What Is Surface Finish and What Is Its Function?

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.

Blue printed circuit board

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 soldering of component leads to wet the surface to form a reliable joint. Therefore, the surface finish is critical for that application.

Why Is Surface Finish So Important?

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 influences:

  • process yield
  • amount of rework
  • ability to test
  • scrap rate
  • field failure rate
  • cost

What are the types of surface finishes?

There are two groups of surface finishes:

  1. Metallic surface finishes which have some metal content such as gold, silver, tin or lead.
  2. Organic – which has no metal part or substances.

1. Metallic surface finishes

Finish Type
AdvantagesDisadvantages
1) HASL (Hot-Air Solder Leveling)

With this method, the solder mask is laid over the bare copper board, it is fluxed, cleaned, and then dipped into a molten solder pot to coat the surface.
Over the past few years, in an effort to reduce or stop using lead, other surface finishes have evolved.
Excellent solderabilityA difference in thickness/topography between large and small pads
Low costNot suited for < 20mil pitch SMD & BGA
Allows large processing windowBridging on fine pitch
Long industry experience / well-known finishNot ideal for High Density Interface (HDI) products

Finish TypeAdvantagesDisadvantages






2) Lead-free HASL

Typical thickness is 1 to 40 um.
Very good for solderingHuge coplanarity difference – not suitable for fine pitch devices
Easy to applyBridging problems on fine pitch devices
Very durable and can withstand multiple thermal cyclesInconsistent coating thickness
Bonds very well to the copper gives good thermal jointNot suitable for High-Density Interconnect (HDI) devicesHigh process temperature – 260 to 270 degrees centigrade
Long shelf life
Easy to inspect
Long shelf life
Low cost

Finish TypeAdvantagesDisadvantages





3) ENIG – Electroless nickel immersion gold

Typical thickness: 3 – 6 um Ni / .05 – 125 um Au
Planar surfaceExpensive
Consistent thicknessBlack pad issues in BGAs
Multiple thermal cyclesWaste treatment of Nickel
Long shelf lifeCannot be reworked at PCB fabricators
Solders easilyComplex PCB fabrication process
Use for wire bonding PCBs
Good for fine pitch devices

Finish TypeAdvantagesDisadvantages



4) ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold

Thicknesses:
Nickel: 5 um, Palladium: 0.15 um, Gold: 0.1 um
Can be stored for a long timeNeed to maintain control over palladium thickness
Solderable and wire bondable (unlike ENIG)Does not wet as well as HASL
No black padNot widely available
Complex process
Thick palladium can also result in a decrease in solder joint strength

Finish TypeAdvantagesDisadvantages


5) Hard (Electrolytic) Gold

This is a very durable finish and is suitable for high wear areas like keypads and edge connector fingers.
Can be stored for a long timeNot solderable when the thickness exceeds 0.43 um
Durable and hard surfaceExpensive
Lead-freeDoes not work well with other surface finishes
Limited rework capability at PCB fabricator

Finish TypeAdvantagesDisadvantages











6) Immersion Silver

Typical thickness .05 to .40 um
Good for fine pitch devicesPlanar micro-voids
Planar surfaceTarnishing must be controlled
InexpensiveCreep corrosion
Easy assembly processPrinted circuit boards made using the immersion silver can under creep corrosion if used in the following environments:
No NickelPaper mills
Does not affect hole sizeRubber manufacturing
Medium shelf likeFertilizer
Can be re-worked/re-applied at the PCB fabrication stageWastewater treatment
Mining/smelting
Cement or asphalt production
Petrochemical
Clay modeling studios
Areas with poor air quality

Finish TypeAdvantagesDisadvantages





7) Immersion Tin

Typical thickness 1 to 1.20 um
Can solder directly to the copperHandling and health concerns
Suitable for fine pitch devicesScratches easily
Good solderabilityDifficult to rework – growth intermettalic whiskers
Planar surfaceSolder mask creeping
Eliminates Nickel
Not too expensive
Good for backplanes and press fit connectors

2. Organic surface finish

Finish TypeAdvantagesDisadvantages






1) OSP – Organic Solderability Preservative

(the only finish that is nonmetal containing)

Typically, this finish is applied in a very thin layer – generally .15 um to .30 um in thickness. So, basically, it is just a lacquer type of spray that is applied over the PCB just to protect the copper surface finish.
The surface is very flat because of the pure copper so it is ideal for fine pitch devices because there’s no coplanar pads and no uneven surfaces for solderingDifficult to inspect and test
It can be re-worked easily during the PCB fabrication processReliability issues – there are some questions about the reliability of the exposed copper after assembly. Some think that there is not enough intermetallics to protect that copper from further corrosion in the field.
It does not affect hole sizesLimited thermal cycles – normally last 1 to 2 cycles and after that the OSP disappears and the protection no longer exists. But advances with OSP are making them for thermally resistant
Low costCannot use solvents on it – for example to clean misprinting of the solder paste will remove the OSP
Good solder mask integrityLimited shelf life and easy to scratch
Environmentally friendly

So, what type of surface finish should you use?

