Adhesives in Electronics
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Transcript of Adhesives in Electronics
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Horten, February 17th
Adhesives in Microelectronics and MEMS Applications
Helge Kristiansen
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Helge KristiansenConpart ASGåsevikvn. 4, KjellerP.O. Box 144, N-2027 KjellerNORWAYPhone +47 64 84 43 00 Mobile +47 930 36 073E-mail: [email protected]
IC Chip
ACF
ConductiveMicrospheres
Substrate
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Monosized polymer particles
Polymer particles with exceptional properties
Mono sized particlesSize 1 – 200 µmPolymer compositionSurface chemistryFunctional groupsPorosityMagneticMetal platedCore & shell
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Adhesion
Adhesion relates to the interface between adherent and adhesiveThe most important mechanism for adhesion is inter-molecular and surface forces
Van-der Waal (dispersion forces)Hydrogen bondsVery short range (sub nm)
Generally no chemical bondsOther factors like mechanical “interlocking” have very little importance in typical electronic systemsCohesion: Internal “strength” of the adhesive (often chemical bonds)
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Adhesive / cohesive
Chip
Substrate
Adhesive failure
Cohesive failureChip
Substrate
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Contact angle
ϑ
γABγAC
γCB
Medium A
C
Medium B
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Criteria for wetting
Force balance (along interface)
γA(B): Surface energy solid (-air) [N/m], [J/m2]
γC(B): Surface energy adhesive (-air)
γAC: Surface energy solid-adhesive
For a good wetting
AB AC CB cosγ = γ + γ ⋅ ϑ
A Bγ > γ
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Polymer properties
Low weight (0.9 - 1.5 g/cm3)High thermal expansionHygroscopic
Affects mechanical and electrical properties
Glass transitionIntroduces new degrees of freedom in the materialIncreases thermal expansionReduced mechanical strength
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Polymer
Long chains of monomersLinear or branchedThermoset
Best chemical and mechanical stabilityEpoxies, polyurethane, silicone
ThermoplasticSimplifies reworkTeflon, polyesters
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Thermoplastics
Reversible melting ↔ solidifyingHeating provides thermal energy for polymer chains to move “freely”Cooling reduces the molecular motion
Linear molecular chainsEntanglementInter-molecular forcesNo chemical cross bonding
Mechanical anisotropy Preferential orientation of molecules
Polyesters, acrylics
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Properties of thermoplastics
Fillers Tg Bond Temp Rework Die Shear Thermal Modulus°C °C min. °C MPa W/m°C GPa
None -40 100 - 150 110 11 0.20 0,4Ag, AIN, None 25 150 - 200 160 14 0.3-3.0 0,4Ag, AIN, None 45 160 - 220 170 17 0.3-3.0 3,2Ag, AIN, None 85 160 - 250 170 19 0.3-3.0 2,5None 145 200 - 230 210 24 0.22 1Ag, Au, AIN, None 180 325 - 400 350 25 0.3-3.0 2,2None 280 350 - 450 400 31 0.25 2,3
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Thermoset
Do not melt upon heatingForms links or chemical bonds between adjacent chains, during curing (intra-molecular bonds) Three dimensional rigid network Properties depend on the molecular units, and the length and the density of the cross-linksThermosetting resins are usually isotropic Epoxies, silicones, urethanes
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Glass temperature
Second order phase transformationDiscontinuous volume expansion, heat capacity and mechanical propertiesThermal energy sufficient to allow rotation about chemical bonds
Depends on bond (Carbon or silicone)Adjacent molecular groups (dipoles, bulkiness and symmetry)
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Different classes of organic adhesives
Solvent-based adhesives: The adhesive is solved in a suitable solvent or dispersed in water. The adhesive solidifies during drying (not curing) Thermoplastic adhesives: Based on thermoplastic materials, that is solid to liquid is a reversible phase change. Typically polyimide- and polyester (easily oxidized)Thermosetting adhesives: Improved mechanical properties, high temperature and chemical resistance
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Structural adhesive
Good load-carrying capability Long-term durabilityResistance to heat, solvent, and fatigue6 main types:
UrethanesAcrylicsEpoxiesSiliconesAnaerobic adhesivesCyanoacrylates
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Structural adhesives
CyanoacrylateCyanoacrylates adhesive bonds quickly to plastic and rubber but have limited temperature and moisture resistance.Cured by humidity present on surfaces
AcrylicsAcrylics, a versatile adhesive family that bonds to oily parts, cures quickly, and has good overall properties.Cure at room temperature when used with activators.
