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EBSD-Measurements in small lead-free solder joints

EBSD-Measurements in small lead-free solder joints. U. Corradi, Chr. Weippert, J. Villain University of Applied Sciences, Augsburg, Germany 17.5.-18.5.2007 COST-Action 531, Final Joint Working Group Meeting, Vienna, Austria. Outline. Introduction to EBSD Sample Assembly Preparation

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EBSD-Measurements in small lead-free solder joints

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  1. EBSD-Measurements in small lead-free solder joints U. Corradi, Chr. Weippert, J. Villain University of Applied Sciences, Augsburg, Germany 17.5.-18.5.2007 COST-Action 531, Final Joint Working Group Meeting, Vienna, Austria

  2. Outline • Introduction to EBSD • Sample Assembly • Preparation • Measurements • Conclusions

  3. Introduction to EBSD (Electron Backscatter Diffraction) • For EBSD, a beam of electrons is directed at a point of interest on a tilted crystalline sample in the SEM • The atoms in the material inelastically scatter a fraction of the electrons, with a small loss of energy, to form a divergent source of electrons close to the surface. • Some of these electrons are incident on atomic planes at angles which satisfy the Bragg equation.

  4. Kikuchi-Pattern Zone axis • The electrons are diffracted to form a set of paired large angle cones corresponding to each diffracting plane. • The regions of enhanced electron intensity between the cones produce the characteristic Kikuchi bands of the electron backscattered diffration pattern. • Each Kikuchi band can be indexed by the Miller indices of the diffracting crystal plane which formed it. The intersections of the Kikuchi bands correspond to zone axes in the crystal. planes indexing

  5. Crystal orientation measurements

  6. Assembly of our Samples Sn-Ag-Cu alloy Au Ni Cu circuit board • 5 different combinations of Sn-Ag(1-5%wt.)-Cu(0,5-1,2%wt.) alloys where used for testing • During the reflow (soldering) process, the gold dissolutes into the solder alloy. • The soldering was carried out with fast and slow cooling rates in order to vary the size of the intermetallic phases • Smallest measurable phase was about 300 nm in diameter.

  7. Reflow process • The reflow was done with a microscope oven with a temperature range from -196° to 350 °C (Linkam) • Fast cooling rates were accomplished by the use of helium gas.

  8. Optical pictures of the samples after different cooling rates Ni Ni Cu Cu slow cooling, 0,1° K/s fast cooling, 440° ± 25° K/s

  9. Preparation • Embedding of the samples in epoxy resin mixed with carbon powder to diminish charging. • Polished sections where highly ion-etched for 2 minutes with the MET-Etch system from Gatan (for better optical analysis). • Final stage was to ion-polish it with MET-Etch to achieve good EBSD pattern. • Sections had to be covered with diluted Conductive-C on the epoxy resin areas for the measurements. • The samples where not coated!

  10. Measurements • EOScan/VEGA 5130XL • HV = 20kV • WD = 20 mm • Tescan MIRA/LM (Field-Emission) • HV = 20kV • WD = 15 mm

  11. Available binary intermetallic phases in databases for indexing with EBSD:

  12. Expected intermetallic phases, indicated through EDX-measurements and literature.

  13. First mappings on „big“ phases Sn = aqua; Cu6Sn5 = red; Ni3Sn4 = blue, Ag3Sn = purple; results without zero solutions, background is a band contrast image.

  14. Measurements in the interactive modus and pattern quality Ni

  15. SEM image of Sample 101-0,4 Ni

  16. Manual indexing of the EBSD Pattern Cu6Sn5 7 Bands (101-0,4-2)

  17. Index problems Cu6Sn5 8 Bands Ni3Sn4 6 Bands

  18. Short summary for measurements • Measurements had to be done very quickly, because the surface deteriorates after a few minutes! • Because of charging and a high topagraphy in the samples the automatic band detection is not working properly. The bands have to be detected manually. • The low symmetry of the phases often leads to more than one solution. For correct indexing it is very important not only to consider the MAD index (fitting index) but also the number of bands. For the monoclinic phases we tryed to get at least up to 7 bands. • The high MAD- index and more than one solution may indicate a lattice distortion within the crystals. • This proves our assumption, that the phases at the interface are not stoichiometric. Further we can be sure, that the intermetallic phases primarily have a Cu6Sn5 structure.

  19. Conclusion • It was possible to do EBSD-Measurements on small intermetallic phases in small solder joints. • The smallest phases (300 nm) where only measurable manually with a field emission microscope. • Though the phases where measurable the indexed patterns only characterize the crystallografic nature of the phase, because there are not enough standards available. • Although the samples were soldered on Ni the the crystallografic structure of the IMC on the interface is based on Cu6Sn5 (monoclinic) with a Copper content of 0.5 % wt. in the alloy.

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