AuSn Soldering

Gold/Tin (Au/Sn) make for hard solder alloys especially used to bond demanding microelectronic and optoelectronic devices. They are available in different forms, such as pre-forms, solder paste or ribbons.

Au/Sn provides a number of advantages, such as high reliability, high melting temperature, excellent resistance to corrosion, good wetting behaviour, high thermal conductivity and high surface tension. When process gas is integrated into the process, chemical flux is not necessary which is beneficial when bonding optical components (diminished risk of pollution).

Au/Sn bonded packages are integrated in many automotive and aerospace applications, in medical devices, military, for lid sealing as well as all devices used for “outdoor” purposes, among others.

As the melting point is rather high, Au/Sn bonding is often combined with other soldering processes such as Indium bonding when performing sequential bonding/step bonding processes. Differing melting points will prevent previous joints from melting.

  • AuSn soldering - overlay of laser bar
    AuSn soldering - overlay of laser bar

What are the Challenges?

  • Highest placement accuracy needed
  • Sensitive components with brittle materials
  • Adapted tooling for delicate edges and facettes
  • Void free bonding required
  • Control force during the bonding process

The Finetech Solution

I. The Principle of AuSn Soldering

After accurate alignment and placement of the chip to the substrate, the chip and substrate are heated up to the melting temperature of the interconnect material. If gold is used, the melting temperature will be about 320°C. Simultaneously force is applied to the chip and controlled during the whole bonding process.

Parameters

Gold (Au): T = 200 … 320ºC

(F = 0.1 … 0.7 N/bump)

II. Adapted Tools & Modules

  • Bonding Force Modules supports optimal force control
  • Adapted tool shapes with custom design to protected defined areas
  • Active and passive equalization tools for even bond line
  • Standard and custom enclosures to provide AuSn joints with optimal bonding atmosphere
  • Mask tools, in combination with 2-point alignment, for face-up and face-down assembly

Integrated Process Management (IPM)

  • Integrated Process Management (IPM)
  • Principle of process gas integration
  • Operating software for bonding

The Integrated Process Management (IPM) is the center piece of a FINEPLACER® system1 - the place where it all comes together. IPM is more than just thermal management. It synchronizes the control of all process modules and their related parameters:

  • Controlled and precisely balanced interaction of top and bottom (pre-)heating and cooling
  • Control of temperature, time, force, power, energy, flow
  • Process-integrated camera and light control
  • Controlled process gas integration for reduced solder contamination, minimized surface tension effects and smooth spherical solder residues

 IPM is very complex, yet easy to access. Via the GUI of the operating software, the user has perfect control of all required adjustments. Just drag 'n drop to define temperature ramps or activate process modules. All settings are represented in only one profile, making for a very intuitive work flow.
 
The operating software provides an ever-growing library of profiles for all kinds of processes. It also offers comprehensive data logging functions essential for statistical process control.

In combination with the system-to-system process transfer capability this is as easy as process development can get.

1 FINEPLACER® core offers co-ordinated top and bottom heating but does not support IPM

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