PCB Surface FinishesIn A Lead-Free World

The Choices

• HASL - Hot Air Solder Level

• ENIG - Electroless Nickel Immersion Gold

• Immersion Tin

• Immersion Silver

• OSP - Organic Solderability Preservative

• Others - ENEPIG, Electroless Pd, Plasma

Decision Factors

• Cost Sensitivity

• Volume of Product

• SnPb, Pb-Free or Both

• Shock/Drop Testing

• Cosmetics

• Product Environment

• Fine Pitch Components (≤0.5mm Pitch)

• Design for Testability

• Shelf Life

Industry Trends

Cost Comparison

*NOTE: Prior to drastic gold price rise in 2011! - Now ~$7/panel in just metals!

HASL(Hot Air Solder Level)

Vertical

Horizontal

HASL Process

Pre-Clean

Pre-Heat

Flux

Solder

Air K

nife

Cool D

own

Post Clean

265°C 280°C42psi

240°C

Variables: Solder Temp, Dip Time, Air Temp, Air Pressure, Knife Angles

HASL Standards

• IPC Standard - 100% Copper Coverage

• Optimum Thickness - 1.5um - 30.5um

• Based on Solder Paste Man. & Lessons Learned

HASL Issues

• Coplanarity

• HASL: 28um

• QFP: 101um

• Stencil: 100-127um

• Copper Dissolution (per Pass)

• SnPb HASL: 1.5um AVG

• Pb-Free HASL: 3.5um AVG

• PTH Barrel: 20um MIN

• Bare Board Cleanliness

• Process Control & Verification

HASL Summary

Advantages Disadvantages• Low Cost• Solder Wetting• Re-Workable• Shelf Life

• Coplanarity• Thermal Shock• Copper Dissolution• Cleanliness• Availability• Process Control

ENIG(Electroless Nickel Immersion Gold)

ENIG Process

CleanerR

inse

Pre-Dip

ActivatorR

inse

Acid D

ip

Au

90°C 90°C50°C

~One Hour Total Process Time

Rinse

Micro-EtchR

inseR

inse

Rinse

Rinse

Rinse

Nickel

Nickel

Rinse

Rinse

Rinse

DI R

inse

ENIG Process

Cu

Pd

Ni

Au

ENIG Standards

• IPC-4552

ENIG Issues

• Skip Plating

• Tin Residues

• Broken Soldermask Tent

• Nickel Foot

• Under-etching

• Palladium Issues

• Brittle IMC

• SnNi vs. NiSnCu

ENIG Issues (con’t)

• Black Pad

• Nickel Hypercorrosion

• Poor Mechanical Bond

• Gold Porosity

• Exposure to N2O

ENIG Summary

Advantages Disadvantages• Planarity• Shelf Life• Good for PTH

• Expensive• Black Pad• Process Control• Damage by ET• Signal Loss (RF)

ImSn(Immersion Tin)

ImSn Process

Cleaner

65°C

~30 Minutes Total Process Time

Rinse

Micro-Etch

ImSn

Rinse

Rinse

DI R

inse

Rinse

Pre-Tin

Dry

ImSn Standards

• IPC-4554

ImSn Issues

• Tin Whiskers

• Inhibitors (Ag)

• IMC Aging

ImSn Summary

Advantages Disadvantages• Planarity• Re-workable

• Tin Whiskers• Corrosion• Shelf Life• Non Multiple Reflows• Carcinogen (Thiourea)

• Availability

ImAg(Immersion Silver)

ImAg Process

Cleaner

55°C

~30 Minutes Total Process Time

Rinse

Micro-Etch

ImA

g

Rinse

Rinse

DI R

inse

Rinse

Pre-Silver

Dry

ImAg Standards

• IPC-4553

ImAg Issues

• Silver Dendrites

• Inhibitors (Organic)

• Creep Corrosion

• Sulfides & Chlorides

ImAg Issues

• Microvoiding

• Ag Plates Void

• Galvanic Cave

• Soldermask Interface Etching

• Poor LPI Adhesion

• Galvanic Corrosion

ImAg Summary

Advantages Disadvantages• Planarity• Re-workable

• Silver Dendrites• Corrosion• Tarnishing• Shelf Life• Handling Issues

OSP(Organic Solderability Preservative)

OSP Process

Cleaner

35°C

~15 Minutes Total Process Time

Rinse

Micro-Etch

Warm

Rinse

DI R

inse

Rinse

OSP

Dry

What is OSP?

• Benzotriazole

• Benzimidazole

• Phenylimidazole

OSP Standards

• IPC-4555 - Announced in 2008

• 2011: IPC Announces “No Standard”

• Too Many Variations Based on Chemistry

• Chairman of IPC Committee

• Round Robin Testing Underway

• Suppliers Not Ready to Release Thickness Data

• Will Require Internal Standards

• Lessons Learned From Companies Using OSP

• J-STD-003A - Solderability Tests

OSP Issues

• Poor Solder Wetting

• No Clean Fluxes (Ask!)

• 1:1 Solder Stencil

• Exposed Copper

• Cupric Oxide

• Statue of Liberty (1886)

• ICT First Pass Yields

• Soldering Increases Yield

• Thinner Coatings

OSP Summary

Advantages Disadvantages• Planarity• Re-workable• Lowest Cost• Simple Process• No Thermal Stress

• Shelf Life• ICT Yields• Handling Issues

Others

ENEPIG

Palladium Over Bare Copper

Plasma

ENEPIG

• The “Universal Finish”

• Wire-bondable

• Palladium Barrier

• Black Pad

• Nickel Diffusion In Gold

• Highest Cost

• ENIG + Palladium

Plasma

• Cleaning & Application - 1 Step

• Outlasts Noble Metals

• Re-workable

• Excellent Solderability

• Wire-bondable

• Less than Noble Metals

• Lot Limitations

• Poor Availability

Electroless Pd

• 15-Minute Process

• Electroless Deposition

• 25.4 Angstroms

• Cheaper than ENIG

• No Black Pad

• Extensive Shelf Life

• Poor Availability

Evolving Market

No Clear Winner!

Testing

• Solderability Testing

• ENIG, ImSn, ImAg, OSP - All OK

• Reliability Testing

• ENIG, ImSn, ImAg, OSP - All OK

• ImAg - Corrosive Gas Issues

• ImSn - Losing Favor

• ENIG - 10-30% Cost-up

• OSP - Best “Compromise”

Quantify OSP

• For example: EAU $40M PCB’s

• Assuming 1.0x HASL Cost Factor

• OSP at 0.7x is a $12M Savings!

• ROI as a System Price

• Not “Plug-and-Play”

• Choose Best Compromise

• Ensure <$12M Annual

• Market Trend vs. Availability vs. Cost

Next Steps

• Single Finish = Less Variation in EA

• Solder Profiles

• Solder Stencil Geometries

• Solder Paste Recipes

• Evaluate OSP ICT Yield Concerns

• OSP-Specific Test Probes

• Reduced Thickness OSP (1500-2000Å)

• Run-At-Rate - Low Risk Volunteers ...