elec 6740 electronics manufacturing chapter 4 substrates ...jevans/elec6740/chapter4bw.pdf · elec...
TRANSCRIPT
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ELEC 6740 Electronics Manufacturing Chapter 4 Substrates for Surface Mounting
R. Wayne [email protected]
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Substrates♦ Rigid laminate♦ Metal core♦ Flexible films♦ Ceramic
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Substrate♦ Provides electrical interconnect and
isolation♦ Mounting surface for components
– Must be compatible with assembly processes
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Laminate Substrate♦ Dielectric
– Polymer • Epoxy• Polyimide• Cyanate Ester
– Reinforcement• Glass fabric• Kevlar• Paper
♦ Conductor– Copper
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Basic Material Property Considerations♦ Glass transition Temperature, Tg
– Hard, brittle, glassy !!!! soft, rubbery
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Tg
♦ Above Tg the modulus (slope of Force vs. Deformation) decreases
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Tg
♦ Above the Tg the coefficient of expansion decreases
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CTE – Laminate Substrates♦ CTE of polymers typically 50-80ppm/oC♦ Reinforcement materials are used to
control the CTE in the X-Y plane– Woven glass fabrics– Kevlar– Etc.
♦ CTE in X-Y plane typically 14-18ppm/oC to match CTE of Copper (16ppm/oC)
♦ CTE in Z-direction 100-200ppm/oC
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Laminate PWB Construction
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Effect of Temperature During Soldering
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CTE♦ Epoxy: 50-80 ppm/oC♦ Glass reinforcement: 0.5 ppm/oC♦ PWB Dielectric: 14-20 ppm/oC♦ Copper: 16 ppm/oC♦ Components: 6-20 ppm/oC
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CTE Effect on Components
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Characteristics of Substrate Options
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Characteristics of Substrate Options
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Characteristics of Substrate Options
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Characteristics of Substrate Options
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Ceramic Substrates♦ Thick Film♦ Low Temperature Cofired Ceramic
(LTCC)♦ High Temperature Cofired Ceramic
(HTCC)
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Thermal Conductivity
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Thermal Conductivity
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Thick Film Processing♦ Sequential printing and firing of:
– Conductor– Dielectric– Resistor
layers onto a base ceramic substrate
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Thick Film Screen Printing Process
Frame
Wire Mesh
Emulsion
SqueegeeInk
Substrate
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Conductor Print
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Cofired Technology♦ Cofired Ceramic
– Firing Temperature:1500 - 1600oC
♦ Glass/Ceramic– Firing Temperature: 850 - 1050oC
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Cofired Process
Top Conductor Layer
Via
Second Layer Conductor
Thick Tape
Thin Tape
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Glass/Ceramic
♦ Conductors– Au, Ag, PdAg, Cu
♦ Dielectrics– Crystallizable Glasses
• Cordierite MgO-SiO2-Al2O3
– Glass Filled Composites• SiO2-B2O3 type glass + Al2O3
• PbO- SiO2-B2O3 - CaO type glass + Al2O3
– Crystalline Phase Ceramics• Al2O3 - CaO - SiO2, MgO - B2O3
• BaSn(BO3)2
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Cofired Ceramic♦ Conductor
– Tungsten– Molybdenum/Manganese
♦ Dielectric– 88 - 92% Al2O3
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Typical Cofired Properties
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
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Typical 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
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Typical 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
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Cofired Glass/Ceramic
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Constrained Core Substrates♦ Copper/Invar/
Copper(Invar – Ni/Fe)♦ Alloy 42♦ SiC-Al
composite♦ Graphite
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Compliant Layers
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Selection Criteria
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Substrate Properties
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Substrate Properties
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Laminate Materials
Same as G-10, but can be used to higher temperatures
Glass fibersEpoxyG-11
General purpose material system
Glass fibersEpoxyG-10
Punchable @ or above R.T. XXXP & XXXPC are widely used in high volume single sided consumer applications
PaperPhenolicXXXPC
