The phase diagram of the binary Au-Sn sytem was
first described by Vogel [1]. In the meanwhile several
papers were published on this topic because of the
complexity of this system. Fig 1 shows the phase diagram
published by Okamoto and Massalski [2].
The Au-Sn system has two eutectic points and four
intermetallic phases. The eutectic alloy used for soldering
applications has an eutectic temperature of 278°C at a
composition of eighty weight percent of gold and twenty
weight percent of tin. This composition is named
Au80Sn20. If an eutectic Au80Sn20 melt freezes an
eutectic reaction occurs with the formation of
intermetallic AuSn phase (also called d-phase) and z-
phase. At temperatures below 190°C a congruent reaction
occurs and the z’-phase is formed. This is a stable
intermetallic phase with the composition Au5Sn. If an
Au80Sn20 solid alloy is heated up to the eutectic
temperature melting starts at each interface between gold
rich and tin rich phases. More details are described in
Vogel1, Hansen [3], Okamoto and Massalski2 and in
Matijasevic et al.