Navigating the Surface Finish Maze…
Navigating the
PCB Surface
Finish Maze
SMTA - Oregon
Date: 18 May 2011
Darren Hitchcock
Field Applications Engineer
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Navigating the Surface Finish Maze…
2
Business Overview: Global Presence
We are the only PCB Shop that has Global Footprint in 4 continents
MINNESOTA
GERMANY
FPC Manufacturing
ITO & Materials Manufacturing
325 K sq ft
Headcount: 350
Rigid PCB Manufacturing
110 K sq ft
Headcount: 450
CHINA - Zhuhai
Rigid PCB Manufacturing
FPC & Rigid Flex Manufacturing
1340 K Sq f
Headcount: 9,100
CHINA - Shenzhen
LCD Displays and
Touch Sensor Manufacturing
291 K Sq ft
BRAZIL
Headcount: 2,900
Rigid PCB Manufacturing
100 K sq ft
Headcount: 250
Electroless Nickel / Electroless Gold (ENEG)
Direct Immersion Gold (DIG)
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Over 13,000 people world wide, ~ 12,000 in China
3
A plating or coating applied to the exposed copper features of a
Printed Wiring Board (PWB).
Plating (metallic)
Coating (metallic or organic)
4
How is the Surface Finish Applied?
What is a Surface Finish?
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Coating
Plating (metallic)
Surface
OSP, HASL, Carbon etc.
Electrolytic
Electroless
Immersion
Horizontal
Finish
Equipment
Metal Plating
Note: (Base) Metal Plating is typically copper (in most cases).
Horizontal or Vertical equipment
Automated or Manual
But for some Nickel or Nickel-Phosphorous serve as the solderable surface.
Vertical
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1
Why is a Surface Finish Required?
Surface Finish Requirements
• Coplanar (flat)
The Surface Finish prepares and protects the conductors of a PWB
for assembly, installation, and use.
• Wire Bonding
• Solderability (wetability)
• In Circuit Test
• Lead-Free (RoHS and WEEE compatible and compliant)
• Compatibility with other Surface Finishes
• Contact Resistance (Compression Connection)
• Multiple Solder Cycles
Daughter Card Edge Fingers
Fuzz Button Pads
Grounding Rails
• Tarnish Resistance
• Wear Resistance
• Hardness
• Chemical Resistance
• Soldermask Plugging of Vias
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Reflow, Wave, Rework
• Multiple Component Types
QFP, BGA, Passives, Press Fit, etc.
• Cost
• Reliability
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Gold
Aluminum
Shock
Vibration
Thermal Cycling
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Surface Finish Types
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Surface Finish Types
Most Common:
•OSP (Organic Solderability Preservative)
• ENIG (Electroless Nickel/Immersion Gold)
• Imm Ag (Immersion Silver)
Under Investigation:
•ENEPIG (Electroless Nickel / Electroless Palladium / Immersion Gold)
•ENIPIG (Electroless Nickel / Immersion Palladium / Immersion Gold)
•Lead Free HASL
•HP ENIG (High Phosphorous Electroless Nickel / Immersion Gold)
•Direct Gold on Copper (Direct Immersion Gold, etc.)
•Lead Free ENIG
•I Ag / I Au (Immersion Silver / Immersion Gold)
•Nanofinish
• Imm Sn (Immersion Tin)
•Sn-Pb HASL (Hot Air Solder Level)
•Electrolytic Ni /Au (Electrolytic Nickel / Gold)
•Hard Gold
•Soft Gold
•Flash Gold (thin, solderable – may be hard or soft)
•ENEG (Electroless Nickel / Electroless Gold)
•more common for packaging industry (chip carrier PWB)
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Surface Finish Types
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Electrolytic Plating
• Electrolytic plating is achieved by passing an electric current through a solution
(electrolyte) containing dissolved metal ions allowing the metal to deposit on the
conductive surface of the PWB.
Less Common
•Carbon Ink (Screened on)
•Electroless Nickel – Immersion Palladium
•Selective Solder Strip (SSS)
•Sn Ni (Tin-Nickel)
•Unfused Tin-Lead
•Reflow Tin-Lead
• Electrolytic Plating requires:
Anode
Cathode
Electrolyte
Metal Ions
• The PWB serves as the cathode and is connected to negative polarity of the rectifier
• The Anode is connected to the positive polarity of the rectifier
Multiple Surface Finishes
•Hard Gold + Immersion Silver
•Hard Gold + ENIG
•Hard Gold + HASL
•ENIG + OSP
•???
