ELEC 6740 ElectronicsELEC 6740 ElectronicsManufacturingManufacturing

Chapter 8: Adhesives and ItsChapter 8: Adhesives and ItsApplicationApplication

R. Wayne JohnsonR. Wayne JohnsonAlumni ProfessorAlumni Professor

Auburn UniversityAuburn University334-844-1880334-844-1880

johnsonjohnson@eng.auburn.@eng.auburn.eduedu

OutlineOutline

•• IntroductionIntroduction•• Ideal Adhesives for SMTIdeal Adhesives for SMT•• General Classification of AdhesivesGeneral Classification of Adhesives•• Adhesives for SMTAdhesives for SMT•• Conductive Adhesives for SMTConductive Adhesives for SMT•• Adhesive Application MethodsAdhesive Application Methods•• Curing of AdhesivesCuring of Adhesives•• Evaluation of Adhesives with DSCEvaluation of Adhesives with DSC

IntroductionIntroduction

•• Adhesives for Wave SolderAdhesives for Wave Solder•• Electrical ConnectionElectrical Connection•• Thermal InterfaceThermal Interface

Ideal AdhesivesIdeal Adhesives•• Pre-cure propertiesPre-cure properties

–– 1-part vs. 2-part1-part vs. 2-part•• 1-part1-part

–– Shelf lifeShelf life–– Storage conditionsStorage conditions–– Warming to room temperatureWarming to room temperature

•• 2-part2-part–– Pot lifePot life–– Air bubblesAir bubbles–– Mixing ratioMixing ratio–– Automated mixing equipmentAutomated mixing equipment

–– ColorColor•• Visual inspectionVisual inspection

–– Tackiness and mechanical strengthTackiness and mechanical strength–– Flow (spreading)Flow (spreading)

Ideal AdhesivesIdeal Adhesives

•• Cure PropertiesCure Properties–– Cure timeCure time–– Cure temperatureCure temperature–– No No outgassing outgassing (void formation)(void formation)

•• Entrap fluxEntrap flux

–– Low shrinkageLow shrinkage•• Minimize stressMinimize stress

Ideal AdhesivesIdeal Adhesives

•• Post Cure PropertiesPost Cure Properties–– ReworkabilityReworkability

•• Low glass transition temperatureLow glass transition temperature

–– NonconductiveNonconductive–– High insulation resistanceHigh insulation resistance–– Moisture resistantMoisture resistant–– NoncorrosiveNoncorrosive

Failure ModesFailure Modes

Adhesive ClassificationAdhesive Classification•• PropertiesProperties

–– Electrically conductive or insulatingElectrically conductive or insulating–– Thermally conductive or insulatingThermally conductive or insulating

•• Chemical natureChemical nature–– EpoxyEpoxy–– AcrylicAcrylic

•• Cure MethodCure Method–– ThermalThermal–– UVUV

•• PhysicalPhysical–– ThermosettingThermosetting–– ThermoplasticThermoplastic–– ElastomericElastomeric

ThermosettingThermosetting

Adhesives for SMTAdhesives for SMT•• EpoxyEpoxy

–– 1 or 2 part1 or 2 part•• 1-part preferred1-part preferred•• Requires refrigerated storageRequires refrigerated storage

–– ThermosettingThermosetting–– Typically thermally curedTypically thermally cured

CH2 CH

O

OCH2CHCH2

OHCH3

CH2

CH3

CO[ ]n

Cross LinkingCross LinkingCH2CH

O

CH2CHO

CH2 CH

O

CH2 CHO

H

HH

H

NRN

Diamine

CH2CH CH2 CH

CH2 CHNRN

CH2CH

OH

OH OH

OH

AcrylicAcrylic

•• ThermosettingThermosetting•• 1 or 2 part1 or 2 part•• UV or thermal cure or UV+thermalUV or thermal cure or UV+thermal•• Most are anaerobic (they can cureMost are anaerobic (they can cure

without air)without air)–– Should not be stored in airtight containersShould not be stored in airtight containers

Other OptionsOther Options

•• UrethanesUrethanes•• CyanoacrylatesCyanoacrylates

–– Cure too fastCure too fast–– Require close fitRequire close fit–– ThermoplasticThermoplastic

•• Lose strength during wave solderLose strength during wave solder

Conductive AdhesivesConductive Adhesives

•• Solder replacementSolder replacement–– Low temperature assemblyLow temperature assembly–– Filled epoxies (isotropic)Filled epoxies (isotropic)

•• SilverSilver•• GoldGold

–– Sufficient loading (60-80% by wt.) forSufficient loading (60-80% by wt.) forparticle-to-particle contact or near contactparticle-to-particle contact or near contact

Conductive AdhesivesConductive Adhesives

•• AnisotropicAnisotropic

Thermally ConductiveThermally Conductive

•• Addition of conductive particlesAddition of conductive particles–– Electrically insulatingElectrically insulating

•• AluminaAlumina, aluminum nitride, boron nitride,, aluminum nitride, boron nitride,diamonddiamond

–– Electrically conductingElectrically conducting•• Silver, gold, aluminum, copper, nickelSilver, gold, aluminum, copper, nickel

Thermally ConductiveThermally Conductive

Adhesion Application MethodsAdhesion Application Methods

•• Adhesives that are kept refrigerated shouldAdhesives that are kept refrigerated shouldbe removed from the refrigerator andbe removed from the refrigerator andallowed to come to room temperature beforeallowed to come to room temperature beforethe containers are opened.the containers are opened.