Punchable @ R.T.PaperPhenolicXXXP
DescriptionBase Material
ResinSystem
Common Designation
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Laminate Materials
Same as FR-4, but has better strength and electrical properties @ higher temperatures
Glass fibersEpoxyFR-5
Same as G-10, but has flame retardant
Glass fibersEpoxyFR-4
Punchable @ R.T. and has flame retardant
PaperEpoxyFR-3
Same as XXXPC, but has a flame retardant (FR) system that renders it self-extinguishing
PaperEpoxyFR-2
DescriptionBase Material
ResinSystem
Common Designation
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Laminate Materials
Controlled dielectric laminate. GX has better tolerance of dielectric properties than GT
Glass fibersTeflonGT or GX
Better strength & demonmstrated stability to a higher temperature than FR-4
Glass fibersPolyimidePolyimide
Designed for low capacitance or high impact resistance; has flame retardant
Glass fibersPolyesterFR-6
DescriptionBase Material
ResinSystem
Common Designation
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Highest Continuous Operating Temperatures (oC)
130130FR-4
220220GX220220GT260260Polyimide105105FR-6180170FR-5
105105FR-3105105FR-2180170G-11130130G-10125125XXXPC125125XXXP
MechanicalElectricalMaterial
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PWB Fabrication
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PWB Fabrication♦ Starting Material: Cu clad Core
Cu
Epoxy Glass Core
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PWB Fabrication:Interlayer Processing
Shear MarkMat. Bake Drill Reg.
Holes Clean
Strip Resist Etch Develop Photo
PrintDry FilmLamiantion
Inspect Test Clean Oxide Treatment Bake
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PWB Fabrication:Apply Dry Film Photoresist
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PWB Fabrication:Expose Photoresist
UV Light
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PWB Fabrication:Develop Photoresist
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PWB Fabrication:Etch Copper –CuCl2/HCl, 2:1
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PWB Fabrication:Strip Photoresist
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PWB Fabrication:Black Oxide Treatment
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PWB Fabrication: Multilayer Processing
Shear‘B’ StagePrepreg
Lay-up Lamination
Cool downRemove FixtureTrim Flash
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PWB Fabrication:Lamination
Release Paper1 oz. CuPrepreg
Prepreg
Inner Layer
1 oz. CuRelease Paper
Top LaminationFixture
Bottom Lam.Fixture
Tooling Holes
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PWB Fabrication:Multilayer Processing
Mark ID Bake Drill Plated Through Holes
DeburrHole Clean:De-smear
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PWB Fabrication:Drill & De-smear
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PWB Fabrication:Multilayer Processing
Pd Seed ElectrolessCu Plate Clean Photoresist
Application
PhotoPrintDevelopElectroplate
CuElectroplateSn/Pb
Strip Resist Etch StripSn/Pb
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PWB Fabrication:Pd Seed & Electroless Cu Plate
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PWB Fabrication:Photo Resist Application, Exposure & Develop
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PWB Fabrication:Electroplate Cu
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Copper Thickness
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Barrel Cracking
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Barrel Cracking
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PWB Fabrication:Electroplate Sn/Pb
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PWB Fabrication:Strip Photoresist & Etch Cu
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PWB Fabrication:Strip Sn/Pb
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PWB Fabrication:Multilayer Processing
SoldermaskApplication & Cure
Legend Print& Cure
Hot Air SolderLevel
CleanDrill Non-PlatedHoles
Route &Bevel
Final Electrical &Mechanical Test
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PWB Fabrication:Solder Mask Application
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Solder MasksTypes of Soldermask
Temporary
PeelableWashable
Aqueous Solvent
Permanent
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Solder MaskPermanent
Dry Film
Aqueous Solvent
Wet Screened
Photoimageable
WetScreen
Curtain Coat ScreenedUVCure
Thermal Cure
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Hole Tenting with Dry Film
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PWB Fabrication:Hot Air Solder Level
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Hot Air Leveled Process (HASL)♦ Solder dip and hot air solder leveling is a common
PCB surface finish for solder attachment.– Sn/Pb coating is applied after the solder mask
application, coating only the contact areas, plated holes and contact pads
– Coated boards are cleaned, fluxed and dipped into molten solder.