• The metal ions in the plating bath are reduced at the conductive surface of the PWB
building up the metal thickness
• The Voltage, Amperage, Temperature, Time, and Purity of the solutions determine
the properties and amount of the deposit.
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Electrolytic Plating
Electroless Plating
• Process is Non-electrolytic and Non-galvanic
Electrolytic Nickel-Gold (Depicted Below)
-
-
+
+
+
+
P
C
B
Then
Solution
Gold
Solution
Hydrogen is released by the reducing agent, oxidized, creating a negative charge at the
surface.
•A catalyst is normally deposited on the copper surface to facilitate further
deposition.
Titanium
Nick el
Nickel
P
C
B
No external electrons are needed (ie: no rectifier)
• Electroless deposition uses chemical reducing agents to supply the electrons
needed for metal deposition.
•Process is Autocatalytic
Deposit continues once the plating starts.
•Examples: electroless nickel, electroless palladium, electroless gold, etc.
Anode
Cathode
Anode
Anode
Cathode
Anode
•Uniform plating
Electrolytic Gold Plating
(Over Nickel)
Electrolytic Nickel Plating
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not prone to differences in circuit density
• Generally harder and more brittle deposit than electroplated
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Electroless Plating
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Immersion Plating
Electroless Nickel (Depicted Below)
• Process is Non-electrolytic
No external electrons are needed (ie: no rectifier)
• Galvanic displacement reaction causes exchange of metal atoms on the
surface with those in the solution.
Ni ++
Nickel is dissolved and Gold replaces the nickel during ENIG process.
•Examples: immersion palladium, immersion gold, etc.
•For ENIG, the phosphorous co-deposited with the nickel is the reducing
agent that allows for the immersion gold plating.
•Uniform plating
Ni
not prone to differences in circuit density
• More prone to localized galvanic etching of sacrificial metal
Cu
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Immersion Plating
• Silver (Depicted Below)
• Tin
Cu ++
Galvanic Displacement - Simply an
Exchange of Copper and Silver Atoms
Ag ++
Pros & Cons of Each
Surface Finish
Cu
Base Foil + Plated Copper
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3
OSP (Organic Solderability Preservative)
OSP
Typical Equipment used for the Coating of OSP
Conveyorized Horizontal OSP and Pre-Flux Line
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ENIG (Electroless and Immersion Plating)
Organic Surface Preservative - OSP
Advantages
•
Flat, coplanar pads
•
Re-workable by PCB supplier
•
Cheap & simple process
•
Short and easy to control process
•
High reliability for SAC based solder
joints
•
High temperature OSP’s are available
•
No additional investment over today
Typical Equipment used for the Plating of ENIG
Disadvantages
•
Limited heat cycle capability
•
•
•
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Newer developments better
PCBA process control critical
Having exposed Cu after PCBA In
Circuit Test (ICT) is a concern
•
But also true today
•
In Circuit Test pins cut through organics
leaving residues on test probes
•
Handling sensitive
•
Overall “tighter” process window for
assembly provider
Automated ENIG Plating Line (PAL)
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Electroless Nickel Immersion Gold - ENIG
Electroless Nickel Immersion Gold - ENIG
Advantages
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23
Disadvantages
•
Flat planar pads
•
Withstands multiple heat cycles
•
Expensive (gold)
•
Long shelf life
•
Process is difficult to control
•
Good solderability
•
Black pad potential
•
No exposed copper
•
High barrel reliability
•
Brittle Fracture for BGA type packages
(under mechanical load)
•
Easy to use for ICT
•
Not handling sensitive
•
May not be suited for high speed signals
(skin effect)
•
Adjustments necessary for Press Fit
technology
•
Great concerns with reliability on SAC
solders (mechanical load)
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4
Immersion Silver Plating
Immersion Silver or Immersion Tin
Typical Equipment used for Horizontal
Immersion Silver Plating
Conveyorized Horizontal Immersion Silver Plating Line
Smaller Proto Shops may use a CONFIDENTIAL
Vertical Batch| Process
Copyright ©2011 Multek. All rights reserved. |
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Immersion Silver
Advantages
•
Flat, coplanar pads
•
Consistent coating thickness
•
Withstands multiple heat cycles
•
Good shelf life
Immersion Tin
Disadvantages
Advantages
Adjustments necessary for press fit
technology (high friction coefficient)
•
Flat, coplanar pads
•
Anti-tarnishing is critical to control
•
Very easy solder able
•
High reliability with Cu Sn IMC
•
Works well at ICT
•
Same metal as CuSnAg alloy (does not
change alloy ratio much)
•
Well suited and proven for press fit
technology
•
•
Likes to build oxides