•• The adhesive should not extend onto theThe adhesive should not extend onto thecircuit pads.circuit pads.

•• Sufficient adhesive should be applied toSufficient adhesive should be applied toensure that when the component is placed,ensure that when the component is placed,most of the space between the substrate andmost of the space between the substrate andthe component is filled with adhesive.the component is filled with adhesive.

Adhesion Application MethodsAdhesion Application Methods

•• It is very important that the properIt is very important that the properamount of adhesive be placed.amount of adhesive be placed.

•• Use proper dot size.Use proper dot size.•• Unused adhesive should be discarded.Unused adhesive should be discarded.•• If 2-part adhesives are used, it will beIf 2-part adhesives are used, it will be

necessary to properly proportion thenecessary to properly proportion theresin and the catalyst materials and mixresin and the catalyst materials and mixthem thoroughly before dispensing.them thoroughly before dispensing.

Dot SizeDot Size

UV CureUV Cure

Stencil PrintingStencil Printing

Frame

Stencil

SqueegeeAdhesive

PWB

Frame

Stencil

SqueegeeAdhesive

PWB

Screen PrintingScreen Printing

Frame

Wire Mesh

Emulsion

SqueegeeAdhesive

PWB

Pin TransferPin Transfer•• Adhesive viscosityAdhesive viscosity•• Multiple pins vs. software controlledMultiple pins vs. software controlled

single pinsingle pin

SyringeSyringe

•• Most commonMost common•• Time and pressureTime and pressure•• ViscosityViscosity•• Stringing or tailing (high viscosity)Stringing or tailing (high viscosity)•• Dot uniformityDot uniformity

Viscosity (temperature)Viscosity (temperature)

Semi-automaticSemi-automatic

AutomaticAutomatic

Adhesive CuringAdhesive Curing

•• Thermal cureThermal cure–– IRIR–– ConvectionConvection

Thermal CuringThermal Curing

•• Thermal cure profile vs. Bond strengthThermal cure profile vs. Bond strength

Thermal CuringThermal Curing

Thermal CureThermal Cure

3-53-537003700100010001271276.06.0

4-54-539003900200020001371375.05.0

4-64-639003900300030001501504.04.0

ReworkReworkTimeTime(sec.)(sec.)

AfterAfterWaveWaveSolderSolder(grams)(grams)

After IRAfter IRCureCure(grams)(grams)

PeakPeakTempTemp((ooCC))

BeltBeltSpeedSpeed(ft/min.)(ft/min.)

Solder MasksSolder Masks

Cure Profile & Void FormationCure Profile & Void Formation

•• Rapid ramp cure profileRapid ramp cure profile•• Entrapped airEntrapped air

–– CentrifugedCentrifuged•• Moisture absorption in the board/solderMoisture absorption in the board/solder

maskmask–– Board bakeBoard bake

Example ProfilesExample Profiles

Example ProfilesExample Profiles

VoidsVoids

UV/Thermal CureUV/Thermal Cure

•• Can be cured by higher temperatureCan be cured by higher temperaturethermal cure without UVthermal cure without UV

•• UVUV–– Correct wattageCorrect wattage–– IntensityIntensity

Differential ScanningDifferential ScanningCalorimetry Calorimetry (DSC)(DSC)

•• Thermal technique to characterize materialsThermal technique to characterize materials•• Based on thermal energy absorption or releaseBased on thermal energy absorption or release•• Ramped temperatureRamped temperature•• Measure of heat input difference between testMeasure of heat input difference between test

sample and controlsample and control•• Control sample should undergo no change inControl sample should undergo no change in

either physical or chemical properties in theeither physical or chemical properties in thetemperature range of studytemperature range of study

•• Used to characterize polymers and determineUsed to characterize polymers and determineproper cure conditionsproper cure conditions

DSCDSC

DSC: Isochronal – constantDSC: Isochronal – constanttemperature increase ratetemperature increase rate

Exothermal

Endothermal

Cure

Tg

Melting

DSC of Epoxy CureDSC of Epoxy Cure

DSC of Cured vs. UncuredDSC of Cured vs. UncuredEpoxyEpoxy

TGA by DSC (Epoxy as Cured)TGA by DSC (Epoxy as Cured)

TGA by DSC (Epoxy afterTGA by DSC (Epoxy afterWave Solder)Wave Solder)

DSC AcrylicDSC AcrylicUV/ThermalUV/Thermal

DSC AcrylicDSC AcrylicUV/ThermalUV/Thermal