Surface Finish 2017-03-26T12:16:48+00:00

The primary function of PCB Surface finish is to protect the copper area from oxidation prior to assembly.
The surface finish forms the foundation of the connection of the electronic component to the PCB solder pad, enhancing the assembly process and promotion a reliable solder joint for long term performance.

Available Surface Finish

  • Sb/Ph Hot Air Solder Level (HASL)
  • Lead Free HASL
  • Organic Solderability Preservative (OSP)
  • Electroless Nickel Immersion Gold (ENIG)
  • Immersion Silver
  • Immersion (White) Tin
  • Electrolytic Nickel/Gold (Hard & Soft Au Wire Bondable)
  • Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

Sn/Pb Hot Air Solder Level (HASL)

  • A very common PCB finish in the industry, as it is easy for assembly.
  • An “Eutectic” composition 63% Tin / 37% Lead
  • Molten solder applied to exposed pads in horizontal or vertical panel orientation.
  • Excess solder is blown off the circuitry with forced hot air “knife”.
  • Typical thickness range of .00003” – .0015” (30-1500 1”).
  • Melts at 183 degrees C.
  • Shelf life 12+ month

Issues

  • Surface Planarity for HDI applications (highly variable topography)
  • Impact of thermal excursion (extreme process temperatures)
  • RoHS/WEEE restrictions and environmental issues with the use of lead

Lead-Free HASL

  • Same application process as Sn/Pb HASL Solder.
  • Environmentally Friendly (RoHS & WEEE compliant).
  • Melting Point 228 degrees C.
  • 12 month shelf life.

Issues:

  • Similar issues as normal HASL
  • Found to be aggressive on Cu. Depends on Alloy

OSP – Organic Solderability Preservative

  • Provide very thin coating (100-4000 Angstroms) of available surface finish.
  • Applied in a vertical (dip tank) or conveyorized chemical process.
  • Flat, planar surface, well suited to tight-pitch pads (BGA, QFP).
  • Shelf Life < 6 month shelf life
  • Advantage : Easy to manufacture
  • Preferred surface finish in Japan.
  • Globally, OSP is the most popular surface finish by area of PCBs manufactured

Issues

  • Difficult to inspect (transparent and colorless).
  • Sensitive to handling.
  • Requires relatively aggressive flux at assembly.

ENIG – Electro-less Nickel Immersion Gold

  • Coating thickness 2-8 micro-inch of Gold over 120-240 micro-inch of Nickel
  • Soldering occurs at the Ni surface. Gold protects the nickel from oxidation during storage.
  • Flat, planar surface, well suited to tight-pitch pads (BGA, QFP).
  • Used for Aluminum wire bonding
  • Shelf Life < 12 months

Issues

  • Can not be used for Gold wire bonding

Immersion Silver

  • Nearly pure silver 8~15 micro-inch thickness
  • Provides surface flat and planar, providing excellent solderability
  • Preferred for bonding Aluminum wire
  • Shelf Life 6~12 months

Issues

  • Extra precaution to be taken in handling, electrical test and packaging as it is sensitive

Immersion Tin

  • Uniform surface finish that is dense, fine grain and non-porous, providing superior hole wall lubricity
  • Suitable for back-plane PCB that have press-fit connectors
  • Shelf Life 6 months

Issues

  • Sensitive to handling
  • Environment concerns due to carcinogen materials present in processing

Micro Units of Measurement

  • Micro-inch (1 1”) = .000001” (one millionth of a inch)
  • Micro-meter (Micron) (11m) = one millionth of a meter
  • One Micron (11m) = .00003937” (39 1in)
  • 25.4 microns = .001” (1 mil)