Issues with process interruptions
Likes to diffuse (and migrate)
Best wet ability of all alternatives
•
•
Easy to repair (PCBA)
Potential issues with µvoids (“champagne”
voids)
•
High solder joint reliability with Cu Sn
inter-metallic
•
Very handling sensitive
•
Easy to probe at ICT
•
Not recommended for wear surfaces
•
Relatively simple process at PCB
manufacturing
•
Creep Corrosion in high sulfur
environments
•
Color and look
•
Fewer heat cycles possible than I Ag due
to IM growth
•
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HASL (Hot Air Solder Level)
Typical Equipment used for the Coating of HASL
Relatively inexpensive
•
Rinsing is highly critical
Likes to build oxides
Diffuses into the copper
•
•
•
Grows inter-metallic (0.1µm/2 months) at
RT and with every heat cycle
Anti-tarnishing is critical to control
•
•
Can be reworked
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•
•
•
•
Disadvantages
•
•
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Exposed Cu
Color and look
Fear of Sn Whiskers
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Tin – Lead HASL
Vertical and
Horizontal HASL
Equipment
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5
Electrolytic Plating of Nickel and Gold
Tin – Lead HASL
Advantages
Typical Equipment used for the Electrolytic Plating
of Nickel and Gold
Disadvantages
•
“Nothing Solders Like Solder”
•
Easily Applied
•
Lengthy Industry Experience
•
Easily Reworked
•
Multiple Thermal Excursions
•
Good Bond Strength
•
Good solder joint reliability
•
•
•
•
Co-Planarity Difference
•
Inconsistent Coating Thickness
•
Contains Lead
•
Not Suited for High Aspect Ratios
•
Not Suited for fine-pitch SMT and Grid
Array Packages
Long Shelf Life
•
PWB Dimensional Stability Issues
Easy Visual Inspection
•
Bridging Problems on Fine Pitch
Cu/Sn Intermetallic (solder joint)
•
Subjects the PCB to High Temp
•
•
Potential Off-Contact Paste Printing
(on Varying Pad Sizes)
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Automated Nickel and Gold Plating Line
PAL and TAB Lines Shown
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Electroplated Nickel / Gold
Electroplated Nickel / Gold
Disadvantages
Advantages
•
Limitation is aspect ratio
Very easy solder able
•
Limitation in line spacing at about 4-5 mil
(100-125 µm)
•
Works well at ICT
•
Gold is expensive
•
Very high barrel reliability (PCB)
•
Recent great concerns with reliability
•
Very easy to rework (PCBA)
•
Flat, coplanar pads
•
•
•
•
•
Very long shelf life
•
Long and proven track record for high end
products
•
Hard gold provides good wear surface
•
10:1 max
Lower joint strength using SAC alloys
compared to OSP
Inter-metallic phase fractures
Sporadic in nature
•
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33
•
Problem is not yet well understood
Not so widely available in the industry
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Lead – Free HASL
Surface Finishes - 2012
Equipment being used for the Coating of Lead-Free HAL
Same as for Leaded Versions but with a few Modifications
• Higher Temp Steel Solder Pots and Stronger - Higher
Temp Pumps
• (Effective heat transfer by improved alloy circulation)
Pre-heat panel (pre-dip)
• Longer contact time with PCB
• High temperature resistant chemistries (oils and fluxes)
• Copper control (Drossing – Dilution and Skimming)
•
*Source: CEMCO / FSL
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Lead – Free HASL
Lead - Free HASL
Disadvantages
Advantages
•
Easily Applied and Reworked
•
Familiar HAL Dynamics
•
Good Bond Strength
•
Long Shelf Life
•
Easy Visual Inspection
•
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Wettability
Subjects the PCB to VERY High
Temperature
•
Copper Feature Dissolution
•
Co-Planarity Difference
•
Inconsistent Coating Thicknesses (on
Varying Pad Sizes)
•
Not Suited for High Aspect Ratios
•
May not be suited for fine-pitch SMT and
Grid Array Packages
•
PWB Dimensional Stability Issues
•
Bridging Problems on Fine Pitch
•
“Dull” and “Grainy” Appearance
•
More Process Controls Req’d
Cu/Sn Solderjoint
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Lead-Free Solder
•
Potential Off-Contact Paste Printing
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LEAD-FREE HAL (Hot Air Level)
Corrosion of Copper Pad
The main considerations in changing a HAL
process from 63/37 Sn/Pb to SN100C
(Ni-stabilized Sn-0.7Cu) is:
Original Pad
18µm Copper
The higher melting point
ALLOY
MELTING
PROCESS
PROCESS
POINT
TEMPERATURE
WINDOW
63/37 Sn/Pb
183°C
250°C(482)
67°C
Sn-0.7Cu+Ni
227°C
265°C(509)
38°C
*Source: Nihon Superior Co., LTD
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After 6 Passes over Wave Soldering Machine
105°°C Preheat, 256°°C Solder Temperature, 4 seconds contact time
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Sn-37Pb
Sn-3.0Ag-0.5Cu
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Sn-0.7Cu+Ni
| Copyright ©2011 Multek. All rights reserved. |
Source: Florida CirTech, Inc.