Choosing the final finish for a printed circuit board may be the most important decision you make because it is going to impact that long-term reliability.

The environments in which the board will be used is a critical factor. Will the boards be used in a harsh environment or where they have to work continuously and can’t fail or, will they be used for consumer goods?

Cost should not be a driving force in the selection of the surface finish. Here are some factors to consider:

  1. The environment where the board will used in
  2. Is cosmetics important? Do you need to have a shiny silvery finish?
  3. Is shock drop and issue? In a case of a smartphone where they are susceptible to be dropped a lot, you are concerned about brittle fracture of the components. So, in this case, you would not see usually see ENIG being used. With ENIG you have a tin-nickel bond, not a tin-copper bond, which is stronger. But you can use ENIG in medical devices.
  4. Corrosion environment – Silver tends to be prone to creep corrosion
  5. What are the reliability requirements – how high is the cost of failure?
  6. Importance of in-circuit test
  7. Presence of fine pitch devices
  8. Product quantity

Surface Finishes by Industry Sector

  • Automotive: OSP, Silver, Immersion Tin
  • High-end consumer products: OSP, ENIG, Silver
  • Basic consumer electronics – OSP, Immersion Tin
  • Mil/Aerospace: HASL, Immersion tin, ENIG, ENEPIG
  • Telecom: Silver, OSP, ENIG
  • Medical: ENIG, ENEPIG, Silver

Summary

Choosing the most suitable surface finish for your board is a complex decision with critical consequences. The selected surface finish will impact quality, reliability, and cost.

There are a number of different surface finishes available each with its own advantages and disadvantages. By understanding the environment where your product will be used in, awareness of reliability and test requirements, consideration of the type of components on the board, and the importance of aesthetics, your selection of finish type will be made easier.

Finish TypeAdvantagesDisadvantages
HASL (Hot-Air Solder Leveling)Excellent solderabilityA difference in thickness/topography between large and small pads
Low costNot suited for < 20mil pitch SMD & BGA
Allows large processing windowBridging on fine pitch
Long industry experience / well-known finishNot ideal for High Density Interface (HDI) products
Lead-free HASLVery good for solderingHuge coplanarity difference – not suitable for fine pitch devices
Easy to applyBridging problems on fine pitch devices
Very durable and can withstand multiple thermal cyclesInconsistent coating thickness
Bonds very well to the copper gives good thermal jointNot suitable for High-Density Interconnect (HDI) devicesHigh process temperature – 260 to 270 degrees centigrade
Long shelf life
Easy to inspect
Long shelf life
Low cost
ENIG – Electroless nickel immersion goldPlanar surfaceExpensive
Consistent thicknessBlack pad issues in BGAs
Multiple thermal cyclesWaste treatment of Nickel
Long shelf lifeCannot be reworked at PCB fabricators
Solders easilyComplex PCB fabrication process
Use for wire bonding PCBs
Good for fine pitch devices
ENEPIG – Electroless Nickel Electroless Palladium Immersion GoldCan be stored for a long timeNeed to maintain control over palladium thickness
Solderable and wire bondable (unlike ENIG)Does not wet as well as HASL
No black padNot widely available
Complex process
Thick palladium can also result in a decrease in solder joint strength
Hard (Electrolytic) GoldCan be stored for a long timeNot solderable when the thickness exceeds 0.43 um
Durable and hard surfaceExpensive
Lead-freeDoes not work well with other surface finishes
Limited rework capability at PCB fabricator
Immersion SilverGood for fine pitch devicesPlanar micro-voids
Planar surfaceTarnishing must be controlled
InexpensiveCreep corrosion
Easy assembly processPrinted circuit boards made using the immersion silver can under creep corrosion if used in the following environments:
No NickelPaper mills
Does not affect hole sizeRubber manufacturing
Medium shelf likeFertilizer
Can be re-worked/re-applied at the PCB fabrication stageWastewater treatment
Mining/smelting
Cement or asphalt production
Petrochemical
Clay modeling studios
Areas with poor air quality
Immersion TinCan solder directly to the copperHandling and health concerns
Suitable for fine pitch devicesScratches easily
Good solderabilityDifficult to rework – growth intermettalic whiskers
Planar surfaceSolder mask creeping
Eliminates Nickel
Not too expensive
Good for backplanes and press fit connectors
OSP – Organic Solderability Preservative

The surface is very flat because of the pure copper so it is ideal for fine pitch devices because there’s no coplanar pads and no uneven surfaces for solderingReliability issues – there are some questions about the reliability of the exposed copper after assembly. Some think that there is not enough intermetallics to protect that copper from further corrosion in the field.
It can be re-worked easily during the PCB fabrication processLimited thermal cycles – normally last 1 to 2 cycles and after that the OSP disappears and the protection no longer exists. But advances with OSP are making them for thermally resistant
It does not affect hole sizesDifficult to inspect and test
Low costCannot use solvents on it – for example to clean misprinting of the solder paste will remove the OSP
Good solder mask integrityLimited shelf life and easy to scratch
Environmentally friendly