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

Outline Introduction to EBSD Introduction to EBSD Sample Assembly Sample Assembly Preparation Preparation Measurements Measurements Conclusions Conclusions

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 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. 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. Some of these electrons are incident on atomic planes at angles which satisfy the Bragg equation.

Kikuchi-Pattern The electrons are diffracted to form a set of paired large angle cones corresponding to each diffracting plane. 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. 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. 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. indexing planes Zone axis

Crystal orientation measurements

Assembly of our Samples circuit board Au Ni Cu Sn-Ag-Cu alloy 5 different combinations of Sn-Ag(1-5%wt.)-Cu(0,5-1,2%wt.) alloys where used for testing 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. 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 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. Smallest measurable phase was about 300 nm in diameter.

Reflow process Heating rate Holding time (above liquidus) Cooling rate Slow cooling 0,5 K/s 360 s 0,1° K/s „normal“cooling 0,5 K/s 60 s 5° ± 2° K/s Fast cooling 0,5 K/s 0 s 440° ± 25° K/s The reflow was done with a microscope oven with a temperature range from -196° to 350 °C (Linkam) 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. Fast cooling rates were accomplished by the use of helium gas.

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

Preparation Embedding of the samples in epoxy resin mixed with carbon powder to diminish charging. 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). 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. 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. Sections had to be covered with diluted Conductive-C on the epoxy resin areas for the measurements. The samples where not coated! The samples where not coated!

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

Available binary intermetallic phases in databases for indexing with EBSD: Sn PhaseCrystal structureDatabase Cu Cu 6 Sn 5 Cu 3 Sn monoclinic orthorhombic NIST ICSD/NIST Ni Ni 3 Sn 4 monoclinicNIST Ag Ag 3 SnrhombicNIST Au AuSn 4 hexagonal (hcp)NIST

CuNi Sn Cu 6 Sn 5 Ni 3 Sn 4 (Cu,Ni) 6 Sn 5 (Ni,Cu) 3 Sn 4 (Cu,Ni,Au) 6 Sn 5 (Ni,Cu,Au) 3 Sn 4 AuCuNiSn atomic radius (nm)0,1440,1280,1250,158 lattice structurefccfccfcctetragonal Expected intermetallic phases, indicated through EDX-measurements and literature.

First mappings on „big“ phases Sn = aqua; Cu 6 Sn 5 = red; Ni 3 Sn 4 = blue, Ag 3 Sn = purple; results without zero solutions, background is a band contrast image.

Measurements in the interactive modus and pattern quality Ni

SEM image of Sample 101-0,4 Ni

Manual indexing of the EBSD Pattern Cu 6 Sn 5 7 Bands (101-0,4-2)

Index problems Cu 6 Sn 5 8 BandsNi 3 Sn 4 6 Bands

Short summary for measurements Measurements had to be done very quickly, because the surface deteriorates after a few minutes! 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. 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 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. 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 Cu 6 Sn 5 structure. 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 Cu 6 Sn 5 structure.

Conclusion It was possible to do EBSD-Measurements on small intermetallic phases in small solder joints. 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. 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. 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 Cu 6 Sn 5 (monoclinic) with a Copper content of 0.5 % wt. in the alloy. Although the samples were soldered on Ni the the crystallografic structure of the IMC on the interface is based on Cu 6 Sn 5 (monoclinic) with a Copper content of 0.5 % wt. in the alloy.