ELEC 6740 Electronics ELEC 6740 Electronics Manufacturing Manufacturing Chapter 4 Substrates for Chapter 4 Substrates for Surface Mounting Surface Mounting R. Wayne Johnson R. Wayne Johnson [email protected]. [email protected]. edu edu
ELEC 6740 Electronics ELEC 6740 Electronics Manufacturing Manufacturing Chapter 4 Substrates for Chapter 4 Substrates for Surface MountingSurface Mounting
R. Wayne JohnsonR. Wayne [email protected]@eng.auburn.eduedu
SubstratesSubstrates
♦♦ Rigid laminateRigid laminate♦♦ Metal coreMetal core♦♦ Flexible filmsFlexible films♦♦ CeramicCeramic
SubstrateSubstrate
♦♦ Provides electrical interconnect and Provides electrical interconnect and isolationisolation
♦♦ Mounting surface for componentsMounting surface for components–– Must be compatible with assembly processesMust be compatible with assembly processes
Laminate SubstrateLaminate Substrate♦♦ DielectricDielectric
–– Polymer Polymer •• EpoxyEpoxy•• PolyimidePolyimide•• Cyanate Cyanate EsterEster
–– ReinforcementReinforcement•• Glass fabricGlass fabric•• KevlarKevlar•• PaperPaper
♦♦ ConductorConductor–– CopperCopper
Basic Material Property Basic Material Property ConsiderationsConsiderations♦♦ Glass transition Temperature, Glass transition Temperature, TTgg
–– Hard, brittle, glassy Hard, brittle, glassy !! soft, rubberysoft, rubbery
TTgg
♦♦ Above Above TTgg thethe modulus (slope of Force vs. modulus (slope of Force vs. Deformation) decreasesDeformation) decreases
TTgg
♦♦ Above theAbove the TTgg thethe coefficient of expansion coefficient of expansion decreasesdecreases
CTE CTE –– Laminate SubstratesLaminate Substrates♦♦ CTE of polymers typically 50CTE of polymers typically 50--80ppm/80ppm/ooCC♦♦ Reinforcement materials are used to Reinforcement materials are used to
control the CTE in the Xcontrol the CTE in the X--Y planeY plane–– Woven glass fabricsWoven glass fabrics–– KevlarKevlar–– Etc.Etc.
♦♦ CTE in XCTE in X--Y plane typically 14Y plane typically 14--18ppm/18ppm/ooC C to match CTE of Copper (16ppm/to match CTE of Copper (16ppm/ooCC))
♦♦ CTE in ZCTE in Z--direction 100direction 100--200ppm/200ppm/ooCC
Laminate PWB ConstructionLaminate PWB Construction
Effect of Temperature During Effect of Temperature During SolderingSoldering
CTECTE
♦♦ Epoxy: 50Epoxy: 50--80 80 ppmppm//ooCC♦♦ Glass reinforcement: 0.5 Glass reinforcement: 0.5 ppmppm//ooCC♦♦ PWB Dielectric: 14PWB Dielectric: 14--20 20 ppmppm//ooCC♦♦ Copper: 16 Copper: 16 ppmppm//ooCC♦♦ Components: 6Components: 6--2020 ppmppm//ooCC
CTE Effect on ComponentsCTE Effect on Components
Characteristics of Substrate Characteristics of Substrate OptionsOptions
Characteristics of Substrate Characteristics of Substrate OptionsOptions
Characteristics of Substrate Characteristics of Substrate OptionsOptions
Characteristics of Substrate Characteristics of Substrate OptionsOptions
Ceramic SubstratesCeramic Substrates
♦♦ Thick FilmThick Film♦♦ Low Temperature Low Temperature Cofired Cofired Ceramic Ceramic
(LTCC)(LTCC)♦♦ High Temperature High Temperature Cofired Cofired Ceramic Ceramic
(HTCC)(HTCC)
Thermal ConductivityThermal Conductivity
Thermal ConductivityThermal Conductivity
Thick Film ProcessingThick Film Processing
♦♦ Sequential printing and firing of:Sequential printing and firing of:–– ConductorConductor–– DielectricDielectric–– ResistorResistor
layers onto a base ceramic substratelayers onto a base ceramic substrate
Thick Film Screen Printing Thick Film Screen Printing ProcessProcess
Frame
Wire Mesh
Emulsion
