New Au-based solder for RoHS-compliant Electronics: AuSn Plus
Years of development and testing have produced a newly formulated Au-based solder for attaching die and sealing lids of RoHS-compliant electronic components. Materion’s new eutectic AuInSn alloy is designed specifically for use in packages that will subsequently be soldered with SAC (tin-silver-copper) alloys in lead-free circuit assembly processes.
The addition of indium to traditional AuSn 80/20 raises its melting point from 278°C to 306°C, providing protection against secondary reflow, loss of hermeticity, or critical component shift during the SMT soldering process.
Materion’s AuSn Plus has comparable physical properties to AuSn and slightly higher processing temperatures near 350°C, similar to those of high-lead solders. The inclusion of indium in the alloy does not affect the AuSn intermetallic formation; the indium substitutes for some of the tin in the Au4Sn intermetallic compound. As a result, the new ternary alloy is slightly harder and less ductile than the original AuSn alloy. The AuSn Plus physical properties are listed in the table below.
|Processing Peak Temp (oC)
|Electrical Resistance (microhms cm)
|Electrical Conductivity (%IACS)
|Thermal Conductivity (W/mK)
|Tensile Strength (PSI)
|Yield Strength (PSI)
Alloy development process
The goal of the research was to formulate a soldering alloy with two key characteristics:
- A melting point between 300° and 309°C
- Single melting point (eutectic) preferred, or the narrowest pasty range possible
Materion scientists began with eutectic 80/20 AuSn, and experimented by doping it with germanium and indium. Screening tests immediately eliminated germanium because it actually lowered the melting point of the resulting alloy. Adding indium raised the melting point, and was carried forward to the next level of experimentation.
The experimental matrix with ten test alloy compositions and their measured melting points is shown below. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC). The test compositions were confirmed with inductively coupled plasma - optical emission spectroscopy (ICP-OES).
Alloy Composition Test Results
|| Liquidus, °C
||*Note that alloy composition is measured in atomic percent, not weight percent.
The baseline Au80Sn20 wt% is expressed as Au70.7Sn29.3 at%
The compositions tested in runs 4, 5 and 6 all met the goal of single melting points between 300 and 309°C. The final composition is known as AuSn Plus.
This process-proven, patent-pending solder alloy is now commercially available for testing and qualification. For more information, contact us.
Side Piece: RoHS driving need for higher temperature AuSn solders
The tin-lead solder used in circuit assembly melts at 183°C and is processed above its melting point for 30-75 seconds at temperatures of up to 230°C. The tin-silver-copper (SAC) solder family used for Pb-free circuit assembly melts at 217-227°C, and is processed above its melting point for 45-120 seconds, at temperatures of up to 260°C, and sometimes higher.
The AuSn solder that is traditionally used to seal hermetic packages and attach die melts at 278°C. If it is used in packages that experience SnPb reflow temperatures, a 48° thermal safety margin insures that the AuSn will not inadvertently melt during the circuit assembly’s soldering cycle. If it is used in packages that experience Pb-free reflow cycles, the safety margin shrinks to only 18°. Improperly profiled PCBs, uncalibrated ovens, or varying thermal densities in PCB layouts can drive peak temperatures up, causing the AuSn to reflow during the circuit assembly process. Secondary reflow of the package’s solder can cause a loss of hermeticity or shift of critical components, impairing the functionality and or reliability of the device.
The new AuInSn solder alloy raises the melting temperature of the solder to 306°C, and expands the safety margin between melting points to a robust 46°C.