FINISH
The finish of a pcb is used to provide a solderable coating for the subsequent soldering operations.
Why have different board finishes, why not just have one finish, there is not one pcb universal cost effective finish, all finishes have advantages and disadvantages…
The list shows the common board finishes available
  • Leaded HASL – Becoming less popular due to RoHS
  • Lead Free HASL
  • Immersion Silver
  • Electroless Nickel/Immersion Gold
  • < >
    Immersion Tin
  • O.S.P. ( Organic Solderability Protection )
  • Electroplated Tin (SMIs/Sculptured Flex)
HASL FINISH
Leaded HASL
Historically this the most common pcb finish, its use now is a lot less due to the restrictions of the RoHS Directive.
We still have this in Vertical and Horizontal HASL processes, and is processed daily due to Aerospace and Military requirements.
Leaded Free HASL
The most cost effective board finish
Advantages and Disadvantages of HASL Finish
Advantages
Widely Available
Inexpensive
Re-Workable
Cu/Sn Solderjoint
Concerns
Not Flat
Paste mis-prints
No fine pitch assembly
Too thin: intermetallics
Thermal Damage to board
Immersion Silver
Picture43
Advantages
Low Cost
Excellent Wetting
Simple Process
Flat/ Fine Pitch
Cu/Sn Solderjoint
Contact Functions
Eliminates Selective Processing
Concerns
Sensitive to Cl-, S-
Sliding Connector Limitation
Poor Handling will affect solderability
Storage and Shelf Life
Packing
Electroless Nickel/Immersion Gold
Picture42
 Advantages
Surface Contact
wirebonding
keypads
Flat/ Fine Pitch
Long Shelf-Life
Widely Available
Concerns
Expensive
Soldermask Attack
Complex Processing
Not Re-Workable
Ni/Sn brittle solderjoint
Immersion Tin
eu'shine
Advantages
Low Cost
Minimal Tarnish
Re-Workable
Flat/ Fine Pitch
Cu/Sn Solderjoint
Concerns
Quick Intermetallic
Reduced Shelf-Life
Surface Conductivity
Thickness Measurement
Thiourea
Soldermask Attack
  • Suspect Carcinogen
    Whisker Concerns
    Solderability Issues if tin not thick enough
    OSP = Organic Solderability Preservative
Advantages
Low Cost
Fast Process
Re-Workable
Cu/Sn Solderjoint
Concerns
Limited Shelf-Life
degrade with temperature
handling sensitive
No Surface Contact
no electrical test/ ICT
no wirebonding
no keypads
Flux Sensitive
Not Inspectable
Specifying board finish thickness
Immersion Silver
State a range of  0.15um to 0.40um, too thin a coating and you will get solderability problems
Electroless Nickel/Immersion Gold
State nickel 3 – 7 um, with gold a minimum of 0.04um nominally 0.06um, typical range is 0.04 – 0.10um, to thin a nickel or gold and you will have solderability problems
Immersion Tin
State a minimum of 1um any thinner, and you might have solderability problems on 2nd side soldering
For HASL type finishes ( leaded and lead free ) : specify a maximum thickness of 20um, minimum thickness is a difficult one, as the thickness of solder at the knee of holes is very thin, but as we all know, you do not get solderability issues with HASL finishes.