability. This method is particularly beneficial for optimizing the process by leveraging the self-limiting nature of aluminum oxide, ensuring effective electron transfer for durable connections.

Medical Devices:

For pacemakers, wire bonding connects the device’s control circuitry to its power source and sensing electrodes, critical for reliable operation. In medical devices, wire bonding is crucial for implantable devices like pacemakers and defibrillators. These devices rely on wire bonding for connecting the silicon chip within the device to its power source and output terminals. This ensures the device can monitor heart rhythms accurately and deliver electrical impulses when needed, with reliability and precision critical for patient safety and device longevity.

Aerospace:

In satellites, wire bonding is used to connect solar cell arrays to the power management systems, essential for power supply in space conditions. In aerospace, wire bonding is used in satellite systems, particularly for connecting solar panels to the power management system. This ensures efficient energy transfer from the solar panels to the satellite’s systems, crucial for the satellite’s long-term operation in the harsh environment of space where reliability and durability are paramount.

Advantage of wire bonding

Precision for small components.
Higher connection density without additional PCB space.
Reduced thermal stress on sensitive components.
Enhanced reliability for high-performance applications.
Ideal for connecting ICs directly to each other or to the PCB in tight spaces.

The main purpose of wire bonding in technology, especially in the realm of high-frequency PCB applications (above GHz), is to ensure reliable and durable connections between chips and connectors. It’s crucial for maintaining the integrity of signals and voltage pulses essential in modern electronics. Wire bonding supports real-time communication between components, minimizing signal distortion and facilitating high-speed data transfer. This makes it indispensable in sectors like EV manufacturing, where performance and reliability are paramount.

Methods for wire Bonding

• Thermosonic Bonding: Combines heat, ultrasonic energy, and force to bond wires, typically gold or copper, to the substrate. It’s the most common method, used extensively in semiconductor packaging.
• Ultrasonic Bonding: Uses ultrasonic vibrations without applying external heat (or minimal heat) to bond aluminum wires, ideal for sensitive components.
• Thermocompression Bonding: Relies on heat and force to achieve the bond, without ultrasonic energy, suitable for specific applications where ultrasonic vibrations might be detrimental.

Common materials used in wire bonding

• Gold (Au): Preferred for its excellent conductivity, corrosion resistance, and ductility, making it suitable for thermosonic bonding.
• Aluminum (Al): Widely used for its good electrical conductivity and lower cost, suitable for ultrasonic and thermocompression bonding.
• Copper (Cu): Offers superior electrical and thermal conductivity compared to aluminum and is increasingly used for its mechanical strength, though it requires careful handling due to oxidation.

Common materials used in wire bonding

• Gold (Au): Preferred for its excellent conductivity, corrosion resistance, and ductility, making it suitable for thermosonic bonding.
• Aluminum (Al): Widely used for its good electrical conductivity and lower cost, suitable for ultrasonic and thermocompression bonding.
• Copper (Cu): Offers superior electrical and thermal conductivity compared to aluminum and is increasingly used for its mechanical strength, though it requires careful handling due to oxidation.

The wire bonding process

• Ball Formation: A small ball is formed at the end of the wire using a flame or electronic discharge.
• Alignment: The wire is aligned with the bonding pad on the chip or substrate.
• Bonding: The ball is pressed onto the pad using heat, ultrasonic energy, or both.
• Loop Formation: The wire is looped to the next bonding point to maintain flexibility and reduce stress.
• Second Bond Formation: The wire is bonded to the next pad, and the excess wire is cut off.

Challenge of wire bonding

Understanding the application and environmental factors is crucial for wire bond reliability. Factors like elevated temperature, humidity, and temperature cycling can lead to failures due to issues such as intermetallic compound formation, corrosion (including galvanic corrosion in Au-Al systems), and thermomechanical stress from CTE mismatches. These challenges necessitate a comprehensive understanding of metal systems and environmental impacts to enhance wire bond durability and performance.