Silver-Indium-Tin: Interaction with Palladium

The interaction of silver-indium-tin alloys (as possible lead-free solders) with palla-dium (as pos-sible substrate in integrated circuits) will be investigated and the corresponding reaction products will be identified. Electrical properties and the strength of the ensuing solder joints will be tested. Due to the toxicity of lead the currently used lead-tin solders will have to be replaced by lead-free materials. The best alternatives are currently alloys based on tin with silver and/or copper. However, the corre-sponding melting temperatures are more than 30 higher than their traditional lead-tin counterparts (183C) which creates problems for the machinery in industry and for the electronic devices them-selves. As a consequence, different additions have been proposed to lower the melting temperature, one of them being indium (mp = 156.6C). The quality and reliability of solder joints depends on the reaction products that are formed during the soldering process. Considering, that palladium is being used as possible metallization in the production of integrated circuits it is necessary to know which products are formed if Ag-In-Sn solders interact with Pd substrates. In the proposed research project, the phase diagram of the quaternary alloy system Ag-In-Pd-Sn will be investigated to reveal the possible reaction products. The phase diagrams of the limiting binary alloy systems as well as of the two ternary systems Ag-In-Sn and Ag-Pd-Sn will be accepted from literature, the other two ternary phase diagrams, Ag-In-Pd and In-Pd-Sn, will also have to be investigated experimentally using thermal analyses, X-ray powder diffraction and electron microprobe analyses. Together with the experimental investigations an optimization of the two ternary sys-tems and of the quaternary system will be performed using the well-established CALPHAD-method (CALcu-lation of PHase Diagrams). This optimization provides the thermochemical properties in para-metric form and permits a calculation of the relevant phase diagrams. Thermochemical data that might turn out useful or necessary for this optimization will be determined experimentally using an emf-method as well as different calorimetric methods. Finally, the electrical properties of Ag-In-Sn alloys and of Ag-In-Pd-Sn alloys represent-ing the actual solder joints will be studied, and the strength and reliability of model alloys as well as of actual solder joints will be tested.

As a consequence of the two Directives of the European Union, RoHS (Restriction of the use of certain hazardous substances in electrical and electronic equipment) and WEEE (Waste Electrical and Electronic Equipment) which went into force on July 1, 2006, no lead is allowed in electrical and electronic equipment anymore within the European Union. This made the development of new lead-free solder materials necessary in order to substitute the traditional tin-lead solders. Besides tin-silver, tin-copper, and tin-silver-copper which are used frequently as substitutes, tin-silver-indium alloys appeared as a possible alternative, especially since their melting temperature could be tuned to be similar to that of the traditional tin-lead solder by varying the composition appropriately. Thus it was the goal of the present project to find out which reaction products are formed if such solders react with contacts containing palladium as metallization, as well as to determine some of the properties of these solders and of their reaction products which are of relevance for the reliability of the corresponding solder joints. With the obtained results it is now possible to answer part of these questions, i.e. the so-called phase diagrams of the ternary systems silver+indium+palladium and indium+palladium+tin were determined which describe the state of an alloy formed from the corresponding three metals depending on composition and temperature. In addition, a number of thermochemical properties (for example, heats of mixing of liquid alloys) were determined for alloys from two or three of the elements silver, indium, tin, and palladium, but also for the four-component alloy. These data were introduced into a European Database (COST 531 Thermodynamic Database) which can be accessed by European scientists participating in a common research action (COST 531, "Lead-free Solder Materials"). This database can be used to calculate optimized and consistent phase diagrams for practically all binary and ternary alloy systems of the metals contained in it, as well as to extrapolate the phase diagrams of higher order systems. From the corresponding extrapolated four-component phase diagram silver+indium+palladium+tin it will be possible to determine which intermetallic compounds are formed if tin-silver-indium solders react with palladium containing contacts. In addition, a number of properties of the tin-silver-indium solders as well as of model solder joints were determined, as for example some mechanical properties, the electrical conductivity of the solders, and their wetting behavior.