Properties of lead-free solders

Due to environmental reasons, lead will be forbidden as a component of solder materials by July 1, 2006. The replacement most companies chose is the ternary Sn-(2-4)Ag-(0.5-2.3)Cu (SAC) alloy. This material has a melting point of 217C, which is much higher than the one of 63Sn-37Pb alloy (183C). This leads to higher stress for the boards to which the alloy is soldered. Additionally has to be said that the toxicity of silver is not fully cleared yet and the SAC solder cannot be used for all possible applications. Therefore it is absolute necessary solve this problem and to find other materials that can be used as lead-free solders. In this project different properties of selected ternary alloys shall be investigated. The wetting behaviour of the ternary Cu-In-Sn alloy with a content of 5 at.% copper on a copper substrate as well as the shear behaviour of this alloy has been investigated previously, but only in a rather macroscopic dimension. Measurements of the wetting angle at a higher copper concentration will be performed during this research. During this project the thermomechanical properties of the ternary system in the copper poor region as well as in a region with a higher copper content shall be investigated on a rather microscopic scale. Special attention will be given to the analysis of the site of fraction by SEM (Scanning Electron Microscopy) dependent on the kind and size of the IMCs (InterMetallic Compounds). The surface tension of the ternary Al-Sn-Zn, Ag-Sn-Zn, and Ag-Bi-Sn system shall be investigated. This quantity has been calculated previously at our institute and with this research experimental data can be used for the controlling of the calculated data as well as for the improvement of the calculation procedure. Furthermore the thermodynamic properties of the ternary Pd-Sn-Zn and Ag-Au-Sn alloys shall be determined with EMF measurements and with a calorimetric method. 3.5Ag-Sn and 91Sn-9Zn are already used as lead free solders and their interaction with different substrate materials like Pd and Au are of great interest. The composition of all of the prepared samples will be investigated with X-ray measurements, and the measurements of the melting points will be carried out either with DSC (Differential Scanning Calorimetry) or DTA (Differential Thermal Analysis), depending on the sample.

Thermodynamic properties of different solder materials have been investigated, as well as the wetting behaviour, the surface tension and the strength of the solder/substrate connection. Also the short range order of the liquid As- Se system has been investigated using synchrotron radiation. Thermodynamic calculations of the thermodynamic properties of the Ag-Au-Sn and the Ag-Bi-Sn system have been performed.