AuSnvalues describe: 80Au-20Sn
Quantum hardwareVacuum electronics & X-ray sourcesSpace test
Why it wins
Below brazing temperatures, hermeticity still exists: AuSn eutectic seals quantum and photonic packages fluxless at 280 C, indium alloys join at temperatures optics survive.
Why not the alternative
SnPb and fluxed solders are banned: flux residues and creep; epoxies are not hermetic over years.
Watch out
AuSn is brittle: design the joint, do not just wet it.
Properties
The values below are candidate: compiled from the sources named, not yet individually validated.
Provisional provenance: AWS Brazing Handbook; Morgan Wesgo alloy data.
Wet cleaning
| Recipe | fluxless reflow under forming gas or vacuum, on gold-plated surfaces; plasma clean immediately before |
|---|
| Forbidden | flux of any kind in a hermetic package |
|---|
| Limit | gold plating thickness, since the tin consumes it |
|---|
Vacuum and outgassing
| Outgassing, unbaked (10 h) | 5e-9 mbar·L/s/cm² |
|---|
| Outgassing, baked | 1e-11 mbar·L/s/cm² |
|---|
| Vapour pressure | gold-tin is stable; avoid anything containing Cd, Zn or Pb |
|---|
Temperature
| Bake, assembled | 200 °C |
|---|
| Vacuum degas | 250 °C |
|---|
| Braze / H2 firing | 280 reflow °C |
|---|
| Metallurgical limit | AuSn eutectic 280 °C |
|---|
Thermal
| CTE | 16 ppm/K |
|---|
| Thermal conductivity | 57 W/m·K |
|---|
| Specific heat | 150 J/kg·K |
|---|
| Emissivity | 0.1 |
|---|
| Melting / softening | 280 °C |
|---|
Mechanical
| Strength | 275 tens MPa |
|---|
| Young's modulus | 68 GPa |
|---|
| Density | 14.5 g/cm3 |
|---|
Electrical and magnetic
| Relative permeability | ~1.0 |
|---|
| Resistivity | 16 µΩ·cm |
|---|
Engineering
| Corrosion | good |
|---|
| Joining | hermetic die attach and package sealing |
|---|
| Process notes | the fluxless alternative to SnPb, and the reason it exists |
|---|
| Availability and cost | preforms from specialist suppliers |
|---|
Grades
| Au80Sn20 eutectic, 280 C | fluxless hermetic lid seal and die attach |
|---|
| In52Sn48, 118 C | gentle joints near aligned optics |
|---|
| Pure In, 157 C | cryogenic compliant joints |
|---|
Sources · 1
- Indium Corporation alloy data
← The Matter