SqueegeeInk
Substrate
Conductor PrintConductor Print
Cofired Cofired TechnologyTechnology
♦♦ Cofired Cofired CeramicCeramic–– Firing Temperature:1500 Firing Temperature:1500 -- 16001600ooCC
♦♦ Glass/CeramicGlass/Ceramic–– Firing Temperature: 850 Firing Temperature: 850 -- 10501050ooCC
CofiredCofired ProcessProcess
Top Conductor Layer
Via
Second Layer Conductor
Thick Tape
Thin Tape
Glass/CeramicGlass/Ceramic
♦♦ ConductorsConductors–– Au, Ag, Au, Ag, PdAgPdAg, Cu, Cu
♦♦ DielectricsDielectrics–– Crystallizable Crystallizable GlassesGlasses
•• CordieriteCordierite MgOMgO--SiOSiO22--AlAl22OO33
–– Glass Filled CompositesGlass Filled Composites•• SiOSiO22--BB22OO3 3 type glass + Altype glass + Al22OO33
•• PbOPbO-- SiOSiO22--BB22OO3 3 -- CaOCaO type glass + Altype glass + Al22OO33
–– Crystalline Phase CeramicsCrystalline Phase Ceramics•• AlAl22OO33 -- CaO CaO -- SiOSiO22,, MgOMgO -- BB22OO3 3
•• BaSnBaSn(BO(BO33))22
Cofired Cofired CeramicCeramic
♦♦ ConductorConductor–– TungstenTungsten–– Molybdenum/ManganeseMolybdenum/Manganese
♦♦ DielectricDielectric–– 88 88 -- 92% Al92% Al22OO33
Typical Typical Cofired Cofired PropertiesProperties
CofiredCeramic
CofiredGlass/Ceramic
CTE 6.5 ppm/oC 3 to 8 ppm/oC
ThermalConductivity
15-20W/m oC 2-6W/m oC
Camber 1-4 mils/in. Conforms to setter
SurfaceRoughness
10-20 µµµµin 8-10 µµµµin
FlexuralStrength
275-400 MPa 15-250 MPA
Typical PropertiesTypical Properties
CofiredAl2O3
CofiredGlass/Ceramic
Line width (min.) 100µµµµm 100µµµµm
Via Dia. (min.) 125µµµµm 125µµµµm
Number ofLayers
1 - 100 1 - 100
ConductorResistance
8 – 12 mΩΩΩΩ/sq. 3 – 20 mΩΩΩΩ/sq.
DielectricConstant
9 - 10 5 – 8 @ 1MHz
Typical PropertiesTypical Properties
CofiredAl2O3
CofiredGlass/Ceramic
DissipationFactor
5 – 15 x 10-4
@1MHz15 – 30 x 10-4
@1MHzInsulationResistance
> 1014 ΩΩΩΩ-cm 1012 - 1015 ΩΩΩΩ-cm
BreakdownVoltage
550V/25µµµµm 800V/25µµµµm
Resistor Values 0.1 ΩΩΩΩ – 1m ΩΩΩΩ
Cofired Cofired Glass/CeramicGlass/Ceramic
Constrained Core SubstratesConstrained Core Substrates♦♦ Copper/Invar/Copper/Invar/
CopperCopper(Invar (Invar –– Ni/Fe)Ni/Fe)♦♦ Alloy 42Alloy 42♦♦ SiCSiC--Al Al
compositecomposite♦♦ GraphiteGraphite
Compliant LayersCompliant Layers
Selection CriteriaSelection Criteria
Substrate PropertiesSubstrate Properties
Substrate PropertiesSubstrate Properties
Laminate MaterialsLaminate Materials
Same as GSame as G--10, but 10, but can be used to can be used to higher higher temperaturestemperatures
Glass fibersGlass fibersEpoxyEpoxyGG--1111
General purpose General purpose material systemmaterial system
Glass fibersGlass fibersEpoxyEpoxyGG--1010
Punchable Punchable @ or @ or above R.T. XXXP above R.T. XXXP & XXXPC are & XXXPC are widely used in high widely used in high volume single sided volume single sided consumer consumer applicationsapplications
PaperPaperPhenolicPhenolicXXXPCXXXPC
Punchable Punchable @ R.T.@ R.T.PaperPaperPhenolicPhenolicXXXPXXXP
DescriptionDescriptionBase Base MaterialMaterial
ResinResinSystemSystem
Common Common DesignationDesignation
Laminate MaterialsLaminate Materials
Same as FRSame as FR--4, but has 4, but has better strength and better strength and electrical properties @ electrical properties @ higher temperatureshigher temperatures
Glass fibersGlass fibersEpoxyEpoxyFRFR--55
Same as GSame as G--10, but has 10, but has flame retardantflame retardant
Glass fibersGlass fibersEpoxyEpoxyFRFR--44
Punchable Punchable @ R.T. and @ R.T. and has flame retardanthas flame retardant
PaperPaperEpoxyEpoxyFRFR--33
