AuSn Solder
Understanding AuSn (Gold-Tin) Solder
A GaN power amplifier die operating at 28 GHz dissipates several watts of heat from a chip the size of a grain of rice. That heat must flow through the die-attach layer to the heat sink within microseconds, or the transistor's junction temperature will exceed its reliability limit. AuSn solder provides the thermal bridge — a thin, void-free metallic joint with the thermal conductivity needed to keep the die alive.
Why Not Standard Solder?
Standard tin-lead (SnPb) and lead-free (SAC305) solders have lower melting points and are easier to process, but they have critical weaknesses for RF power die attach:
- Creep: Soft solders flow slowly under sustained thermal stress, creating voids that degrade thermal conductivity over the device's lifetime.
- Thermal conductivity: SnPb at 50 W/m·K and SAC305 at 58 W/m·K are comparable to AuSn, but the void-free joints achievable with AuSn's flux-free process provide superior effective thermal conductivity in practice.
- Reliability: AuSn's high elastic modulus resists fatigue cracking during thermal cycling, providing the 20+ year lifetimes required for aerospace and defense RF modules.
Key Equations
AuSn (Gold-Tin) solder is a eutectic solder alloy with a composition of 80% gold / 20% tin by weight (Au₈₀Sn₂₀) and a eutectic melting point...
Key specifications:
80 % | 20 % | 280 °C | 57 W | 58 W | 5 W
Yield: Y = e−AD (Poisson defect model)
Comparison
| Aspect | AuSn Solder Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | AuSn (Gold-Tin) solder is a eutectic sol... | Application-dep. | Critical | Verify in sim |
| Operating range | Understanding AuSn (Gold-Tin) Solder A G... | Application-dep. | Critical | Verify in sim |
| Performance | That heat must flow through the die-atta... | Application-dep. | Critical | Verify in sim |
| Integration | AuSn solder provides the thermal bridge... | Application-dep. | Critical | Verify in sim |
| Trade-off | Reliability: AuSn's high elastic modulus... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
How is AuSn die attach performed?
AuSn die attach uses a eutectic die bonder in a controlled atmosphere (nitrogen or forming gas). A preform of AuSn solder (typically 25–50 μm thick) is placed on the gold-metallized heat sink surface. The die (with gold or platinum back-metallization) is placed on the preform. The assembly is heated above 280°C in an inert atmosphere, the solder melts and wets both surfaces, and the assembly cools to form a solid, void-free joint. No flux is used, eliminating the cleaning step that would be required inside a hermetic package.
What is the void percentage requirement?
For high-reliability RF power die attach, the industry standard is less than 10% void area in the AuSn solder joint, verified by Scanning Acoustic Microscopy (SAM). Voids are regions where the solder did not wet the surface, creating local thermal barriers. A void directly under a transistor's active region causes a local hot spot that can reduce device lifetime by orders of magnitude. Some defense programs require less than 5% voiding.
Why is AuSn expensive?
AuSn solder is approximately 80% gold by weight. A 1mm × 1mm × 25μm AuSn preform contains approximately 0.4 mg of gold. At $60/gram gold price, the gold content alone is approximately $0.024 per die — seemingly trivial. However, the controlled-atmosphere eutectic bonding equipment, the gold-metallized heat sinks, and the precision placement required for void-free joints add significant process cost. For commercial applications (consumer electronics, automotive), silver epoxy or sintered silver die attach provides adequate thermal performance at a fraction of the cost.