Low-Temperature, Flux-Free VCSEL Flip-Chip: A Thermosonic Route Case

Ultrasonic bonding is a bonding technique that uses pressure and high-frequency mechanical vibration energy to plastically deform the metal interface and form a solid-state joint. Its advantages are solder-free, lower required temperature and pressure, and faster bonding rate; it is commonly used for gold-bump (Au bump) flip-chip.

Adding moderate heating on top of ultrasonic bonding gives thermosonic bonding (Thermosonic Bonding) — similar to thermocompression, but the introduced ultrasonic energy markedly lowers bond force and temperature. Pure thermocompression often needs >300°C; thermosonic, combining thermocompression + ultrasonic welding, brings temperature and force down to below 200°C, 20–50 g/bump, targeting placement scenarios sensitive to temperature and demanding on interface cleanliness, achieving a gentle, reliable, high-precision microelectronic connection.

一、Application Case: VCSEL and Glass Interposer Precision Interconnect

In board-level optical interconnect, VCSEL and PD convert between electrical and optical signals. To shorten the electrical-interconnect path and couple the beam into the on-board polymer waveguide, the optoelectronic chip can be flip-chipped onto a transparent glass interposer first, then integrated with driver/receiver circuits and the waveguide structure. This step must complete electrical connection while also handling optical-surface cleanliness, device placement, and post-bond stability.

This study used Au stud bump thermosonic flip-chip to interconnect the VCSEL with the glass substrate. The test assembly used 3 gold bumps, completed at 120°C, total force 1 N, ultrasonic power 1 W, pulse 500 ms. The flux-free solid-state process helps reduce residue risk near the optical coupling surface, and the lower temperature with shorter action time also leaves room to control assembly thermal input.

二、Ultrasonic Bonding Challenges: Align, Withstand, Flatten

Challenge ① · Place it accurately, and keep it from shifting after bonding

The light emitted by the VCSEL must align to the micro-mirror and then enter the waveguide. Even if placement is accurate, bump compression and ultrasonic micro-vibration during bonding may shift the die slightly; once the offset is too large, less light enters the waveguide. Therefore, high placement accuracy is required, and more importantly, the actual post-bond accuracy and optical-coupling effect must be examined.

Challenge ② · Precise control of temperature, pressure, and ultrasonic parameters

If temperature, pressure, or ultrasonic energy is insufficient, the gold bump may not bond firmly; if parameters are too high, the bump may over-deform and even raise the risk of chip and thin-glass damage. Process development must tune temperature, pressure, ultrasonic power, and duration together to find a window that bonds firmly without over-stressing.

Challenge ③ · Flatness control of multiple bumps: simultaneous contact, uniform force

Multiple gold bumps are like the legs of a table: if one is too high or too low, or the collet is not flattened, one side may already be pressed while the other has not yet contacted. Before assembly, bump-height control is needed and the collet must stay parallel to the substrate so every joint connects stably.