Same as XXXPC, but Same as XXXPC, but has a flame retardant has a flame retardant (FR) system that (FR) system that renders it selfrenders it self--extinguishingextinguishing
PaperPaperEpoxyEpoxyFRFR--22
DescriptionDescriptionBase Base MaterialMaterial
ResinResinSystemSystem
Common Common DesignationDesignation
Laminate MaterialsLaminate Materials
Controlled dielectric Controlled dielectric laminate. GX has better laminate. GX has better tolerance of dielectric tolerance of dielectric properties than GTproperties than GT
Glass fibersGlass fibersTeflonTeflonGT or GXGT or GX
Better strength & Better strength & demonmstrated demonmstrated stability stability to a higher temperature to a higher temperature than FRthan FR--44
Glass fibersGlass fibersPolyimidePolyimidePolyimidePolyimide
Designed for low Designed for low capacitance or high capacitance or high impact resistance; has impact resistance; has flame retardantflame retardant
Glass fibersGlass fibersPolyesterPolyesterFRFR--66
DescriptionDescriptionBase Base MaterialMaterial
ResinResinSystemSystem
Common Common DesignationDesignation
Highest Continuous Operating Highest Continuous Operating Temperatures (Temperatures (ooCC))
130130130130FRFR--44
220220220220GXGX220220220220GTGT260260260260PolyimidePolyimide105105105105FRFR--66180180170170FRFR--55
105105105105FRFR--33105105105105FRFR--22180180170170GG--1111130130130130GG--1010125125125125XXXPCXXXPC125125125125XXXPXXXP
MechanicalMechanicalElectricalElectricalMaterialMaterial
PWB FabricationPWB Fabrication
PWB FabricationPWB Fabrication
♦♦ Starting Material: Cu clad CoreStarting Material: Cu clad Core
Cu
Epoxy Glass Core
PWB Fabrication:PWB Fabrication:Interlayer ProcessingInterlayer Processing
Shear MarkMat. Bake Drill Reg.
Holes Clean
Strip Resist Etch Develop Photo
PrintDry FilmLamiantion
Inspect Test Clean Oxide Treatment Bake
PWB Fabrication:PWB Fabrication:Apply Dry Film Apply Dry Film PhotoresistPhotoresist
PWB Fabrication:PWB Fabrication:Expose Expose PhotoresistPhotoresist
UV Light
PWB Fabrication:PWB Fabrication:Develop Develop PhotoresistPhotoresist
PWB Fabrication:PWB Fabrication:Etch Copper Etch Copper ––CuClCuCl22//HClHCl, 2:1, 2:1
PWB Fabrication:PWB Fabrication:Strip Strip PhotoresistPhotoresist
PWB Fabrication:PWB Fabrication:Black Oxide TreatmentBlack Oxide Treatment
PWB Fabrication: PWB Fabrication: MultilayerMultilayer ProcessingProcessing
Shear‘B’ StagePrepreg
Lay-up Lamination
Cool downRemove FixtureTrim Flash
PWB Fabrication:PWB Fabrication:LaminationLamination
Release Paper1 oz. CuPrepreg
Prepreg
Inner Layer
1 oz. CuRelease Paper
Top LaminationFixture
Bottom Lam.Fixture
Tooling Holes
PWB Fabrication:PWB Fabrication:MultilayerMultilayer ProcessingProcessing
Mark ID Bake Drill Plated Through Holes
DeburrHole Clean:De-smear
PWB Fabrication:PWB Fabrication:Drill & DeDrill & De--smearsmear
PWB Fabrication:PWB Fabrication:MultilayerMultilayer ProcessingProcessing
Pd Seed ElectrolessCu Plate Clean Photoresist
Application
PhotoPrintDevelopElectroplate
CuElectroplateSn/Pb
Strip Resist Etch StripSn/Pb
PWB Fabrication:PWB Fabrication:Pd Seed & Pd Seed & Electroless Electroless Cu PlateCu Plate
PWB Fabrication:PWB Fabrication:Photo Resist Application, Photo Resist Application, Exposure & DevelopExposure & Develop
PWB Fabrication:PWB Fabrication:Electroplate CuElectroplate Cu
Copper ThicknessCopper Thickness
Barrel CrackingBarrel Cracking
Barrel CrackingBarrel Cracking
PWB Fabrication:PWB Fabrication:Electroplate Electroplate SnSn//PbPb
