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Ken Gilleo, PhD

Ocean State - Rhode Island

L-FKen@ET-Trends.com

SMTA Boston 2000

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Outline

• Lead-Free Solders: Success or Failure?

• Polymers Vs. Metallurgical Solders

• Conductive Adhesive Technology

• Pros and Cons of CAs for SMT

• Conclusions and Prognosis

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Metallurgical Solder

phase Cu6Sn

5

component Tin phase Sn

-phase Cu6Sn5

-phase Cu3Sn

Copper - PWB

(shown with Sn only for simplicity)

Binary and tertiary alloys form complex structures with many organometallic compounds

Binary and tertiary alloys form complex structures with many organometallic compounds

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L-F Alloy Political Issues• Japan

– Using their own proprietary alloys– Alloys; too many and too diverse?– A patent “minefield” from global perspective?

• USA– Industry: WHY are we doing this?– Government: electronic Pb, minor envir. issue– ITRI and NEMI are endorsing TAC (Tin-Silver-Copper)

• World in General– No global cooperation mechanism?– Low consensus, too many “my alloy” agendas?– Ambiguous deadlines and not in synch!

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AlloysAlloy Melting Range63Sn-37Pb 183

Au-20Sn 280

Sn-5Sb 232-240

Sn 232

Sn-0.7Cu 227

Sn-3.5Ag 221

Sn-4Ag-0.5Cu 217 Sn

CuZn? MgBa Ce AgInBi

Mostly tin plus, hold the lead

Mostly tin plus, hold the lead

Some 5-metal alloys

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Popular Alloys• “TAC”: Sn-Ag-Cu

– Promoted by several organizations, appears to be viable

– Patent issues, “Doctrine of Equivalence” issues in USA?

• Sn-Cu, Sn-Ag; appears to be viable, mostly patent-free

• Sn-Ag-Cu-Sb– Much process experience with this alloy

– Many have licensed from AIM as Castin®

• Sn-Ag-Cu-Bi– Majors in Japan are using this alloy system

– Bi supply no issue - low % Bi used

– Process at 20oC lower than Sn-Ag-Cu

– Licensed from Oatey by Alpha-Fry who will sublicense

Compiled by Dr. Alan Rae

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The Real L-F Problem L-F

30 - 60oC Higher - was Sn/Pb already too hot?

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Collateral Damage?

• Equipment– oven; N2, faster ramp, different materials– tester & probes; harder flux residue

• Substrate; warp, delamination, degradation

• Coatings; solder masks; degradation

• Board Cleaning; more difficult

• Package– substrate; warp, delamination– encapsulant; popcorn– devices like photo, MEMS, low k dielectric?

Can they take the heat and thermal effects

Can they take the heat and thermal effects

L-F

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Flux Issues• Lead-free alloys don’t wet as well:

– Sn without Pb oxidizes more easily

– Nitrogen probably required

– High temperature fluxes; more voids in reflow?

• Fluxes - halogen-free for rel. and environ

• Newer epoxy fluxes may not be viable

• Expect new fluxes and processes

• They may be harder to clean

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Cleaning• Baked-on fluxes?

– Many users still clean, even no-clean flux, to improve conformal coating adhesion, etc.

– Nitrogen atmosphere can reduce residue toughness

– “Stronger” cleaning machines

from Speedline Electrovert

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Laminates

• Laminates can degrade at 260oC on multiple passes

• Dimensional stability issues

• Solder masks; adhesion, color change, degradation

• Issue - finding an effective compromise on electrical properties, flammability, mechanical properties

Before or after?

FR4 in jeopardyFR4 in jeopardy

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Packaging Materials

• Epoxy Molding Compounds (EMC) may need to be modified or changed to totally different resins.

• Formulations are very complex; many formulations have >10 components.

Additives

O

OCH2CH CH2

O

OCH2CH CH2

Epoxy Resin

OH

OH

OH

OH

Hardener

Catalyst

A

Silica Fillers

CouplingAgent

A

AA

AA

O

OCH2CH CH2

Epoxy Molding Compound (EMC); could be in jeopardy

Epoxy Molding Compound (EMC); could be in jeopardy

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Long-chained macromolecules of small repeating block monomers. Properties like strength, flexibility, melting point and electricals can be engineered into the polymer.

Put the fire out with COOLCONDUCTIVE ADHESIVES

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The inventor of the IC

Jack Kilby, uses

Conductive Adhesives

The inventor of the IC

Jack Kilby, uses

Conductive Adhesives

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SOLIDS

LIQUIDS (or meltable solid)

Resin, hardener. accelerator

Resin, hardener. accelerator

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Conductive Adhesives• Metal-filled composite• Thermoset (a few thermoplastics) • Filler determines type• Very different than solder• Used commercially for SMT and FC• Two types

– Isotropic (best for SMT)– Anisotropic; random or patterned

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Classes of Joining Materials

Solders - metallurgical

POLYMER-BASED

Isotropic Cond. Adh. (ICA)

Anisotropic Con. Adh. (ACA)

Hybrids

Non-cond.