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ENEPIG
Copper Dissolution Rate
Alloy #1
Electroless Nickel / Electroless Palladium / Immersion Gold
40 sec contact
120 sec contact
Alloy #2
15 sec contact
• Boards were waved and reworked with 2 different alloys. Simulated mini-pot
rework (no component removal) was performed from 0 – 120 second exposure.
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7
ENEPIG
• Electroless Nickel / Electroless Palladium /
Immersion Gold
ENEPIG
• Advantages:
E
• Sometimes called the Universal Finish
• Ni is 3-8 um thick (120 – 315 microinches)
• Pd is 0.1 – 0.3 um thick (4 – 12 microinches)
• Au is 0.02 – 0.05 um thick (0.8 – 2.0 microinches)
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ENEPIG
Gold and Aluminum Wirebonding
Good Solderability
No bussing bars needed to plate (not electrolytic)
No concern with “Black Pad” because the Phosphorous is plated on the
Electroless Nickel by chemical reduction, not galvanic like in ENIG, so there is no
attack of the Electroless Nickel layer during ENEPIG.
No excessive corrosion of the EP layer by the IG due to low phosphorus content
of the EP layer and low thickness of the gold.
Good surface wear resistance
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ENIPIG
Electroless Nickel / Immersion Palladium / Immersion Gold
• Disadvantages:
Less common in the industry
doesn’t have the track record of the other finishes. So don’t know if there are no
issue, or no issues…yet.
• Further Investigation:
Thin IMC layer – good or bad?
High volume processing window – large or narrow?
Surface finish cost?
Immersion Gold is still galvanic with Electroless Phosphorous, are there any
hidden issues with galvanic corrosion of the EP layer?
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Direct Gold on Copper
ENIPIG
• Electroless Nickel / Immersion Palladium / Immersion Gold
• Advantages:
Gold and Aluminum wire bondable
Good solderability (also in lead-free assembly)
Palladium bath is less sensitive to impurities than with ENEPIG
Faster processing time than ENEPIG
Wider process window than ENEPIG
Thinner Palladium deposit 0.3-0.5 um (1-2 microinches)
Lower cost than ENEPIG
I
• Disadvantages:
replacement / galvanic deposition of Palladium on the Electroless Nickel.
Unknown issues with black pad type phenomenon
however there is a very thin deposit of the palladium, so the corrosion attack should be
minimized
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8
Direct Gold on Copper
Direct Gold on Copper
• Gold is plated directly on the copper without a nickel barrier layer.
• Advantages
• Direct Immersion Gold
Immersion Gold and Electroless Gold type deposition is used (not intuitive from
DIG acronym)
A mixed autocatalytic reaction and displacement reaction
Uyemura study (>80% autocatalytic) – www.uyemura.com/library-3.htm
Good for mixed surface finish PWB
Multiple solder exposure during assembly
Compatible with press fit connectors
Good for flexible circuits
Eliminate lossy nickel in high speed / RF applications
Better coverage than standard immersion gold
• Disadvantages
Shelf Life – gold / copper migration concerns
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Tight gold deposit is supposed to produce a non-porous deposit that resists copper
migration into the gold.
