Thermodynamics of lead-free high temperature solder alloys

Lead and lead containing alloys are considered to be toxic and harmful for the environment. Thus, in the European Union and parts of Asia laws have been enacted in order to ban the application of lead in solders for electric and electronic products. However, there are still many exemptions for some applications caused by serious problems to find a proper substitute for hitherto used lead-containing solders. Such an application is the so-called "High- temperature soldering" at approximately 230 - 350 C. The main application for high temperature solders in the electronic industry concerns advanced packing technologies, e.g. die-attach and BGA (Ball Grid Array) solder spheres, chip scale package and multi chip modeling. Regarding the solder alloys candidates (see detailed application) and frequently used contact materials (Cu, Ni, Ag, Au) the alloy systems Cu-Ni-Sn, (Ag,Au,Cu)-Sb-Sn, Au-Sn as well as Ga-Ni and Bi-Ni have been chosen for investigation within this project. A basic knowledge of thermodynamic properties, e.g. phase relations, diffusion behavior, thermochemistry, of these and respective higher order systems is crucial for a systematic design of solder materials. The intermetallic systems mentioned above will be experimentally investigated in terms of X-ray diffraction, thermal analysis, metallography and calorimetry. Thermodynamic calculations of the CALPHAD type will be employed to prove the consistence of our data and experimental literature data. A simultaneous performance of experiments and calculations will reduce the experimental efforts. Finally a contribution to a data base with all this information for binary and ternary alloys needs to be created. Well known CALPHAD procedures will be applied for the assessment and presentation of experimental data. This allows the extrapolation of thermodynamic calculations to higher order intermetallic systems. Within the COST-531 Action, which was run from 2002-2007, such a database was created for soldering at 180 - 240 C. The following COST-602 action, initiated by A. Kroupa, a cooperation partner of this project, was started in 2007. It is dedicated to high-temperature soldering and one of the aims is the creation of such a database for these kinds of solder systems. Thus the expected results of the present basic research project will be a valuable contribution within the framework of COST-602.

Guidelines for the production and disposal of electronic devices lead us to a general ban of lead in solder materials. However, contrary to low-temperature soft solders for high-temperature soft solders sufficient lead-free alternatives are not available. In order to systematically design such materials the knowledge of thermodynamic properties is basically necessary. Experimental thermodynamic investigations of binary and ternary intermetallic alloy systems relevant for lead-free solders have been successfully performed. This comprises phase diagram work as well as the determination of thermochemical properties like enthalpy of formation or heat capacity of various phases. The main focus was on Ni-Zn and Ni-Sn-Zn but also systems like Cu-Sn-Zn, In-Ni-Sn, Ni-Sb-Sn and several others have been investigated. The data generated are crucial for a valid thermodynamic description of the alloy systems which is the basis of various types of calculations, e.g., phase diagrams, inter-diffusion, or solidification simulations. There was an extensive co-operation with other researchers of this field within COST-Action MP0602 High-temperature lead-free solders. Most of our work has been published yet in well reputable, peer reviewed journals. Our achievements will contribute to a systematic, computer based development of new high-temperature lead-free solders.