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Classes of Joining Materials

• Solders - metallurgical

• POLYMER-BASED

– Isotropic Cond. Adh. (ICA)

– Anisotropic Cond. Adh. (ACA)

– Hybrids

– Non-conductive

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Iso- vs. Anisotropic

ISOTROPIC:Conducts Equallyin X, Y, Z planes

ANISOTROPIC:Unidirection Conductivity;

in Z-Axis

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Conductive Particles

silver powder silver flake

+ Metal coatings: Ag, solder

custom penetratingparticles

+ Anti-corrosion films

still the standard

+ Conductive Polymer Coating

No real success yet!

copper

use blend

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Typical CA Composition

• Pre-polymers or monomers (epoxy is common)

• Hardener and accelerators

• Silver filler; major component (>79% wt.)

• Additives for wetting, flow, adhesionMix under vacuum

Mix under vacuum

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Silver is Dominant

• Moderate cost (some impact)

• Oxide is electrically conductive

• Malleable; can be shaped

• Only moderately abundant

• Chemically reactive

• Safe for humans (biocide)

(but why silver?)

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ICA Adhesive Model

Polymerbinder

oxide&

surfactant

Electrical pathways

The many interfaces add electricalresistance

Mechanical & Electrical properties are mostly independent

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Adhesive Assembly Processes

StencilAdhesive

Place Component

CureAdhesive

StencilSolder

Remove or deactivate

flux

Place Component

ReflowSolder

Runs on today’sSMT lines

Cookson - Foxboro, MA

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Adhesive Solder

50% reduction

AdhesiveOxide-Tolerant

Solder Dewetting

Oxide

<1% reduction

Half as much volume

Half as much volume

Much more compatible

Much more compatible

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Digitizer made by SOLDERLESS SMT

Poly-Flex Circuits - Parlex

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Test the System• MECHANICAL

– Bond Strength– Die Shear– Shock; drop

• Bond Strength After:– Heat Age– Chemical Exposure– Temp. & Humidity

• ELECTRICAL– Volume Resistivity– Electrical Stability– Heat Age– Thermal Cycle– Temp. & Humidity

• REWORKABILITY

SOLDER: 4-6 m Per junction

CA: 8-12 m Per junction

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Junction Instability

• Many PWB finishes oxidize

• Most metal oxides are insulators

• Oxidation can occur under CAs

• Most CAs can form unstable junctions

OXIDIZED METAL

AFTER AGING

METAL

INITIAL

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One Proven Solution: Penetrating

Particles

Penetrating

Particles

Metaloxide

METAL

Oxide-Penetrating particles

Poly-Solder used by Poly-Flex a Parlex Co.

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Junction Resistance - 85%RH/85oC

Sn/Pb solder, the reference, shows no change

Sn/Pb solder, the reference, shows no change

1000 hrs

+18% incr.

+15% incr.

-8%

-3% -10%

-22%

1000 hrs1000 hrs 500 hrs

-8%

-3%

PLCC-44

1206 resistor

PWB Finish Bare Cu Silver Ink SolderGold

85%rh/85oC

Note resistance drop

Components are solder-coated

Components are solder-coated

Penetrating Particle Type Adhesive:

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+ -

Silver Migration?

Silver is encapsulated by resin.

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Flip Chips with CAs

• Work well, in production

• Underfill provides high strength

• Underfill eliminates migration concernsUsed for memory, RFIDs

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Advantages• Low Thermal Processing Stress

• Eliminate Solder Mask

• Excellent Fine Pitch

• Wide Process Window

• Less Used: (1/4 to 1/6 as much by wt.)

• No Flux, No Residues, No Cleaning

• No Lead

• No -Particle Emission

Laptop mouse pad,polyester’ max 150oC

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Limitations• Lower mechanical shock• No self-alignment• Some require non-oxidizing surfaces• Higher electrical resistance• Higher thermal resistance• Much more difficult to rework• Higher cost (than Sn/Pb; L-F?)

• Silver: limited supply, aquatic toxicity

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Comparison

Characteristic SOLDER

Volume Resistivity .000015 ohm.cm .00006 ohm.cm

Best Adhesive

Typical Junction R 8 - 15 m <25 m

Thermal Conductivity 30 - 50 W/m-deg.K 3 - 8 W/m-deg.K

Shear Strength (1206) >2000 PSI > 1600 PSI

no change no change

Mechanical Shock Pass 6 ft. x 6 Fail 6 ft., once

Thermal Fatigue Yes Lower, gradual

Proc. Temp. 220 260oC 130 - 160oC

Fine Pitch Good Excellent

T & H (85%/85C)

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Conclusions

• Adhesives run on std. SMT lines

• Much lower process temperatures

• Low mechanical shock issue

• Need to move away from silver

• A niche product without a breakthrough

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Prognosis• Copper adhesives will succeed

– metal-coated

– conductive polymer coated - boost strength?

– Mixtures with solder; now used in inks

• Mechanical properties will be the key

• Cost probably lower than L-Fs

• Metallurgical joints likely to dominate