Meaning
A solid-state joining process connects two metals by applying heat and pressure simultaneously without melting the materials. In semiconductor packaging, thermocompression bonding is used to attach microbumps on a die to a substrate or another die. This technique relies on atomic diffusion across the contact interface to create a solid metallic joint.
The temperature and pressure boundaries of the process are set to avoid damaging the delicate circuit structures, providing a reliable alternative to conventional reflow soldering for fine-pitch applications.
Thermal Profile
The heating cycle must raise the interface temperature quickly to promote atomic mobility at the joint. For thermocompression bonding, the tool head holds the die at a elevated temperature while the substrate is preheated to a lower level. This temperature differential minimizes thermal expansion mismatch and prevents warping.
Mechanical Force
A controlled compressive force is applied to deform the microbumps plastically, establishing intimate contact. During thermocompression bonding, the applied load must be high enough to break through surface oxides but low enough to prevent die cracking. This force is held constant for a specified dwell time to complete the diffusion process.
Interface Integrity
Voids or incomplete diffusion at the boundary can lead to poor electrical conductivity. When thermocompression bonding is executed incorrectly, joint strength decreases, making the assembly vulnerable to thermal cycling failures. Non-destructive acoustic microscopy is often used to inspect the bonded interface for voids.
