EBSD-Measurements in EBSD-Measurements in small lead-free solder small lead-free solder

jointsjoints

U. Corradi, Chr. Weippert, J. VillainU. Corradi, Chr. Weippert, J. VillainUniversity of Applied Sciences, Augsburg, University of Applied Sciences, Augsburg,

GermanyGermany

17.5.-18.5.200717.5.-18.5.2007COST-Action 531, Final Joint Working Group COST-Action 531, Final Joint Working Group

Meeting,Meeting, Vienna, AustriaVienna, Austria

OutlineOutline

Introduction to EBSDIntroduction to EBSD Sample Assembly Sample Assembly PreparationPreparation MeasurementsMeasurements ConclusionsConclusions

Introduction to EBSD Introduction to EBSD (Electron Backscatter (Electron Backscatter

Diffraction)Diffraction)

For EBSD, a beam of electrons is directed at a point of interest on a tilted For EBSD, a beam of electrons is directed at a point of interest on a tilted crystalline sample in the SEMcrystalline sample in the SEM

The atoms in the material inelastically scatter a fraction of the electrons, 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 with a small loss of energy, to form a divergent source of electrons close to the surface.the surface.

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

Kikuchi-PatternKikuchi-Pattern

The electrons are diffracted to form a set of paired large angle The electrons are diffracted to form a set of paired large angle cones corresponding to each diffracting plane.cones corresponding to each diffracting plane.

The regions of enhanced electron intensity between the cones The regions of enhanced electron intensity between the cones produce the characteristic Kikuchi bands of the electron produce the characteristic Kikuchi bands of the electron backscattered diffration pattern.backscattered diffration pattern.

Each Kikuchi band can be indexed by the Miller indices of the Each Kikuchi band can be indexed by the Miller indices of the diffracting crystal plane which formed it. The intersections of the diffracting crystal plane which formed it. The intersections of the Kikuchi bands correspond to zone axes in the crystal.Kikuchi bands correspond to zone axes in the crystal.

indexing

planes

Zone axis

Crystal orientation Crystal orientation measurementsmeasurements

Assembly of our SamplesAssembly of our Samples

circuit board

AuNiCu

Sn-Ag-Cu alloy

5 different combinations of Sn-Ag(1-5%wt.)-Cu(0,5-1,2%wt.) 5 different combinations of Sn-Ag(1-5%wt.)-Cu(0,5-1,2%wt.) alloys where used for testingalloys where used for testing

During the reflow (soldering) process, the gold dissolutes During the reflow (soldering) process, the gold dissolutes into the solder alloy.into the solder alloy.

The soldering was carried out with fast and slow cooling The soldering was carried out with fast and slow cooling rates in order to vary the size of the intermetallic phasesrates 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 processReflow process

Heating Heating raterate

Holding time Holding time (above (above

liquidus)liquidus)

Cooling rateCooling rate

Slow coolingSlow cooling 0,5 K/s0,5 K/s 360 s360 s 0,1° K/s0,1° K/s

„„normal“coolingnormal“cooling 0,5 K/s0,5 K/s 60 s60 s 5° ± 2° K/s5° ± 2° K/s

Fast coolingFast cooling 0,5 K/s0,5 K/s 0 s0 s 440° ± 25° K/s440° ± 25° K/s

The reflow was done with a microscope The reflow was done with a microscope oven with a temperature range from -oven with a temperature range from -196° to 350 °C (Linkam)196° to 350 °C (Linkam)

Fast cooling rates were accomplished Fast cooling rates were accomplished by the use of helium gas.by the use of helium gas.

Optical pictures of the samples Optical pictures of the samples after different cooling ratesafter different cooling rates

NiCu

NiCu

slow cooling, 0,1° K/s fast cooling, 440° ± 25°± 25° K/s

PreparationPreparation

Embedding of the samples in epoxy resin mixed Embedding of the samples in epoxy resin mixed with carbon powder to diminish charging.with carbon powder to diminish charging.

Polished sections where highly ion-etched for 2 Polished sections where highly ion-etched for 2 minutes with the MET-Etch system from Gatan minutes with the MET-Etch system from Gatan (for better optical analysis).(for better optical analysis).

Final stage was to ion-polish it with MET-Etch to Final stage was to ion-polish it with MET-Etch to achieve good EBSD pattern.achieve good EBSD pattern.

