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PCB Surface Processes
 

Purpose of PCB Surface Finish

Provides a solderable surface for Component assembly.
Protect Copper Surface from Oxidation.
Provide a surface appropriate for wear related parts.



High Performance Surface Finish

Mixed Technology Board Designs
Solderable,SMT and Thru-hole
Wire Bondable
Contact Resistance
Co-Planar Surfaces
Compatability with variety of fluxes & paste
Cost effective
Environment


Available Surface Finishes


Electroless Nickle-Immersion Gold
Immersion Silver
Hot Air Solder Level
Organic Solderability Preservative
Electrolyte Nickel - Hard Gold
Electrolyte Nickel - Soft Gold

Surface Finish Details

Electroless Nickel-Immersion Gold (ENIG)

A two layer, gold over nickel, metallic surface finish plated onto the copper base by means of a chemical decomposition process.


 

Advantage Disadvantage
  • Typical Thickness: 3-8 uin gold over 150-250 uin nickel
  • Excellent Corrosion resistance
  • Good for aluminium wire bonding
  • Excellent flatness for fine-pitch technology
  • Excellent Solderability
  • Excellent shelf life(12 mo)
  • Good surface contrast
  • Good contact resistance
  • High Aspect Ratio Capability
  • Narrow process window
  • Higher cost than HASL(1.5 to 2.0 times)
  • Back pads: brittle solder joints
  • Fatigue failures on large BGA packages
  • Skip plate
  • Extraneous nickel plating
  • Solder mask capability with the nickel bath

Immersion Silver

A co-deposit of silver and organics

 

 

 

Advantage Disadvantage
  • Typical coating thickness: 5-25 uin
  • Very good alternative to HASL (lead free)
  • Similar cost to HASL
  • Excellent Solderability,(Comparable to Solder)
  • Excellent for fine pitch and BGA's technologies
  • Holes Capability over 10:1 aspect ratio
  • Press fit connector's capability
  • Mulitple heat cycles during assembly process
  • Reworkable product(at fabricator)
  • Selective gold compatible
  • Good Shelf life ,6 months to 12 months
  • Conductive during PCB testing
  • Low % Silver dissolved into silver joint
  • No thermal cycle after Immersion
  • Silver at the Fabricator
  • Vias cap:UV curable only
  • Handling
  • Some resistance from the OEM to use
  • Immersion Ag with press fit connectors
  • Micro-void in the solder joints
  • Also found on other surface finishes
  • Discoloration due to reaction with sulfur in the air.
  • (Does not impact solderability)


Hot Air Solder Level (HASL)

  • A molter solder immersion and hot air leveling(blasting) of surfaces and holes to provide a "pretinned" coating on exposed copper surfaces.

 

 

Advantage Disadvantage
  • Typical coating thickness specifications
  • 100-1000 uin with average of 150-300 uin fine pitch
  • 40-1000 uin on discrete pads with average 80 uin
  • coverage on large surface (ground plane) 15-40 uin
  • Excellent solder "Nothing solders better than solder"
  • 20mils pitch capability; dependent on technology
  • Board thickness up to 200 mils
  • Panel size up to 24 x 28 inches
  • One of the less expensive surface finish
  • Good shelf life
  • Uniformity of the coating
  • Thermal shock
  • Limited capability on high density borads
  • Limited capacity to clear vias holes
    (>6:1 aspect ratio)
  • Difficulty to maintain hole size tolerance along
    plated edges
  • Difficulty to process thin material
  • Require a different drill selection.
    (Press fit connector dictate the drill)
  • Process very dependant of the board technology
  • Intermetallic layer grows with time and heat cycle
  • Use of Lead


Organic Solderability Preservative

  • A water based Organic compound that selectively bonds with copper to provide an organomettalic layer that protects the copper for soldering

 

 

Advantage Disadvantage
  • Typical coating thickness: 8-20 uin
  • Major advantage: Gold doesn't need to be protected
  • The least expensive surface finish
  • Reworkable
  • Shelf life (6 months)
  • New lead free OSP formulations available
  • Difficult or impossible to inspect the final product
  • Skip Plating
  • Residue on Copper
  • Coating thickness
  • Difficult to soldermask via cap
  • Electrical testing prior to coating
  • Micro-void in the solder joints
  • also found on other surface finishes
  • Solder may not wet to corners of large SMT pads
    (still good solder joint)

Electrolyte Nickel - Hard Gold

  • A two layer,gold over nickel,metallic surface finish plated onto the copper base by means of a electrolytic deposition process.

 

 

Advantage Disadvantage
  • Good Connector. Typical thickness 30-50 uin
    minimum gold (99.7 %)
    over 150 uin minimum nickel
  • Solderable Surface. Typical thickness 5-15 uin
    minimum gold (99.7 %)
    over 150 uin minimum nickel
  • Excellent corrosion resistance
  • 130 to 200 Knoop hardness
  • Excellent wear resistance for surface rotary switches, on-off contacts,and edge connectors
  • Excellent shell life

  • Gold Distribution
  • Throwing power in the holes
  • The copper is exposed on the sides of the traces
  • Porosity
  • Difficulty to measure the true line width
  • Soldermask adhesion on gold

 

Electrolyte Nickel - Soft Gold

Advantage Disadvantage
  • Good Connector. Typical thickness 30-50 uin
    minimum gold (99.7 %)
    over 150 uin minimum nickel
  • Solderable Surface. Typical thickness 5-15 uin
    minimum gold (99.7 %)
    over 150 uin minimum nickel
  • Excellent corrosion resistance
  • 130 to 200 Knoop hardness
  • Excellent wear resistance for surface rotary switches, on-off contacts,and edge connectors
  • Excellent shell life

  • Gold Distribution
  • Throwing power in the holes
  • The copper is exposed on the sides of the traces
  • Porosity
  • Difficulty to measure the true line width
  • Soldermask adhesion on gold


Immersion Tin

A co-deposit of Tin and organics

 

 

 

Advantage Disadvantage
  • Back Plane
  • Press fit connectors Technology
  • Reworkable
  • Reworkable

  • Soldermask compatability
  • Thiourea (TU)
  • High temperature of the Tin bath
  • Process control:Solderability Test (Tin porosity)
  • Intermetallic growth
  • Solderabilty goes down significantly with each reflow cycle
  • Unable to analyze chemistry directly
  • solderabilty test(6-8 hr test)

PCB Surface Finish Developments

Lead Free Compatible OSP

Enthone 106 HT Designed for Lead Free re-flow Profiles
Drop in Alternative to 106 AX (for Merix and CMS)
Survive Extra re-flow Cycle in a tin/lead Assembly
Easy Control Process

UIC Immersion Silver

Investigating to Reduce or Eliminate Tarnishing
Testing Planned

 
 
     
   

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