Die Attach and Wire Bonding — How Does a Chip “Settle Into” Its Package?

I. Die Attach Process: Eutectic Bonding vs. Conductive Adhesive

The package housing is done — how does the chip go in? The first step is die attach — fixing the chip onto the header’s mounting surface so that the chip and the package form thermal, electrical, and mechanical connections. The two mainstream methods in the industry each have their role:

Eutectic bonding— uses Au-Sn (gold-tin eutectic) or Au-Ge solder to solder the chip to the mounting surface. It offers excellent thermal conductivity and high strength and is the standard for high-power lasers. The gold-plated metallized mounting surfaces of Xuri’s TO56, BTF-14PIN and other headers are designed precisely to meet the wetting requirements of eutectic bonding.

Conductive adhesive— silver-filled epoxy. Its process temperature is lower than eutectic bonding and residual stress is small, making it suitable for temperature-sensitive devices, but its thermal conductivity is only one-tenth that of eutectic bonding.

The selection logic is simple: look at the power. The higher the power and the heat-dissipation demand, the stronger the preference for eutectic bonding.

II. Wire Bonding: Gold Wire or Aluminum Wire?

The chip is attached — now how do the electrical signals come out? Through wire bonding — using fine metal wires to connect the chip electrodes to the header pins. Each of the two wire materials has its own territory:

Gold wire— excellent conductivity, corrosion resistance, and high bond strength make it the first choice for hermetic packaging. Thermosonic bonding requires heating, and the gold ball-bonding process is mature and stable.

Aluminum wire— low cost; ultrasonic bonding needs no heating and suits high-current applications.

Bond quality is judged by two key parameters: bond pull strength and shear strength. Poor bonding is the number-one cause of internal opens and shorts in packages — which is why bond appearance must be 100% inspected before finished products ship.

[Next Episode] Next time: the “air conditioner” inside the butterfly package — how the TEC cools the laser chip.

[Glossary]

1. Die Attach: the process of fixing a chip onto the header’s mounting surface, typically achieved with eutectic bonding or conductive adhesive

2. Eutectic Die Attach: a die-attach process completed at the eutectic-point temperature with eutectic alloy solder, delivering excellent thermal conductivity and strength

3. Wire Bonding: an electrical interconnection process that connects chip electrodes to external pins with fine metal wires

4. Thermosonic Bonding: a bonding method combining heat, ultrasonic energy, and pressure; the standard process for gold-wire bonding