The adhesive and activator can be applied separately to the bonding surfaces. Often a low viscosity activatorThe adhesive and activator premixed in a static mixer prior to application. Anaerobic
Anaerobics, or surface-activated acrylicsBonding threaded metal parts etc.
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Silicones
Solvent born resinsEvaporation of solventCuring at elevated temperature
Room Temperature Vulcanisation RTV One part RTV systems generally cured by moisture
Give of by-products during cure
Two part RTV generally cured by a platinum complex
UV curedVery wide temperature range
-50 – 200 0C
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Silicones (II)
Soft, low modulus materialVery low Tg (typically -65 0C)
Very high CTE (Typically 350 ppm/ 0C)Very low dielectric constant
Low dielectric loss
Easily modified to provide a range of physical and mechanical properties, cure system, and application techniques.Very low surface tension (24 mN/m)Very mobile, vapor attaches to all surfaces
Wire bonding problems
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Silicones (III)
Good water and moisture resistanceHowever, very high water permeability If adhesion is good, a continuous water film is not obtained at the interfaceNB! Oxygen bridge bonding to die surface prevents Al corrosion)Relatively low equilibrium moisture contentVery low levels of mobile ions
Good sealing propertiesVery durable for glass sealing
Weather well out-of-doorsLow surface tension expels liquid water
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(Poly)urethanes
One or two component systemsOften solvent basedLow stress materials
Good flexibility and elasticityHigh thermal shock resistanceGood peeling characteristics
Moisture curing system (~ 50%RH)Good adhesion to many organic materials
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Epoxies
ThermosetThermal cure (UV or light cure optional)Relatively high E-modulus
Brittle
Generally good adhesionHydrogen bonding (NB! Except gold, due to lack of oxygen at the surface))Mechanical interlocking
Typical shrinkage upon curing is in the order of 1 to 2 percentRelatively low moisture permeability
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Curing
The selection of a curing agent is as important as the choice of the resin
Rate of reactivityDegree of exothermalGel timeCuring time Mechanical properties of the cured adhesive.
The number of epoxide groups in the molecule determines the “functionality” of the resin
Increasing glass transition temperature Decreasing Thermal Coefficient of Expansion (TCE)
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Epoxide group
O
C C
H
C
H
HH H
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Modification of properties
Choice of resin systemChoice of hardener“Fillers”
Electrical (dielectric) propertiesMechanical propertiesPossible ionic contamination
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Fillers
Plasticizers, flexibilizers or elastomersPlasticizers (an additive that is physically blended into the resin (do not become part of the polymer). Plasticizers in epoxies are typically rubber or thermoplastic particles. They have to be > 1 micron to be efficient Flexibilizers are chemically reacted into the epoxy system (often thermoplastic polymers)
Reduces solvent and moisture resistance and lowers Tg
Elastomers remains as a distinct second phase (two separate cross-linked networks)
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Plasticisers
Absorbed liquids (soft solids), decrease the glass temperatureIncreased “free volume”
Reduces the internal viscous friction
Ex. Glass temperature of water : approx. –135 0C.
)()()(
1Lg
L
Pg
p
PFg Tw
Tw
T+=
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Influence of water
0.0
20.0
40.0
60.0
80.0
100.0
0 2 4 6 8 10 12
Water content [%]
Gla
ss te
mpe
ratu
re [C
]
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Silver epoxies
Low impact strengthHigh loading of silver particles
SolutionsReducing filler rate by use of porous silver particles (Fraunhofer)Randomise particle orientation, by making conductive composite particles (Georgia Tech)Introduce plastizisers, to increase the ability to absorb mechanical energy (Ablestik)Polymer spheres with silver coating
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Mechanical properties
15
20
25
69 72 75
20
E-m
odul
us[G
Pa]
Coefficientof therm
alexpansion
[ppM/K
]
Filler content silver [weight %]
15
10
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Criteria for adhesive selection
Means of “dispensing” of adhesiveScreening, film, dispensing, stamping etc…
Viscosity / rheology: Unwanted flow (capillary driven), leaching
Good adhesion to parts: What is the main components to be bonded (surface!!)Surface cleaning
Demands for mechanical propertiesStrength - flexibility
E-module / Elongation before break
Sensitivity to mechanical stress Requirements for temperature cycling
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Criteria for adhesive selection
Processing temperatureThermal expansion adhesive:
High CTE will often increase stress in adhesive joint due to the increased miss-match with the components to be bondedHigh thermal expansion is likely to increase diffusion of humidity etc..