Sections had to be covered with diluted Sections had to be covered with diluted Conductive-C on the epoxy resin areas for the Conductive-C on the epoxy resin areas for the measurements.measurements.

The samples where not coated!The samples where not coated!

MeasurementsMeasurements

EOScan/VEGA EOScan/VEGA 5130XL5130XL

HV = 20kVHV = 20kV WD = 20 mmWD = 20 mm

Tescan MIRA/LMTescan MIRA/LM((Field-Emission)Field-Emission)

HV = 20kVHV = 20kV WD = 15 mmWD = 15 mm

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

SnPhase Crystal structure Database

CuCu6Sn5

Cu3Snmonoclinicorthorhombic

NISTICSD/NIST

Ni Ni3Sn4 monoclinic NIST

Ag Ag3Sn rhombic NIST

Au AuSn4 hexagonal (hcp) NIST

   Cu Ni

Sn

Cu6Sn5 Ni3Sn4

(Cu,Ni)6Sn5 (Ni,Cu)3Sn4

(Cu,Ni,Au)6Sn5 (Ni,Cu,Au)3Sn4

Au Cu Ni Sn

atomic radius (nm)

0,1440,144 0,1280,128 0,1250,125 0,1580,158

lattice structure

fccfcc fccfcc fccfcc tetragontetragonalal

Expected intermetallic phases, Expected intermetallic phases, indicated through EDX-measurements indicated through EDX-measurements

and literatureand literature..

First mappings on „big“ phasesFirst mappings on „big“ phases

Sn = aqua; Cu6Sn5 = red; Ni3Sn4 = blue, Ag3Sn = purple; results without zero solutions, background is a band contrast image.

Measurements in the interactive Measurements in the interactive modus and pattern qualitymodus and pattern quality

Ni

SEM image of Sample 101-SEM image of Sample 101-0,40,4

Ni

Manual indexing of the Manual indexing of the EBSD PatternEBSD Pattern

Cu6Sn5 7 Bands (101-0,4-2)

Index Index problemsproblems

Cu6Sn5 8 Bands Ni3Sn4 6 Bands

Short summary for Short summary for measurementsmeasurements

Measurements had to be done very quickly, because the Measurements had to be done very quickly, because the surface deteriorates after a few minutes!surface deteriorates after a few minutes!

Because of charging and a high topagraphy in the samples Because of charging and a high topagraphy in the samples the automatic band detection is not working properly. The the automatic band detection is not working properly. The bands have to be detected manually.bands have to be detected manually.

The low symmetry of the phases often leads to more than The low symmetry of the phases often leads to more than one solution. For correct indexing it is very important not one solution. For correct indexing it is very important not only to consider the MAD index (fitting index) but also the only to consider the MAD index (fitting index) but also the number of bands. For the monoclinic phases we tryed to number of bands. For the monoclinic phases we tryed to get at least up to 7 bands. get at least up to 7 bands.

The high MAD- index and more than one solution may The high MAD- index and more than one solution may indicate a lattice distortion within the crystals.indicate a lattice distortion within the crystals.

This proves our assumption, that the phases at the This proves our assumption, that the phases at the interface are not stoichiometric. Further we can be sure, interface are not stoichiometric. Further we can be sure, that the intermetallic phases primarily have a Cuthat the intermetallic phases primarily have a Cu66SnSn55 structure.structure.

ConclusionConclusion It was possible to do EBSD-Measurements on It was possible to do EBSD-Measurements on

small intermetallic phases in small solder joints.small intermetallic phases in small solder joints. The smallest phases (300 nm) where only The smallest phases (300 nm) where only

measurable manually with a field emission measurable manually with a field emission microscope.microscope.

Though the phases where measurable the Though the phases where measurable the indexed patterns only characterize the indexed patterns only characterize the crystallografic nature of the phase, because there crystallografic nature of the phase, because there are not enough standards available.are not enough standards available.

Although the samples were soldered on Ni the the Although the samples were soldered on Ni the the crystallografic structure of the IMC on the crystallografic structure of the IMC on the interface is based on Cuinterface is based on Cu66SnSn55 (monoclinic) with a (monoclinic) with a Copper content of 0.5 % wt. in the alloy.Copper content of 0.5 % wt. in the alloy.