An Elementary Introduction to Intermetallics in Ball Bonds
-
Upload
christopher-breach -
Category
Engineering
-
view
910 -
download
4
Transcript of An Elementary Introduction to Intermetallics in Ball Bonds
ProMat
Consultants
© 2015
An Elementary Introduction to Intermetallics in Ball Bonds
1
ProMat
Consultants
© 2015
The following slides give an elementary, non-rigorous introduction
to intermetallics in ball bonds
2
ProMat
Consultants
© 2015
ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary
3
ProMat
Consultants
© 2015
Ball Bonding Process
4
1st Bond
2nd Bond
Looping
ProMat
Consultants
© 2015
The Ball Bond CycleUltrasound softens the ball and makes it easier to compress AND activates a chemical reaction between the ball and bond pad
5
J. Schwizer, M. Mayer, O. Brand. Force Sensors for Microelectronics Packaging. Springer Series in Microtechnology and MEMS 2005.
ProMat
Consultants
© 2015
1st Bond & Intermetallics: 1
6
(not to scale)
ProMat
Consultants
© 2015
1st Bond & Intermetallics: 2
7
(not to scale)
ProMat
Consultants
© 2015
1st Bond & Intermetallics: 3
8
(not to scale)
ProMat
Consultants
© 2015
1st Bond & Intermetallics: 4
9
Au
Cu
(not to scale)
ProMat
Consultants
© 2015
Intermetallics Join Balls to Bond Pads
10
Au, Cu (and Ag) wires form intermetallics with Al alloy bond pads
Intermetallics easily visible with Au wires, not so easy to see with Cu wires
Au
Cu
ProMat
Consultants
© 2015
ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary
11
ProMat
Consultants
© 2015
ELEMENTARY METAL PHYSICS 12
ProMat
Consultants
© 2015
Electrons in Elemental Metals
13
Simple metal Complex metals
Increasing valence electron energy
‘bonding’ or valence electrons
The valence electron energies of Al and Cu or Au are very different with the lower energy valence electrons of Al moving more slowly than the higher energy valence electrons in Au and Cu
ProMat
Consultants
© 2015
Electrons in Solid Metals
14
Valence electrons in Au, Al, Cu, Ag are highly mobile
Electrons are free to move throughout the metal
The electrons are not bonded to any single atom
Bonding and cohesion is due to attraction between all electrons and atoms
Ions
Gas of electrons shared by all ions
ProMat
Consultants
© 2015
Solid Metal Elastic Deformation
15
Al, Au, Cu, Ag easily deform (ductile)
The electron gas-ion cores resist the motion
When the atoms move the low viscosity electron gas easily follows the atom movement
Stretching of Planes of Atoms
ProMat
Consultants
© 2015
Solid Metal Plastic Deformation
16
When forces are high enough to overcome the resistance of the electron gas-ion cores, layers of atoms slide
There is permanent shape change
Electron gas rearranges relatively easy and follows the ion cores
Sliding of Planes of Atoms
ProMat
Consultants
© 2015
Deformation: Ductile Metals
17
Elongation (%)
Ten
sile
Str
ess
(MPa
)
Plastic Elastic
Planes of atoms slide
Mobile electrons follow the atoms
ProMat
Consultants
© 2015
ELEMENTARY ALLOY PHYSICS 18
ProMat
Consultants
© 2015
Electrons in Alloys
19
Metals that have valence electrons similar in energy mix easily and form alloys with each element sharing electrons like a gas over the whole alloy
Grey & blue circles represent Ag and Au atoms*
*this is not a representation of the true crystallographic structure of Ag-Au alloys
ProMat
Consultants
© 2015
Alloy Deformation
20
Valence electrons in deformed alloys behave similarly to metals
Valence electrons follow the movement of the ions
Elastic Deformation
Plastic Deformation
ProMat
Consultants
© 2015
Deformation: Ductile Alloys
21
Elongation (%)
Ten
sile
Str
ess
(MPa
)
Elastic Plastic
Planes of atoms slide
Mobile electrons follow the atoms
ProMat
Consultants
© 2015
INTERMETALLIC COMPOUNDS 22
ProMat
Consultants
© 2015
Electrons in Intermetallics
23
Electrons with large energy differences interact via a complex process of energy exchange and localised sharing of electrons to form stable compounds
Electrons may be localised over regions of space between clusters of atoms
ProMat
Consultants
© 2015
Mixed Bonding
24
The electronic structure of intermetallics may be covalent-like, ionic-like, metallic or a mixture of each
Intermetallics with strongly covalent character are often brittle
ProMat
Consultants
© 2015
Intermetallic DeformationElectrons in intermetallics resist deformation and localised bonds are stretched
Electrons in the localised regions are free to move in a limited spatial volume 25
Elastic Deformation
ProMat
Consultants
© 2015
Intermetallic DeformationElectrons in intermetallics resist deformation and localised bonds are stretched
Electrons in the localised regions are free to move in a limited region of space
Electrons cannot easily redistribute and plastic deformation is limited or cannot occur 26
Strained Bonds
‘Snapped’ Bonds-Brittle Failure without Plastic Deformation
ProMat
Consultants
© 2015
Deformation: Brittle Intermetallics
27
Elongation (%)
Ten
sile
Str
ess
(MPa
)
Elastic
Material ‘Snaps’: Brittle Brittle intermetallics show little or no plastic deformation
When the chemical bonds are strained to the limit they snap
Electrons are not mobile
ProMat
Consultants
© 2015
Intermetallic Crystal Structures
28
The crystal structures are often very different from the individual components
Very complex structure
Al Au
Simple structure Simple structure
+
Au8Al3
ProMat
Consultants
© 2015
ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary
29
ProMat
Consultants
© 2015
Au-Al intermetallics
30
Au2Al Au4Al AuAl
Au8Al3
AuAl2
Increasing Al
Representations of Au-Al crystal structures
ProMat
Consultants
© 2015
Cu-Al IntermetallicsCu9Al4
Cu3Al2
Cu4Al3
CuAl
CuAl2
Increasing Al
Representations of Cu-Al crystal structures
ProMat
Consultants
© 2015
Resistivity of Cu-Al and Au-Al intermetallics
32
Intermetallic bonding is often complex compared with metals
Some electrons may be involved in chemical bonding and others may be available for conduction
Electrical conductivity is usually lower than metals
Au-Al intermetallics are poorer electrical conductors than Cu-Al
For the same bonding conditions Au-Al intermetallics are thicker than Cu-Al
ProMat
Consultants
© 2015
ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary
33
ProMat
Consultants
© 2015
SummaryIntermetallics commonly form between metals with very different electronic structures
Bonding in intermetallics is complex and can be mixed (covalent/ionic/metallic)
Strongly covalent/ionic Intermetallics are often brittle with poor electrical conductivity
Intermetallics with more metallic character may show some plastic deformation and higher electrical conductivity
34