Low ionic contaminationThermal (and electrical) conductivityVoid free bond line
Local stress concentration (crack initiation)Poor local thermal contact
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Requirements from application (I)
Glass transition temperature relative tooperational (and storage) temperature of the device.
Radical change of mechanical propertiesReduced “E-modulus”Increased thermal expansion
Increased diffusivity above Tg.
Water absorption:Changes mechanical properties (plasticizer)
Reduced TG
Changes dielectric properties
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Resin bleed
Resin bleed is the unwanted capillary action of low viscous parts of the adhesiveContamination of bond pads
Destroy the ability to perform wire bonding
Relatively low molecular weight thermosetDuring initial heatingUltra clean surfaces (high surface tension)Rough surfaces (large surface area)
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E-modulus of different die attach adhesives
Adhesive T_g [C]
@ -65 @ 25 @ 150 @ 250
A Epoxy 53 2,68 2,30 0,21 0,17
B Epoxy 30 1,81 1,63 0,01
C Epoxy 88 5,78 4,98 0,36 0,27
D Epoxy 40 5,40 2,87 0,12 0,05
E Epoxy 67 6,27 5,30 1,18 0,45
F Thermoplastic 98 2,40
G Cyanate Ester 245 6,90
E-modulus (dynamic) [GPa]
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Stress in “Die attach”
Chip Substrate Adhesive
E-modulus 190 GPa 20 GPa -
50 ppM/K
0,03 mm
Poisson ratio 0,25 0,4 0,45
CTE 2,6 ppM/K 18 ppM/K
Thickness 0,4 mm 2,0 mm
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Shear stress in adhesive
0,00E+00
5,00E+06
1,00E+07
1,50E+07
2,00E+07
2,50E+07
0 1 2 3 4 5
Distance from centre [mm]
Shea
r stre
ss [P
a]30 GPa10 GPa3 GPa1 GPa0,3 GPa0,1 GPa
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Normal stress in die
-9.00E+07
-8.00E+07
-7.00E+07
-6.00E+07
-5.00E+07
-4.00E+07
-3.00E+07
-2.00E+07
-1.00E+07
0.00E+00
0.0 1.0 2.0 3.0 4.0 5.0Distance from centre [mm]
Nor
mal
stre
ss [
Pa]
30 GPa10 GPa3 GPa1 GPa0,3 GPa0,1 GPa
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Measured stress in silicon die
Silicon chip onto copper substrateCompressive stress along the centre-lineMeasured with Piezo-resistors
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Measured stress in silicon die (II)
Compressive stress normal to the centre-lineMeasured with Piezo-resistors
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Measured and calculated stress
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Comments: Measured stress
Observed strain (stress) much lower than calculated!K/5022-81 (polyimide based): problems with the curing of the adhesiveWhat is the effective ”Zero-stress” temperature?Plastic deformation?
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Transient Hot Wire
y = 0.006375 ln(x) + 0.36764
0.34
0.35
0.36
0.37
0.1 1ln(t)
V(t)
b
bhIR⋅⋅⋅⋅
=π
αλ4
32
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Results
λ increase with increased amount of conductive filler
For Nano scale particles λ increase more than theory suggest;
0% 0.20W/mK10% 0.35 W/mK20% 0.56 W/mK
Surface treated alumina better:Untreated: 0.26W/mK Treated: 0.32W/mK
Nanoscale particles better than micro scale particles:
Micro: 0.26 W/mKNano : 0.35 W/mK
10vol%, untreated(10%)
(10%)
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Silver epoxy
Silver flake distribution in Ablebond 952-8Elongated particles
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Thermal interface resistance
Ablebond 958-2Silicon chip onto molybdenum substrate
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Thermal interface resistance (II)
Epo-Tek H20E-175Silicon chip onto copper substrate!
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Temperature cycling of die attachTh
erm
al re
sist
ance
[Kcm
2/W
]
Number of cycles
500 1000 1500 2000
0,5
1,0
1,5
2,0 Adhesive 1Adhesive 2
10 - 150 0C, 8x8 mm test-chip oncopper substrate
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Adhesive as underfill
Bump lifetime under thermal cycling decreases with induced strainBumps without underfill:
CTE mismatch => cyclic shear strainCrack growth
Bumps with underfillStrain reductionLifetime increase 10X
Si chip (CTE=2.6 ppm /K)
FR4 (CTE= 18 ppm /K)
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Effect of Underfill
Substrate
Chip
Bump
Mechanical coupling chip/substrate
Reduces bump shear strainStrain highest at chip edgeStrain depends on shear modulus of underfill
Introduces axial strain CTE mismatch solder/underfillDepends on underfill CTEAffects all bumps equally
Impact on lifetime?
