Tin gold solders
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 280⁰C) 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.
2. 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
Soldering temperature: The soldering temperature is only 20-30 °C above its melting point (about 300 ~ 310 ⁰C). 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 °C.
|Coefficient of thermal expansion*||16 X10¯6 /°C @20-250°C.|
|Thermal conductivity||57.5 W.m¯¹K¯¹|
|Shear modulus||25 GPa|
|Electrical resistivity||16.4 X10-8 Ω.m|
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.
3. Thermodynamic Properties
The superior properties of Au/Sn result from its thermodynamic properties. The Au/Sn alloy system forms a eutectic alloy at the composition of 80 wt. % (or 71at. %) Au and 20 wt. % (or 29 at. %) Sn as shown in Figure The reaction L ↔έ + δat 280°C provides the basis of the eutectic solder. The interesting phases for Au-Sn solder include; έ (Au5Sn), έ and δ (AuSn). The έ phase has a composition of 10.7 wt. % Sn and exists up to 190 °C . It has a hexagonal structure. The έ phase exists from the peritectic β+ L ↔έ at 5.7 wt. % Sn at 521°C to 11.3 wt. % Sn at 280 °C, and to 8.8 wt. % Sn at 190°C. This phase has a Mg-type close packed hexagonal structure. The δ phase is an intermetallic compound with a melting point of 419.3°C anda NiAs-type hexagonal structure. It is a non-stoichiometric compound with a Sn composition of 50.0 to 50.5 at. % (or 37.5 to 37.9 wt. %).
Preforms: Typically punched, these thin pieces of solder are manufactured as squares, rectangles, frames, disks, washers, and custom geometries. Solder preforms can be applied in surface mount technology (SMT), which is common to the manufacture of most consumer electronics such as cellular phones and computers. Preforms separately attach a component to a pad, or they augment the solder volume of the solder paste. Washers serve as pin connectors or other through-hole components.
The advantages of preforms are:
- Accurately controlled solder volume, composition, and surface condition, enabling a wide soldering-process window and optimal finished assembly quality, resulting in a high-reliability of consistent solder joints, which are required by the high Cpk-demands by the industry, while assuring lower, bottom-line, quality assurance cost.
- Preforms used in a controlled atmosphere eliminate the need for the messy and uncontrolled use of flux. A proper control over the soldering conditions eliminates the need for a costly, post-soldering cleaning.
- Preforms are often the best solution for high-reliability and excellent heat transfer to meet the ever-increasing demand for making challenging joints.
- On the requirements of the change of substrate materials for their specific properties and/or their environmentally friendly behavior, there is almost no limit for AuSn preforms.
- When correctly designed and applied, preforms can have a high ratio of performance to cost, creating extremely high yields of sound and electrically reliable soldered joints.