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 ...
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