Knowledge Center
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Creating interconnects between IC and package using a thin wire.


Wirebonding is a commonly used method for creating interconnections between an IC on a silicon die and its packaging, in which a thin wire is connected from a bond pad on the device to a corresponding pad (i.e. lead) on the package. This connection establishes the electrical path from the internal circuits of the chip to the external pins that connect to a printed circuit board (PCB).

Wirebonding process

The process begins by positioning a wire beneath the end of a slender bonding instrument. A precise force is exerted, pushing the wire against the electrode’s surface, leading to an initial deformation at the point of contact. This energy is derived from an ultrasonic device that produces mechanical oscillations around 60 kHz. These oscillations are channeled through the bonding instrument to the fusion zone for a duration of about 100 milliseconds.

The friction generated by the ultrasonic vibrations produces localized heating at the interface, which when combined with the pressure applied by the bonding tool, facilitates atomic diffusion across the interface. As the wire and substrate materials’ atoms come into close proximity due to the mechanical deformation and frictional effects, they begin to form intermetallic compounds. These compounds are often more stable than the parent materials and ensure a strong bond between the wire and the substrate.

There are two primary types of wire bonds, ball bonds and wedge bonds. Ball bonds are the most prevalent, constituting about 90% of all bonds, and are typically formed using a thermo-sonic bonding technique for gold and copper wiring. Wedge bonds, which are often created using ultrasonic or thermocompression techniques, are more common with aluminum alloy and gold wires and are frequently used in specific applications.

Fig. 1: A comparison of three types of wirebonding technologies. Source: NASA.

Fig. 1: A comparison of three types of wirebonding technologies. Source: NASA


Gold (Au) bonding wire was the industry standard for decades. Its unparalleled chemical stability, corrosion resistance, and dependability made it the logical material for semiconductor packaging. That began to change as gold prices increased 478% in the decade between 2001 and 2011.

Copper (Cu) was adopted for its cost-effectiveness and superior electrical and thermal conductivity. It can conduct the same current as gold with a smaller diameter wire, without succumbing to overheating. Additionally, its slower reaction rate with aluminum enhances its reliability, especially under prolonged high-temperature storage conditions. However, it poses several challenges, including advanced equipment and specialized techniques, high heat required to achieve a bond, stiffness, and oxidation.

Silver (Au) alloyed with palladium (Pd) and gold has a cost similar to palladium-coated copper, with superior thermal conductivity and reduced electrical resistivity compared to copper, making it an ideal choice for power electronics. Its elasticity and hardness strike a balance between gold and copper, simplifying the bonding process, and its lower melting point helps protect fragile bond pads. Silver wire bonds are also less susceptible to corrosion compared to copper.

Aluminum (Al) is an industry standard wirebonding material that has been used almost as long as gold and has unique properties that make it ideal for a wide range of applications. Its compatibility with aluminum pads on dies allows for bonding at room temperature with low energy levels, which helps prevent damage to sensitive devices. To enhance its mechanical properties for specific applications, it is often alloyed with elements such as silicon, typically around 1%, or sometimes magnesium. These aluminum alloys result in wires with finer sizes and increased strength.

Fig 2: Some characteristics of 0.7 mil gold, aluminum, copper and silver wire. Source: ASME.

Fig. 2: Some characteristics of 0.7 mil gold, aluminum, copper and silver wire. Source: ASME

Adapted from Wirebonding Is Here To Stay.