Understanding Thermo-Compression Bonding

Key Takeaways

• Thermo-compression bonding is used for creating atomic-level metal bonds. It employs force and heat to facilitate atom migration between crystal lattices, resulting in clean, highly conductive, and robust bonds.

• Thermo-compression is used in CMOS processes for vertically integrated devices, in compliant bonding for solid-state bonds between gold leads and surfaces, in flip-chip applications for bonding chip bumps to substrates, and in thermo-pressure bonding for connecting micro-components.

• The bonding temperature usually ranges between 300-400 degrees Celsius, and the process can be quick, lasting only a fraction of a second. 

Example diagram of wire bonded to circuit board pad thru thermo-compression bonding

Thermo-compression bonding is a method that brings two metals into atomic contact by simultaneously applying force and heat causing diffusion to occur. Atoms migrate between crystal lattices due to lattice vibrations and bind the two surfaces together at the interface. This creates clean bonds, free from flux or lead, and results in bonds that are highly thermally conductive, electrically conductive, and structurally robust. Other names include diffusion bonding, pressure joining, thermocompression welding, or solid-state welding. 

Thermo-Compression Bondings Steps 

Thermocompression Die Bonding Methodology 

In thermocompression die bonding, or die attach, the process does not need any adhesives for joining the die to the package. Instead, in applying intense heat and force to the die, a metallic bond with the substrate is formed. This also provides a hermetic seal to the package's internal structures and electrical interconnects in just one step. Since they have high diffusion rates, gold (Au), aluminum (Al), and copper (Cu) are the most commonly used materials in this process.

Bonding Temperature and Cycle

The temperatures used for interfacial bonding typically range between 300 and 400 degrees Celsius. The bonding cycle, not including the time for bond positioning, is especially quick, lasting only a fraction of a second. Heat is applied either through a heated capillary that feeds the wire or by placing the substrate or package on a heated stage, known as a column. Most contemporary thermocompression bonders employ a hybrid approach, utilizing both capillary and column heat for optimal bonding efficiency.

The diffusion process includes the following three types: 

1. Surface diffusion is the movement of atoms along a material's surface, crucial in surface-related processes like catalysis and coatings, deposition and patterning of thin films onto wafer.
2. Grain boundary diffusion involves atomic migration along the grain boundaries within polycrystalline materials used in gate electrodes and interconnects, essential in sintering and microcrack healing in metals.
3. Bulk diffusion involves atoms moving through the bulk of a material, significantly impacting alloying, phase transformations, heat treatments of metals and used in doping.

Thermo-Compression Bonding Applications

Thermocompression bonding is used in a variety of applications –some notable ones are discussed below.

• Thermocompression bonding is used in CMOS processes, enabling vertically integrated devices and producing wafer-level packages with reduced sizes. This technique is used in manufacturing various components, such as pressure sensors, accelerometers, gyroscopes, LEDs, laser diodes, and RF MEMS.
• Compliant bonding is a submethod that allows for the formation of solid-state bonds between a gold lead and a gold surface by transmitting heat and pressure through a compliant or deformable medium. This keeps the physical integrity of the lead by controlling wire deformation. Additionally, it allows for simultaneous bonding of multiple gold wires of different dimensions, as it ensures contact and deformation of all lead wires.
• In flip-chip applications, thermocompression bonding is regularly employed to bond bumps on a chip's surface to a substrate with corresponding pads. The contact points, typically metallic bumps, are pressed against their opposing pads to create a bond, while a second metallic bond is formed at the point of contact with the package metallization. This technique often requires high temperatures of around 350 to 400 degrees celsius and forces up to 100 g/bump. Dies that are thin, brittle, or heat-sensitive may not be suitable for attachment using this method.
• Thermo-pressure bonding is also useful in connecting miniature copper thermocouples to ultra-fine wires, facilitating metal-to-metal connections of micro-coils to printed circuit boards using thicker intermediate wires, and attaching micro coils to support systems using metal-plated terminals.

Thermocompression Wirebondoing

In this thermo-compression bonding, the ductility of the wire is a crucial factor contributing to the successful formation of the bond. The thermocompression bonding of gold to aluminum metallization is a particularly well-researched area of interdiffusion mechanisms, given its widespread use. The plastic deformation occurring at the bonding interface plays a significant role. It ensures close surface contact between the wire and the pad, leading to an expansion of the bonding area. This deformation can also break down any layers of interfacial film, such as oxide or contaminants, creating a more effective bond.

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