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What are the different finishes for PCBs and their impact on Assembly?

It is very important that an ideal surface finish be selected for a PCB. The high assembly density and Ball Grid Array (BGA) have led to the higher usage of electronic packages for commercial applications. Besides the Hot Air Solder Level (HASL / HAL), there are many other surface finishes too to fulfill the advanced assembly needs. With each finish having its individual benefits and limitations, the ideal choice on a product’s application.

Major decision drivers for Ideal Surface finish:

  • Compatible with the assembly process
  • Co-planarity – Flat and uniform
  • Good solder ability
  • An extended shelf life
  • Lowest overall cost
  • Easily rework able
  • Highest yield in production
  • Complying with all ROHS requirement
  • Testability

Surface finishes available with PCB Power:

PCB Power offers following surface finishes:

  • HAL
  • Lead free HAL
  • Immersion Tin
  • ENIG (Electroless Nickel and Immersion Gold)

HAL (Hot Air Leveling):

  • Most commonly used surface finish in automotive and industrial applications
  • “Nothing can solder like solder”
  • Low cost
  • Easily applied and reworked
  • Long shelf life
  • Good bond strength
  • Easy visual inspection
  • Considered the standard surface finish as related to assembly needs.
  • Huge co-planarity difference
  • In consistent surface finish thickness
  • Not suited for fine pitch SMTs and BGA package
  • Fabrication process itself introduces thermal excursion
  • Exhibit issues for high aspect ratio holes
  • Dimensional stability issues
  • Contain Lead

Lead free HAL:

  • “Nothing can solder like solder”
  • Low cost
  • Easily applied and reworked
  • Long shelf life
  • Good bond strength
  • Easy visual inspection
  • Complying with all ROHS requirement
  • Huge co-planarity difference
  • In consistent surface finish thickness
  • Not suited for fine pitch SMTs and BGA package
  • Fabrication process itself introduces thermal excursion
  • Exhibit issues for high aspect ratio holes
  • Dimensional stability issues

Immersion Tin:

  • Flat / Planar surface
  • Suitable for fine pitch applications
  • Suitable for horizontal and vertical application
  • Can be a substitute for reflowed solder
  • Good solder joint reliability
  • Fairly good shelf life
  • Relatively low-cost deposit
  • Suitable for compliant pin (press fit) connection
  • Complying with all ROHS requirement
  • Handling concerns
  • Horizontal process needs Nitrogen blanket
  • Not suitable for contact switching applications
  • Ionic contamination for automotive applications

ENIG (Electroless Nickel and Immersion Gold):

  • The second most commonly requested finish
  • Flat / Planar surface
  • Easy to solder
  • Long shelf life
  • Multi reflow approved
  • Can be used for wire bonding
  • Can be used for contact pads
  • Complying with all ROHS requirement
  • Aesthetically very pleasing
  • The process is very difficult to control and is laboratory intensive
  • Higher cost
  • Black pad
  • Risk of brittle solder joint
  • Not optimal for higher speed signals
  • Manufacturing process uses cyanide and other unpleasant chemicals

OSP (Organic Solderability Preservaive):

  • Flat / Planar surface
  • Easily rework able
  • Doesn’t affect finished hole size
  • Low cost
  • Easy process
  • Good solder mask integrity
  • Difficult to inspect
  • Limited thermal cycles
  • Limited self life
  HAL (SNPB) LEAD FREE HAL IMMERSION TIN ENIG OSP
ROHS compliant No Yes Yes Yes Yes
Cost Low Low Medium High Low
Shelf life 1 Year 1 Year Up to 3 Months 1 Year Up to 3 Months
Fine pitch quality Poor Poor Best Best Best
Assembly cycle capacity Multiple Multiple Multiple Multiple Multiple
Multiple rework capacity Limited Limited No Limited No
Solder Wattebility Excellent Good Good Good Good
Co-planarity Poor Good Excellent Excellent Excellent
Solder joint integrity Excellent Good Good Good Good
Low resistance / High speed No No No No N/A
Wave soldering Excellent Excellent Good Good Good
Reflow soldering Excellent Excellent Good Good Good
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