an elementary introduction to intermetallics in ball bonds

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An Elementary Introduction to Intermetallics in Ball Bonds

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The following slides give an elementary, non-rigorous introduction

to intermetallics in ball bonds

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ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary

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Ball Bonding Process

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1st Bond

2nd Bond

Looping

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The Ball Bond CycleUltrasound softens the ball and makes it easier to compress AND activates a chemical reaction between the ball and bond pad

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J. Schwizer, M. Mayer, O. Brand. Force Sensors for Microelectronics Packaging. Springer Series in Microtechnology and MEMS 2005.

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1st Bond & Intermetallics: 1

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(not to scale)

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1st Bond & Intermetallics: 2

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(not to scale)

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1st Bond & Intermetallics: 3

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(not to scale)

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1st Bond & Intermetallics: 4

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Au

Cu

(not to scale)

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Intermetallics Join Balls to Bond Pads

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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

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ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary

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ELEMENTARY METAL PHYSICS 12

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Electrons in Elemental Metals

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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

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Electrons in Solid Metals

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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

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Solid Metal Elastic Deformation

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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

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Solid Metal Plastic Deformation

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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

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Deformation: Ductile Metals

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Elongation (%)

Ten

sile

Str

ess

(MPa

)

Plastic Elastic

Planes of atoms slide

Mobile electrons follow the atoms

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ELEMENTARY ALLOY PHYSICS 18

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Electrons in Alloys

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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  

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Alloy Deformation

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Valence electrons in deformed alloys behave similarly to metals

Valence electrons follow the movement of the ions

Elastic Deformation

Plastic Deformation

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Deformation: Ductile Alloys

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Elongation (%)

Ten

sile

Str

ess

(MPa

)

Elastic Plastic

Planes of atoms slide

Mobile electrons follow the atoms

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INTERMETALLIC COMPOUNDS 22

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Electrons in Intermetallics

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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

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Mixed Bonding

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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

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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

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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

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Deformation: Brittle Intermetallics

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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

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Intermetallic Crystal Structures

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The crystal structures are often very different from the individual components

Very complex structure

Al Au

Simple structure Simple structure

+

Au8Al3

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ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary

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Au-Al intermetallics

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Au2Al  Au4Al  AuAl  

Au8Al3  

AuAl2  

Increasing Al

Representations of Au-Al crystal structures

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Cu-Al IntermetallicsCu9Al4  

Cu3Al2  

Cu4Al3  

CuAl  

CuAl2  

Increasing Al

Representations of Cu-Al crystal structures

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Resistivity of Cu-Al and Au-Al intermetallics

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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

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ContentThe Ball Bonding Process & Intermetallic FormationAbout IntermetallicsIntermetallics in Ball BondsSummary

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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

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