PWB Fabrication:PWB Fabrication:Strip Strip Photoresist Photoresist & Etch Cu& Etch Cu
PWB Fabrication:PWB Fabrication:Strip Strip SnSn//PbPb
PWB Fabrication:PWB Fabrication:MultilayerMultilayer ProcessingProcessing
SoldermaskApplication & Cure
Legend Print& Cure
Hot Air SolderLevel
CleanDrill Non-PlatedHoles
Route &Bevel
Final Electrical &Mechanical Test
PWB Fabrication:PWB Fabrication:Solder Mask ApplicationSolder Mask Application
Solder MasksSolder MasksTypes of Soldermask
Temporary
PeelableWashable
Aqueous Solvent
Permanent
Solder MaskSolder MaskPermanent
Dry Film
Aqueous Solvent
Wet Screened
Photoimageable
WetScreen
Curtain Coat ScreenedUVCure
Thermal Cure
Hole Tenting with Dry FilmHole Tenting with Dry Film
PWB Fabrication:PWB Fabrication:Hot Air Solder LevelHot Air Solder Level
Hot Air Leveled Process Hot Air Leveled Process (HASL)(HASL)♦♦ Solder dip and hot air solder leveling is a common Solder dip and hot air solder leveling is a common
PCB surface finish for solder attachment.PCB surface finish for solder attachment.–– Sn/Pb coating is applied after the solder mask Sn/Pb coating is applied after the solder mask
application, coating only the contact areas, plated application, coating only the contact areas, plated holes and contact padsholes and contact pads
–– Coated boards are cleaned, fluxed and dipped into Coated boards are cleaned, fluxed and dipped into molten solder. molten solder.
–– While the alloy is still in the liquid state, excess While the alloy is still in the liquid state, excess material is blown off the contact surface with hot material is blown off the contact surface with hot air, leaving a solder coated surface finish.air, leaving a solder coated surface finish.
Issues related to HASLIssues related to HASL
♦♦ Uneven surface platingUneven surface plating♦♦ Crowning of solder on fine pitch and CSP sitesCrowning of solder on fine pitch and CSP sites♦♦ Solder paste uniformitySolder paste uniformity♦♦ Tin/Copper intermetallic migrationTin/Copper intermetallic migration♦♦ Extreme Thermal shock Extreme Thermal shock
–– Board warpBoard warp–– DelaminationDelamination–– Damage to the plated holesDamage to the plated holes–– Defects that may effect long term reliability.Defects that may effect long term reliability.
Ni/Au Ni/Au ElectrolessElectroless ProcessProcess
♦♦ Electroless Ni is applied over the exposed bare Electroless Ni is applied over the exposed bare copper after solder mask coating processcopper after solder mask coating process..–– The fabricator will typically use the Sn/Pb plated The fabricator will typically use the Sn/Pb plated
circuit pattern as an etch resist and strip the circuit pattern as an etch resist and strip the SnSn//PbPbafter etching.after etching.
–– Exposed attachment sites and holes are plated with Exposed attachment sites and holes are plated with the Ni using the Ni using electrolesselectroless plating process followed by a plating process followed by a layer of gold by immersion process as well.layer of gold by immersion process as well.
–– TypicalTypical•• ElectrolessElectroless Ni thickness : 125 Ni thickness : 125 -- 200 µ in200 µ in•• Immersion Gold thickness : 3 Immersion Gold thickness : 3 -- 8 µ in8 µ in
–– Ni improves plated through hole reliabilityNi improves plated through hole reliability
Ni/Au Electroplating ProcessNi/Au Electroplating Process
♦♦ Electroplated Ni/Au is applied after hole Electroplated Ni/Au is applied after hole plating. plating.
