PCB Surface Finish (Treatment)

The surface finish is the part of the printed circuit board that prevents oxidation of the copper. It is the coating laid over the base copper surface of the outer layers of a PCB after the copper has been etched.

When manufacturing PCBs you have to make the copper solderable and preserve the solderability of the board. So, the copper has to be made pristine with a finish that does not oxidize. The purpose of the surface finish is to protect the surface until the components are assembled onto it.
This allows for the soldering of component leads to wet the surface to form a reliable joint Therefore, the surface finish is critical for that application.

The importance of PCB Surface Finish

The selection of the surface finish could be the most important material decision made for the component assembly. Each surface finish has its own advantages and disadvantages, which influence:

  • wettability
  • solderability
  • shelf life
  • appearance
  • solder joint strength
  • intermetallic
  • void formation
  • reliability
  • process yield
  • rework and ease of use
  • ability to test
  • scrap rate
  • field failure rate
  • cost
PCB surface finish types

Considerations When Selecting Your PCB Finish

Choosing the appropriate PCB finish depends on the following factors:

  • Cost
  • Shelf life
  • The environment in which the end product will be used in (humidity, temperature, etc.)
  • If the design includes fine pitch components
  • If the PCB is for a high-frequency application and the importance of signal integrity
  • Thermal resistance
  • The level of ruggedness required
  • Whether lead-free assembly is required
  • Build quantity
  • Ease of probing for electrical test

Types of PCB Surface Finish

  • ENIG – Electroless Nickel Immersion Gold
  • HASL – Hot-Air Solder Leveling
  • Lead-free HASL
  • ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold
  • Hard (Electrolytic) Gold Plating
  • ImAg – Immersion Silver
  • ImSn – Immersion Tin
  • OSP – Organic Solderability Preservative

Cost Comparison of PCB surface finish types

  • Hard (Electrolytic) Gold Plating 100% 100%
  • ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold 91% 91%
  • ENIG – Electroless Nickel Immersion Gold 39% 39%
  • ImAg – Immersion Silver 23% 23%
  • ImSn – Immersion Tin 18% 18%
  • HASL – Hot-Air Solder Leveling 14% 14%
  • OSP – Organic Solderability Preservative 6.8% 6.8%

This cost comparison considers the Hard (Electrolytic) Gold Plating PCB finish to be the most expensive and rates other finishes compared to the cost of Hard Gold plating finish.

Comparison of PCB surface finish types

Finish typeWhen to UseAdvantagesDisadvantages

ENIG (Electroless Nickel Immersion Gold) Surface Finish

A 2 layer metallic coating where the electroless plating is covered with a thin layer of immersion gold, which protects the nickel from oxidizing during storage. The nickel functions as a protective layer for the copper circuit, and a surface on which the components are soldered.

  • Designs containing fine pitch devices
  • RoHS requirement
  • Hard and durable
  • Long shelf life
  • Results in a very flat surface finish
  • Lead-free
  • Good for PTH (Plated Through-Hole)
  • Expensive
  • Possibility of black pad syndrome
  • Finish itself is not reworkable

HASL (Hot Air Solder Leveling) and lead-free HASL Surface Finish

The PCB is dipped into a bath of molten solder, which covers all exposed copper surfaces. Excess solder is removed by passing the PCB through a hot air knife system that blows off and removes excessive solder from the PCB.

  • Design containing primarily through-hole or larger SMT components.
  • Low cost
  • Hard & durable
  • Long shelf life
  • Easy to inspect visually
  • Great for soldering
  • Prevents copper oxidation
  • Not suitable for Plated Through Hole (PTH)
  • Not suitable for fine pitch SMT components
  • Susceptible to thermal shock

ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold Surface Finish

Is a finish consisting of a metal plating process, which consists of an electroless nickel plating of the PCB which is then covered with a thin layer of gold.

  • Aerospace
  • Military
  • Medical
  • High-performance applications
  • Hard and durable
  • Long shelf life
  • Results in a very flat surface finish
  • Lead-free
  • Good for PTH (Plated Through-Hole)
  • Good oxidization resistance
  • Ideal for wire bonding
  • Expensive
  • Slower to wet

Hard (Electrolytic) Gold Surface finish

Is a PCB finish where a layer of gold is plated over a layer of nickel. Gold plating is usually applied to high wear or high friction locations of a PCB,. This type of finish can only be used in PCB locations where soldering is not needed.

  • Keypads
  • Edge connectors (also known as gold fingers).
  • Hard and durable
  • Wear resistance
  • Good shelf life
  • Expensive
  • Poor solderability

Immersion Silver Finish (ImAg)

Is a PCB finish where a layer of silver is plated over the copper

 

  • High-speed designs
  • Designs having fine pitch components
  • Excellent solderability
  • Good shelf life
  • Results in a very flat surface finish
  • RoHS compliant
  • Economical
  • Alternative for ENIG
  • Impacted by temperature and humidity
  • Needs good packaging
  • Must be used quickly after removal from packaging
  • Formation of silver whiskers under electric fields

Immersion Tin Finish (ImSn)

Is a PCB finish where a layer of tin is plated over the copper

  • Press-fit connectors
  • Fine pitch components
  • Results in a flat surface finish
  • Lead-free
  • Good solderability
  • Formation of tin whiskers under electric fields
  • Health and safety concerns
  • Not suitable for multiple passes in the reflow oven
  • Scratches easily

Organic Solderability Preservative (OSP) Finish

This is a thin, uniform, protective layer on the copper

  • Copper pad applications
  • Fine pitch components
  • Results in a flat surface finish
  • Lead-free
  • Good solderability
  • Low cost
  • Difficult to inspect and test
  • Not suitable for PTH (Plated Through-Hole)
  • Not suitable for multiple passes in the reflow oven
  • Short shelf life