– While the alloy is still in the liquid state, excess material is blown off the contact surface with hot air, leaving a solder coated surface finish.
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Issues related to HASL
♦ Uneven surface plating♦ Crowning of solder on fine pitch and CSP sites♦ Solder paste uniformity♦ Tin/Copper intermetallic migration♦ Extreme Thermal shock
– Board warp– Delamination– Damage to the plated holes– Defects that may effect long term reliability.
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Ni/Au Electroless Process
♦ Electroless Ni is applied over the exposed bare copper after solder mask coating process.– The fabricator will typically use the Sn/Pb plated
circuit pattern as an etch resist and strip the Sn/Pbafter etching.
– Exposed attachment sites and holes are plated with the Ni using electroless plating process followed by a layer of gold by immersion process as well.
– Typical• Electroless Ni thickness : 125 - 200 µ in• Immersion Gold thickness : 3 - 8 µ in
– Ni improves plated through hole reliability
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Ni/Au Electroplating Process
♦ Electroplated Ni/Au is applied after hole plating.
♦ Ni/Au is resistant to the acid used to etch away copper.– This replaces the plating and subsequent stripping
of Sn/Pb.♦ This method can furnish finer lines and spaces.♦ Typical
– Electroplated Ni thickness : 100 - 150 µ in– Electroplated AU thickness : 3 - 5 µ in
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A word of caution…
♦ The gold plating volume within the solder joint should be less than 3% and preferably less than 1% to avoid embrittlement of the joint andintermetallic formation.– Gold thickness will depend on solder volume
♦ Current industry issue with ElectrolessNi/Immersion Au. – Low occurrence rate of failures in
mechanical shock related to the immersion gold process
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Solder Ball with Crack at Pad Interface
Courtesy:Bruce HoughtonCelestica
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Black Pad
Courtesy:Bruce HoughtonCelestica
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Electroless Ni/Immersion Au♦ Root cause (current theory): nickel is
attacked or excessively corroded in the gold bath.– Somewhat design dependent– Somewhat chemistry dependent– Not 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.
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Pd or Ni/Pd (electroless) Plating♦ Pd coatings have been developed as an
alternative to solder and Ni/Au.♦ Process is relatively new, but proven to be
compatible with solder attachment processes.♦ Pd is applied to the exposed circuit features
using electroless plating method and is compatible with either Ni alloy as a base plating or the bare Cu alloy surface.
♦ Low cost, low stress process♦ Pd metal cost is high
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Immersion Ag♦ Provides a solderable coating♦ Ag dissolves into molten solder♦ Growing in popularity
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Immersion or White Tin♦ Good initial solderability♦ Sn-Cu intermetallic formation and
oxidation limit use with multiple soldering cycles
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Alternatives to Alloy Plating♦ As an alternative to plating, many companies
have had success and economic advantage as well as a flat attachment surface with organic preservatives or pre-flux coatings over bare copper.
♦ As a means of retarding oxide growth on the bare copper attachment sites and via/test pads, a preservative or inhibitor coating is applied to the board. Organic/Nitrogen coatings such as, Benzotriazole or Imidazole are used instead of alloy finishes.
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Advantages of OSP♦ Multiple exposure capability♦ Ease of visual inspection of deteriorated
copper (if any)♦ Excellent pad coplanarity♦ Consistent solderability
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Concerns of OSP Coated Boards♦ Degrades in high humidity/temperature♦ Limited (6-12 months) shelf life♦ Physical contact can degrade coating♦ Exposed copper will (in time) tarnish