Transient Liquid Phase Bonding (TLPB) on Cu/In-48Sn/Cu bonds as alternative Pb-free interconnection technology improving service temperature

Abstract: An alternative lead-free solder alloy In–48 at%Sn with a melting point of 120 °C and its implementation to bond Cu substrates in a diffusion soldering joining method, which below to the Transient Liquid Phase Bonding (TLPB) family, are presented. This method consists in the complete transformation of the original soft solder in intermetallic phases by the solid-liquid interaction with the substrates.
According to the EPMA, TEM/EDX and electron diffraction analyses, two different behaviours were observed in the interconnection zone depending on the temperature range: (i) below 200°C a single layer consisting of η phase; (ii) above 200°C a Cu-poor region consisting of η phase and a Cu-rich layer formed by a mixture of thin alternate regions of ζ-CuSn3 and δ-Cu7In3 phases perpendicular to the interconnection plane. The η layer shows two morphologies: large grains and fine grains at the η/In–48Sn (liquid) and at the η/Cu-rich interfaces, respectively. Additionally, the η region shows a gradual change in composition, suggesting a change from the Cu6Sn5 to the Cu2In structures.
Regarding the reliability properties, kinetic, mechanical, electrical, thermal and corrosion behaviour were investigated. Thermal stability tests indicate that the thermal resistance of the bonds is about 750 °C, offering high service temperatures, about 200-400°C above the production temperature. The thickness of the Cu-rich layer shows a constant growth rate (linear growth) between 290 to 400°C. The temperature dependence of the growth rate constant of the Cu-rich layer is described by an Arrhenius relationship. The joints show a very good mechanical behaviour, especially when the interconnection zone was formed only by the Curich layer, reaching values of 150 MPa for ultimate tensile strength. The electrical resistivity and the corrosion behaviour seem to be better than the one of traditional Pb-Sn solders.

date: 17.12.2009
place: UHW 11/1102
time: 10 am

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