Soldering is an important technique in the assembly of electronic products. To make a sound solder joint, the choice of solder materials is very important. Solder ability, melting point, strength and Young’s modulus, thermal expansion coefficient, thermal fatigue and creep properties and creep resistance will all affect the quality of a solder joint. The eutectic Au80Sn20 solder alloy (melting point 280â°C) has been applied in semiconductor and other industries for years. Due to some superior physical properties, Au/Sn alloy gradually becomes one of the best materials for soldering in optoelectronic devices and components packaging
Physical Properties
Some principal physical properties of Au80Sn20 are in table 1, by which the advantages of Au/Sn solder could be identified as follow
| Density |
14.51 gm/cm³@20℃.
|
| Coefficient of thermal expansion* |
16 X10¯6 /℃ @20-250℃. |
| Thermal conductivity |
57.5 W.m¯¹K¯¹ |
| Tensile strength |
270MPa |
| Young's modulus |
70GPa |
| Shear modulus |
25 GPa |
| Poisson’s ration |
0.405 |
| Electrical resistivity |
16.4 X10-8 Ω.m |
| Elongation |
2 % |
Soldering temperature:
The soldering temperature is only 20-30 ℃ above its melting point (about 300 ~ 310 ℃). Because the alloy is eutectic, minimal superheat is needed for the wetting and flowing in the soldering process. It also freezes quickly resulting in a shorter soldering cycle. This temperature range is suitable for high-reliability component packaging. The components could also be safe for subsequent assembly with a Pb-free assembly solder. These solders typically require an assembly temperature up to 260 ℃.
High strength:
The alloy has high yield strength at ambient temperature, and even at assembly temperatures of 250-260 °C it is still strong enough to maintain hermeticity. Material strength is comparable to that of high temperature brazing materials, but with the benefit of much lower processing temperatures.
Good wettability:
meanwhile, due to similar compositions, Au/Sn solder has a good compatibility with Au metallization due to low leaching rate to thin Au coatings; no migration problems like Ag; etc.
Fluxless:
The alloy allows for fluxless soldering due to the minimal surface oxidation of the high content of Au (80 wt. %). If used in a vacuum or under a forming gas (N2/H2 or Ar/H2-mixture), soldering can be achieved without the use of a chemical flux.
Low viscosity:
the alloy has low enough viscosity in liquid form that it can fill large gaps.
In addition
Au80Sn20 solder has high corrosion and creep resistances and good thermal and electrical conductivities.