♦♦ Ni/Au is resistant to the acid used to etch away Ni/Au is resistant to the acid used to etch away copper.copper.–– This replaces the plating and subsequent stripping This replaces the plating and subsequent stripping
of Sn/Pb.of Sn/Pb.♦♦ This method can furnish finer lines and spaces.This method can furnish finer lines and spaces.♦♦ TypicalTypical
–– Electroplated Ni thickness : 100 Electroplated Ni thickness : 100 -- 150 µ in150 µ in–– Electroplated AU thickness : 3 Electroplated AU thickness : 3 -- 5 µ in5 µ in
A word of caution…A word of caution…
♦♦ The gold plating volume within the solder joint The gold plating volume within the solder joint should be less than 3% and preferably less than should be less than 3% and preferably less than 1% to avoid1% to avoid embrittlementembrittlement of the joint andof the joint andintermetallicintermetallic formation.formation.–– Gold thickness will depend on solder volumeGold thickness will depend on solder volume
♦♦ Current industry issue with Current industry issue with ElectrolessElectrolessNi/Immersion Au. Ni/Immersion Au. –– Low occurrence rate of failures in Low occurrence rate of failures in
mechanical shock related to the immersion mechanical shock related to the immersion gold processgold process
Solder Ball with Crack at Solder Ball with Crack at Pad InterfacePad Interface
Courtesy:Bruce HoughtonCelestica
Black PadBlack Pad
Courtesy:Bruce HoughtonCelestica
ElectrolessElectroless Ni/Immersion AuNi/Immersion Au
♦♦ Root cause (current theory): nickel is Root cause (current theory): nickel is attacked or excessively corroded in the attacked or excessively corroded in the gold bath.gold bath.–– Somewhat design dependentSomewhat design dependent–– Somewhat chemistry dependentSomewhat chemistry dependent–– Not related to phosphorous content in NiNot related to phosphorous content in Ni
Ref: F. D. Bruce Houghton, “Solving the ENIG Black Pad Problem: An ITRI Report on Round 2,” Future Circuits International, 2000, pp.121-128.
Pd or Ni/Pd (Pd or Ni/Pd (electrolesselectroless) ) PlatingPlating♦♦ Pd coatings have been developed as an Pd coatings have been developed as an
alternative to solder and Ni/Au.alternative to solder and Ni/Au.♦♦ Process is relatively new, but proven to be Process is relatively new, but proven to be
compatible with solder attachment processes.compatible with solder attachment processes.♦♦ Pd is applied to the exposed circuit features Pd is applied to the exposed circuit features
using electroless plating method and is using electroless plating method and is compatible with either Ni alloy as a base compatible with either Ni alloy as a base plating or the bare Cu alloy surface.plating or the bare Cu alloy surface.
♦♦ Low cost, low stress processLow cost, low stress process♦♦ Pd metal cost is highPd metal cost is high
Immersion AgImmersion Ag
♦♦ Provides a Provides a solderable solderable coatingcoating♦♦ Ag dissolves into molten solderAg dissolves into molten solder♦♦ Growing in popularity Growing in popularity
Immersion or White TinImmersion or White Tin
♦♦ Good initial Good initial solderabilitysolderability♦♦ SnSn--Cu Cu intermetallicintermetallic formation and formation and
oxidation limit use with multiple oxidation limit use with multiple soldering cyclessoldering cycles
Alternatives to Alloy PlatingAlternatives to Alloy Plating♦♦ As an alternative to plating, many companies As an alternative to plating, many companies
have had success and economic advantage as have had success and economic advantage as well as a flat attachment surface with organic well as a flat attachment surface with organic preservatives or prepreservatives or pre--flux coatings over bare flux coatings over bare copper.copper.
♦♦ As a means of retarding oxide growth on the As a means of retarding oxide growth on the bare copper attachment sites and via/test pads, bare copper attachment sites and via/test pads, a preservative or inhibitor coating is applied to a preservative or inhibitor coating is applied to the board. Organic/Nitrogen coatings such as, the board. Organic/Nitrogen coatings such as, Benzotriazole or Imidazole are used instead of Benzotriazole or Imidazole are used instead of alloy finishes.alloy finishes.
Advantages of OSPAdvantages of OSP
♦♦ Multiple exposure capabilityMultiple exposure capability♦♦ Ease of visual inspection of deteriorated Ease of visual inspection of deteriorated
copper (if any)copper (if any)♦♦ Excellent pad coplanarityExcellent pad coplanarity♦♦ Consistent solderabilityConsistent solderability
Concerns of OSP Coated Concerns of OSP Coated BoardsBoards♦♦ Degrades in high humidity/temperatureDegrades in high humidity/temperature♦♦ Limited (6Limited (6--12 months) shelf life12 months) shelf life♦♦ Physical contact can degrade coatingPhysical contact can degrade coating♦♦ Exposed copper will (in time) tarnishExposed copper will (in time) tarnish