Shear mode
Substrate
Chip
BumpUnderfill
Tensile mode
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Measurement of bump lifetime - Aims
Measure number of thermal cycles to failureTwo underfill materials (different CTE)Measurements as function of distance to chip centre
No variation: Axial strain dominatesLifetime decreases with distance: Shear strain
Establish simple analytical model for bump lifetimeCompare experimental lifetimes with model
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Experimental method
Test sample:Silicon test chip with eutectic solder bumps Reflowed to FR4 substrateUnderfill applied and cured
Thermal cycling-55 - 145 oC2 cycles per hour
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Lifetime measurements
Daisy chain connections on chip/substrate
Grouped in “rings” with similar distance to centreContinuity of each ring monitored
In situFor each 50 cycles (at RT)
Resistance increase = failure
Chip connectionSubstrate connection
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Underfill characteristics
Two different types used:Filled: CTE = 28 ppm/K Unfilled: CTE = 58 ppm/KTg: 120 oCCTE and Tg measured by DMA
Curing: 150 oCMaximum cycling temperature (145 oC) exceeds Tg!
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ACA technology
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Overview
IntroductionAnisotropic conductive adhesive
Z-axis conductive film
Typical productsJoint qualityReliability Different FlipChip techniquesConclusions
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ACF History
Calculatordisplay LCD COG
Flip Chipon PCB
First ACFin industry
Fine pitchconnection
250 µm
First thermo-setting ACF
200 µm
COG, ACF with insulated layer
100 µm
Fine pitch COGconnection
50 µm
1976 1980 1984 1988 1992 1996 2000
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Anisotropic Conductive Adhesive
The adhesive film is applied uniformlyPressure is applied during curing, giving conduction only between padsThermoplastic or thermosetting Film (tape) or paste
Chip
Glass
Chip
Glass
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Particle compressed between bump and pad
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Driving force
Possibly higher reliabilityRight choice of adhesive and bonding surface
Fewer process stepsNo fluxing or cleaning
Finer pitchAnisotropic Conductive AdhesiveNon-conductive Adhesive
Environmental friendlyNo lead
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Introduction
Lower temperature processingLower thermal stress
New materials in packagingPolyester based flex circuitsLow cost plastics
Non solderable surfacesITO conductors (LCD’s)
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Optimising process conditions
Large number of particles on padLow particle density between neighbouring padsFast process timeReliable and uniform connection resistance
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Curing cycle
T 1
T 2 T 3T 4
T 5T 6
Con
nect
ion
resi
stan
ce
Pressure
Heat Time
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Curing cycle (II)
T 1: Start of pressurisationT 2: Start of heatingT 3: The resistance increase depending on electrode materialT 4: End of heatingT 5:T 6: End of pressure
If curing is insufficient, the resistance may start to increase
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Bonding force
The applied bonding force is counter balanced by
Squeezing of the adhesive filmCompression of particles
Squeeze film pressureFilm thicknessChip dimensionViscosity of adhesiveRate of squeezing
p
Chip
Substrate
F
∫ ∑+=ChipA i
ppdAF εκ
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Squeezing of adhesive film
Flat silicon surfaces1st stage: squeeze of adhesive only2nd stage: compression of particles
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Factors influencing on joint quality
Coplanarity.
Bump height and uniformity
Pressure and pressure distribution.