Slow deposition time
High process bath temperature
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High Phosphorous ENIG
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HiPhos ENIG
• Phosphorous >10% codeposited with the Electroless Nickel
• Advantages
Corrosion resistance
Supposed to eliminate Black Pad risk
Solderability and Reliability as good as Medium Phosphorous ENIG
Drop-in to existing ENIG plating lines
Lower internal stress in the Nickel deposit (better for flexible circuits)
• Disadvantages
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Not well known.
Needs more testing
Slower deposit rate than standard ENIG
Nickel bath sometimes reacts with Sulfur content in some soldermasks
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Lead Free ENIG
Lead – Free ENIG
• No Lead as a stabilizer in the Electroless Nickel bath
The main reactions in the Electroless Nickel bath are
the deposit of Nickel and Phosphor
buildup of orthophosphite and other byproducts
• Orthophosphite causes higher stress in nickel deposit which increases the
risk of nickel corrosion
• Other by products lead to breakdown of the soldermask which leads to
sulphur buildup which increases the risk of nickel corrosion.
• Heavy metals (Lead) and organic stabilizers are used to limit these buildups
and corner attack.
• Removal of Lead reduces corner attack, therefore a reduction in the amount
of sulphur accelerator needed. This leads to an increase in corrosion
resistance and a reduction in the associated risk of black pad.
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9
ENAG
ENAG
Electroless Nickel / Immersion Gold / Electroless Gold
• ENIG with additional Electroless Gold plated on top.
• Used for thicker gold deposit
• Advantages
excellent for gold wirebonding (packaging, camera modules, etc.)
reason for the thicker gold
• Disadvantages
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Not recommended for soldering – solder joint embrittlement risk due to amount
of gold.
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Immersion Silver / Immersion Gold
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Imm Silver / Imm Gold
• Immersion Silver deposit followed by Immersion Gold deposit
Immersion Gold
Immersion Silver
• Currently under investigation
Copper
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PCB MATERIALS
FINAL FINISH
2011
90
Electrolytic Ni/Au
12.0Mm 2 4% Reflowed Sn/Pb
1.5Mm 2 0.0%
ImSn
2
14.6Mm 5%
Electrolytic Ni/Au Reflowed Sn/Pb
8.5Mm 2 3.4%
1.9Mm 2 0.7%
ImSn
OSP
9.0Mm 2 3.6%
115.0Mm 2 46%
80
70
OSP
183.7Mm 2 58%
Lf HASL
57.3Mm2 18%
Lf HASL
46.0Mm 2 18%
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2011 Surface Finish – select factories
Final Finish Market Survey
2006
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50
40
30
HASL
17.1Mm 2 5%
20
10
CAAGR 5.0%
0
ImAg
25.3Mm 2 8%
HASL
52.1Mm 2 21%
ImAg
10.4Mm2 4.2%
Imm ENIG
HASL Elec.
Silver
OSP
Ni/Au
ENIG
7.5Mm 2 2%
ENIG
7.1Mm 2 2.8%
TOTAL: 250Mm2
TOTAL: 319Mm2
Handheld
MFF
LFF
Imm
Tin
ENIG
+
OSP
Source: Prismark Partners
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10
Conclusion Finish I
Surface Finish Comparison
There is no SINGLE perfect finish that “fits all”
HASL (Sn-Pb)
ENIG
Imm Ag
OSP
Flatness
poor
good
good
good
Solderability
good
good
good
fair
Rework (Sn-Pb)
good
fair
good
good
n/a
fair
good
good
Rework (Lead-free)
Finishes should be selected based on specific product
application (OEM-EMS key)
US market tends to favor I Ag (larger and thicker boards)
EU tends to favor I Sn (smaller and thinner boards)
Handhelds prefer OSP/ENIG
OSP is used in all product application areas
All other finishes will be niche (currently)
acceptable in
some
good - button,
good - ICT,
applications
poor - wiping
poor - wear
poor
No
Yes
Yes
Yes
oxidizes
yes
fair
fair
Solder Joint Reliability (thermal cycle)
good
fair
good
good
Solder Joint Reliability (shock)
good
poor
good
good
low
high
low
low
Contact Resistance (Wear)
RoHS (compliant)
Tarnish Resistance
Cost
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Thank You
Thank You
11
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