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:
- shelf life
- solder joint strength
- void formation
- process yield
- rework and ease of use
- ability to test
- scrap rate
- field failure rate
Considerations When Selecting Your PCB Finish
Choosing the appropriate PCB finish depends on the following factors:
- 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 type||When to Use||Advantages||Disadvantages|
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.
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.
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.
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.
Immersion Silver Finish (ImAg)
Is a PCB finish where a layer of silver is plated over the copper
Immersion Tin Finish (ImSn)
Is a PCB finish where a layer of tin is plated over the copper
Organic Solderability Preservative (OSP) Finish
This is a thin, uniform, protective layer on the copper