Particle distribution
Cure temperature and cure time
Temperature ramp rate
Alignment accuracy
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Contact stability (I)
Stable contactUniformly deformed particles Adequate pressure
Unstable contactToo low pressure during connection
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Contact stability (II)
Uneven particle sizeNot uniformly deformed particles
Uneven bump-height Not uniformly deformed particles
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Degradation mechanisms
Oxidation and hydration of conductors
Polymer degradation by moisture or UV-light
Adhesive failure due to humidity adsorption
Crack formation
Thermal and mechanical fatigue
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Failure mechanisms I
AdhesiveThermal and mechanical fatigueHumidityUV light
CohesiveHumidityThermal and mechanical fatigue
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Failure mechanisms II
Oxidation of metal surfacesHumidityCorrosive gases
Expansion / swellingThermal and mechanical fatigueHumidity
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Typical reliability tests
Temperature cycling from -40 to +125 0C
Constant humidity test: 85 0C, 85%RH
High temperature ageing at 125 0C
Temperature cycling from -40 to +125 0C
NB!: Tests are typically adopted from solderDifferent failure mechanisms
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The use of ACA Technology
Typical applications:Flat panel displaysSmart cardsSingle or multi-chip modulesPiezo electric components (printer heads etc.)Micro mechanical and Micro optical components
Low temperature bondingPlastic, clothing
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Mounting trends
COG
Large sizeLCD
Super slimTAB
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TAB connection
ITOPWB
Chip
ACA connection
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Chip on Glass
LCD panel
Chip
ACA connection
Flexprint
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Chip on Glass; Sharp DVD player
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ACF applications
Chip on FlexFlex on Glass
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ACF Flip Chip on rigid board
Personal digital assistant (PDA) by Casio ComputerSix IC´s (Microcontroller, Gate Array, Memory, decoderand amplifier) are mounted with flip-chip.Minimum pitch is 124 µm Sequential build-up substrate.
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Key factors for Chip on Glass
More than 5 particles per bumpEnsure insulation
Insulated particles for low pitch?
Development of more elastic binder materialHigher Tg materialLess moisture absorption
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Sharp; “ELASTIC”
IC
Cured adhesiveTacky adhesive
Conductive particle
IC
Adhesive
Glass substrate Glass
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Sharp; “ELASTIC “
ELASTIC: Electrical interconnection using light-setting adhesivePlacing particles on tacky adhesive
Photo processGold plated plastic spheresNo need for bump plating
50 micron pitch demonstrated
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Mitsubishi
Photo process with conductive particles
UV transparent substrateNon transparent pads
Waste of conductive particles? UV Light
Opaque electrode pad Uncured photoresist
Glass substrate
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Matsushita; Stud “wire” bumps“Isotropic Conductive Adhesive”
Stud bumpInsulating adhesive
Electrode Isotropic conductive adhesive
IC
Glass substrate
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Casio; “Microconnector”
Insulating layer
Metal layer
Polymer ball
Chip
Substrate
Bump Adhesive
Electrode pad
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Mitsubishi
UV Light
Opaque electrode pad Uncured photoresist
Glass substrate
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Hitachi: Double layer ACA
Chip
Bump
Adhesive layer
Particle layer
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Conductive particles
Volume fraction 5 to 10 % Responsible for the electrical contactPure metals such as gold, silver or nickelMetal-coated particles with plastic or glass cores.
Typically 3 to 10 micron in size,Treating particles separately from the adhesive.
Small volume fractions of particles in the ACA, gives minor changes in mechanical behaviour, at least in the case of metallised polymer particles.
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Conductive particle
250 500 750 1000 Å
Elem
enta
ry c
ompo
sitio
n
50styrene
Nickel
Gold
Styrene
Diam. 5-10 µm
%
100
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Particle development
1st generation (Dyno Specialty Polymers), focus on
Different polymer compositionsCross-linking densities
2nd generation, focus onAdhesion between metal and polymer coreAdded chemical groups for bonding to the metal
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Testing of mechanical properties
LCR meter
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Results, mechanical testing (@RT)
0
0,1
0,2
0,3
0,4
0,5
0 200 400 600 800 1000Force [mg]
Def
orm
atio
n
AU 54
Commercial
AU 128
AU 15
AU 11
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2nd generation particle (@ 150 0C)
0,000,050,100,150,200,250,300,350,400,450,50
0 100 200 300 400 500 600 700 800
Force [mg]
Def
orm
atio
n
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SEM picture of 2nd generation particle (tested at 150 0C)
Very good adhesion between polymer and metal
Adhesion promoters included in polymerisation process
Highly reliable contacts due to integrity of particles
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Commercial particles (testing at 150 0C)
Particles fully des-integratedLack of adhesion between polymer and metal
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Measurement of contact resistance
V
4-wire measurementEnsemble of free particlesAu-Pt electrodes
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Electrical resistance AU-54
1
10
100
0 500 1000 1500 2000Force per particle [mg]
Res
ista
nce
[Ohm
]
Contact resistance versus “contact force”4-point measurement“Free” particlesGold electrodes
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Contact resistance versus cycling
2,00
2,20
2,40
2,60
2,80
3,00
3,20
3,40
0,1 1 10 100 1000 10000
Compression cycles
Res
ista
nce
[Ohm
]