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Investigating the Microstructure and Physical and Mechanical Properties of Composite Lead-Free Solder Containing Ferromagnetic Particles

Nourmohammadi, Amir Hossein | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55655 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. The use of tin-lead solder alloys (Pb-Sn) has been limited due to environmental problems in recent years, and several types of research are being carried out on lead-free solder alloys. However, these solders have weaker physical and thermal properties than Pb-Sn solders. In recent years, researchers have been trying to improve these solders' physical and mechanical properties by adding reinforcing particles to the solder and using a magnetic field during melting and solidifying the solder. On the other hand, with the growth path of miniaturization and, simultaneously, compaction and integration of electronic equipment, electronic circuits should have higher current density, better efficiency, and longer life. One of the approaches in this field is the emergence of a new generation of composite and nanocomposite solders. Higher current density can be used in electronics industries by using soft composite solders that show more microstructural stability at high temperatures. In this research, Sn-0.3Ag-0.7Cu solder alloy was used as a paste along with cobalt powder (with ferromagnetic properties) with a size distribution of 8-10 micrometers in the range of 0 to 2% by weight.Cobalt particles did not significantly reduce the solder's melting point and electrical resistance. Also, during the solidification of the solder, they play the role of nucleate, which caused the reduction of undercooling during the soldering process. However, solder embrittlement decreased with increasing cobalt. By using the optimal amount of these particles (cobalt 0.2% by weight), the thickness of the IMCs compounds at the interface between the solder and the copper substrate was reduced by 45%, and their morphology was changed from a scallop to a smooth state. At the optimal percentage of cobalt, the solder strength increased by 35%. It also increased the strength of the solder by 50% in the aging test and 10% in the thermal fatigue test. Using the magnetic field, we tried to reduce the problems of adding cobalt particles to the solder, including reducing the solder's brittleness and the cobalt's repulsion in the thermal fatigue test. Using a magnetic field during soldering changed the distribution of cobalt in the solder matrix from random distribution to consecutive columns. This caused an increase in the mechanical properties of the solder, brittleness, and an increase in the properties of the solder in the thermal fatigue test due to the stability of the cobalt particles in the field.
  9. Keywords:
  10. Microstructure ; Magnetic Fields ; Soldering Paste ; Lead Free Solders ; Reflow Process ; Cobalt Particles ; SAC Solder Alloy

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