What is PCB 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.
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
What are the types of surface finishes?
There are two groups of surface finishes:
- Metallic surface finishes which have some metal content such as gold, silver, tin or lead.
- Organic – which has no metal part or substances.
1. Metallic surface finishes
|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 solderability||A difference in thickness/topography between large and small pads|
|Low cost||Not suited for < 20mil pitch SMD & BGA|
|Allows large processing window||Bridging on fine pitch|
|Long industry experience / well-known finish||Not ideal for High Density Interface (HDI) products|
2) Lead-free HASL
Typical thickness is 1 to 40 um.
|Very good for soldering||Huge coplanarity difference – not suitable for fine pitch devices|
|Easy to apply||Bridging problems on fine pitch devices|
|Very durable and can withstand multiple thermal cycles||Inconsistent coating thickness|
|Bonds very well to the copper gives good thermal joint||Not suitable for High-Density Interconnect (HDI) devicesHigh process temperature – 260 to 270 degrees centigrade|
|Long shelf life|
|Easy to inspect|
|Long shelf life|
3) ENIG – Electroless nickel immersion gold
Typical thickness: 3 – 6 um Ni / .05 – 125 um Au
|Consistent thickness||Black pad issues in BGAs|
|Multiple thermal cycles||Waste treatment of Nickel|
|Long shelf life||Cannot be reworked at PCB fabricators|
|Solders easily||Complex PCB fabrication process|
|Use for wire bonding PCBs|
|Good for fine pitch devices|
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 time||Not solderable when the thickness exceeds 0.43 um|
|Durable and hard surface||Expensive|
|Lead-free||Does not work well with other surface finishes|
|Limited rework capability at PCB fabricator|
6) Immersion Silver
Typical thickness .05 to .40 um
|Good for fine pitch devices||Planar micro-voids|
|Planar surface||Tarnishing must be controlled|
|Easy assembly process||Printed circuit boards made using the immersion silver can under creep corrosion if used in the following environments:|
|No Nickel||Paper mills|
|Does not affect hole size||Rubber manufacturing|
|Medium shelf like||Fertilizer|
|Can be re-worked/re-applied at the PCB fabrication stage||Wastewater treatment|
|Cement or asphalt production|
|Clay modeling studios|
|Areas with poor air quality|
7) Immersion Tin
Typical thickness 1 to 1.20 um
|Can solder directly to the copper||Handling and health concerns|
|Suitable for fine pitch devices||Scratches easily|
|Good solderability||Difficult to rework – growth intermettalic whiskers|
|Planar surface||Solder mask creeping|
|Not too expensive|
|Good for backplanes and press fit connectors|
2. Organic surface finish
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 soldering||Difficult to inspect and test|
|It can be re-worked easily during the PCB fabrication process||Reliability 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 sizes||Limited 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 cost||Cannot use solvents on it – for example to clean misprinting of the solder paste will remove the OSP|
|Good solder mask integrity||Limited shelf life and easy to scratch|
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:
- The environment where the board will used in
- Is cosmetics important? Do you need to have a shiny silvery finish?
- 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.
- Corrosion environment – Silver tends to be prone to creep corrosion
- What are the reliability requirements – how high is the cost of failure?
- Importance of in-circuit test
- Presence of fine pitch devices
- 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
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.