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Lead-Free Bismuth-Containing Solders

#3387


Lead-Free Bismuth-Containing Solders

Tech Area / Field

  • CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry
  • CHE-SYN/Basic and Synthetic Chemistry/Chemistry
  • CHE-THE/Physical and Theoretical Chemistry/Chemistry
  • MAN-MAT/Engineering Materials/Manufacturing Technology

Status
3 Approved without Funding

Registration date
02.11.2005

Leading Institute
Institute of General and Inorganic Chemistry (IONKh), Russia, Moscow

Supporting institutes

  • VNIIEF, Russia, N. Novgorod reg., Sarov\nTsNIIChermet (Ferrous Metallurgy), Russia, Moscow

Collaborators

  • Eindhoven University of Technology, The Netherlands, Eindhoven\nUniversity of Vienna / Institute of Inorganic Chemistry, Austria, Vienna

Project summary

The project is devoted to development of a bismuth-containing solder to replace the 37Pb-63Sn solder now in general use.

A lot of various characteristics (physical, chemical, technological) and market potential as well must be taken into account at the solder design. The major one is the melting temperature. Recently performed researches allow us to establish a range of the most advanced composition of solders with melting temperature that approaches that of the 37Pb-63Sn alloy. However in order to select a desirable composition with characteristics required for use in electric and electronic assemblies, a complex study of physical, chemical and mechanical properties of the perspective alloys in solid and liquid states should be carried out. The most important are thermodynamic and thermal properties, electrical resistance in a wide temperature range, melt surface tension, wetting angle for different substrate materials, possibility of melt amorphization, chemical properties, microhardness, microstructure and some others.

Practical application of selected solders will become possible only after the complex tests on strength, reliability, corrosion resistance, compatibility with materials of electrical and other facilities, with coatings of components, integrated circuits (IC) and printed circuit boards (PCB). Moreover, it is necessary to test the possibility of using now available fluxes and washing liquids.

In the planned project a modern approach will be used for determination of the solder melting conditions and other necessary characteristics connected with the phase diagrams. It is based on thermodynamic modeling and calculations of phase equilibria coupled with simultaneous experimental studies of both the thermodynamic properties of selected phases and the coordinates of crucial phase equilibria at the diagrams. Thermodynamic modeling will be founded on the universally accepted scheme of joint consideration and analysis of all published information on the thermodynamic properties and phase equilibria in each system under investigation. This will allow us to obtain the self-consistent and, consequently, most adequate characteristics of both types. In the case of the lack of data and/or insufficient precision of available data on thermodynamic properties and phase diagrams, the analysis will indicate correctly the optimal volume and types of experimental research required to obtain the above - noted information. Then the procedure of matching the data on thermodynamic properties and phase diagrams will be repeated and it will allow deciding whether further experimentation is necessary. As the result, the complete and adequate description of the thermodynamic functions and phase diagrams will be found. The described approach has a number of advantages in comparison with the classic physical and chemical analysis. It provides a means for:

  • planning the research in such a way as to sufficiently decrease the volume of experimental studies required for obtaining both the thermodynamic functions with necessary precision and the information about the melting and other regularities connected with phase diagrams;
  • securing the complete, plausible and consistent description of the thermodynamic properties of alloys in solid and liquid states;
  • prediction both stable and metastable states in the alloys;
  • application of the developed thermodynamic descriptions for solution of other problems of the project.

The concentration and temperature dependence of surface tension , which is one of the main property of any solder, will be analyzed on the basis of the developed thermodynamic description of liquid phases. Butler equation will be used for the purpose.

The thermodynamic characteristics of liquid alloys, obtained in the described way, will be used as the basis for description of the alloys ability to transition to amorphous state and dependence of this ability on concentration, for assessment of the critical cooling rates required for this transition. This type process is known to depend mainly on two thermodynamic characteristics – melting entropy and excess heat capacity of liquid. The results of the theoretical consideration will be tested on a few selected samples by melt spinning technique. All these will permit conclusion about possibility of producing the solder in the form of thin amorphous tape that is in the form most advantageous for technological applications.

Thermodynamic database, developed in solving the first goal of the project, will also form the basis for further research directed at elucidating the solder chemical properties such as resistance to oxidation; characteristics of interaction with fluxes, contact materials and metals (Cu, Ag, Au, Pd), possibilities of formation of intermetallic compounds. Thermodynamic analysis and speculations will permit us not only to plan the experimental activity in the above directions, but also to interpret the experimental results and confirm their reliability. Moreover, while accomplishing the mentioned tasks, thermodynamic database will be supplemented and extended through both solving the reverse problems of chemical thermodynamics and obtaining new experimental data.

After establishing the optimal solder, special attention will be given to the study of its interaction with the parts of real electronic and electric assemblies and to the ways of fluxing IC surfaces before and washing after the soldering as well. This will be performed using the available equipment. The designed prototypes of products will be tested on the resistance to “thermal shock” and vibration.

Finally, the expert's assessment and expert’s report on the environmental security of use of the proposed solder in PCB production.

Environmental-friendly lead-free solders must be compatible with the metal-resistive coating of printed circuit boards (PCB). Furthermore, it is necessary to ensure both a set of processing properties (melting temperature, chemical composition, applicability to existing technological processes etc.) and high performance characteristics (reliability).

The following issues will be worked out during the project realization: development of the composition and the technology of applying (i) final lead-free coating for PCB, compatible with the proposed lead-free solder; (ii) composition of environmental-friendly water-soluble flux. Complex tests of properties of soldered joints, PCB coatings, and flux compositions, including the imitation test of long-term storage, will be conducted.

In our opinion, the most advantageous substance for the final lead-free coating of PCB is a Sn-Bi-based alloy with additions of modifying components. It can provide the required mechanical strength and reliability of soldered joints, compatibility with the solder on melting temperature and wetting. What is important that no re-tooling of existing equipment and, consequently, no corresponding expenses are necessary in this case. Modified Sn-Bi coatings demonstrate good metal-resistive characteristics in the process of etching by copper chloride-based ammonia solutions used in manufacturing of PCB. Application of these type alloys for coating current-conducted prints allows production of PCB with clearance between circuits near 0.1 mm.

At the process of soldering, the final coating of PCB can be made compatible with the lead-free solder only, if a specific flux is used. A special polyethylene glycol brazing flux with surfactant additives will be elaborated. The main criteria for choosing the flax optimal composition will be: (i) possibility of use of warm water for PWB washing, (ii) high wettability, and (iii) resistance to corrosion.

In the process of the project realization, a new technology of soldering of electronic arrangements will be devised and the test batch of PCB will be produced, including the production with the automatically operated equipment.

Complex tests of the soldered joints between PCB and electronic assemblies, including imitation tests for long-term storage, will be carried out. Characteristics such as shear strength, resistance to direct pull and corrosion, electrical stability will be determined.

Project will be performed with the participation of high skilled specialists, most of which are involved in the design of weapons. Specialists in chemistry, physics, materials science, catalysis, automation of experimental studies, computer simulation, specialists who have the experience in materials design and soldering technology.

Total effort of weapon scientist and engineers in the project will be 8 335 person*days (84% of total efforts).

Technical problems will be continuously discussed with collaborators to obtain the result ready for use just after the project complete. The joint experimental activity is possible.


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