Just like the clothes we wear to shield us, a surface finish on a circuit board protects it from any damage due to operating or storage environments. Selecting the right PCB surface finish plays an important role in enhancing the shelf-life of the PCB. Let us have a detailed look at the different board surface finishes available.
Why do you need a surface finish on a circuit board?
A PCB surface finish is a coating between the bare copper at the solderable area and the components on the circuit board. It protects the copper surface from oxidation. During assembly, the surface coating prepares the board surface for soldering components and also enhances the PCB’s shelf life.
A PCB assembly’s shelf life depends on the following:
1. Components used in the assembly.
2. Bare boards.
3. Various materials used in the board assembly process such as solder, adhesives, and more.
Types of surface coating
Surface coating for printed circuit boards can be classified broadly into metallic and organic.
Metallic surface finishes
Metallic surface finishes use metallic layers. Some examples of these finishes are discussed below.
Hot air solder leveling (HASL)
HASL is a traditional surface finish. In this method, the PCB is dipped into a molten solder bath to encase all the exposed copper surfaces with solder. The extra solder is removed by passing the circuit board between hot-air knives. The solder used here is a mix of tin and lead. HASL can make a device drop-resistant.
Lead-free HASL
Lead-free HASL is identical to the HASL method. The only difference is that the solder is made up of a lead-free alloy such as Sn/Cu/Ni/Ge, Sn/Ag/Cu (SAC), and Sn/Cu/Co.
Electroless nickel immersion gold plating (ENIG)
ENIG surface finish refers to electroless nickel plating covered with a thin coating of immersion gold.
Electroless nickel deposition is an auto-catalytic process. Here nickel is deposited on the copper surface which is catalyzed using palladium. To obtain a consistent coating, the reducing agent that contains nickel ions needs to be restored. This also provides a proper concentration and temperature. Subsequently, gold will adhere to the areas that have nickel plating. ENIG can be used to protect medical devices from shock and drop issues.
Electroless nickel electroless palladium immersion gold (ENEPIG)
ENEPIG is similar to ENIG. The difference is a coating of palladium is implemented as a protective layer to prevent nickel oxidation and to stop diffusion to the copper layer.
ENIG and ENEPIG provide great solderability at a higher cost compared to other surface finishes.
Hard gold
Hard gold includes a layer of gold that is plated over a nickel coating. The hard gold finish consists of a gold alloy including nickel, cobalt, or iron. It is best suited for components with a higher probability of wear and tear such as edge connectors, interconnected carrier boards, contacts, and keypads.
Immersion silver (ImAg)
Immersion silver surface finish provides a lead-free layer of silver and protects the copper traces from corrosion. An electroless immersion reaction is used to apply ImAg to the copper traces.
Immersion tin (ImSn)
Immersion tin surface finish consists of a thin layer of tin. It is RoHS-compliant and is a great choice when it comes to small geometries and components. It is also well-suited for flat surface requirements and fine-pitch components.
These are all the commonly used metallic surface finishes. Now let us take a look at the organic ones.
Organic
An organic finish includes organic compounds (compounds with carbon). Some of the popular ones are listed below.
Organic solderability preservative (OSP)
OSP is a water-based organic finish used for copper pads. It protects the copper pad prior to soldering.
Carbon ink
This consists of a layer of carbon ink. This method is used in welding equipment, remote controllers, automotive, keyboards, keypads, and RF shielding. The primary factors involved in board production with the carbon ink surface finish are printing, baking, and resistance control.
Choosing the right surface finish for a board
Selecting the right surface finish is essential since it decides the reliability of the board. The operating environment also affects the PCB’s reliability, especially if it is exposed to extreme temperatures.
Some important factors to keep in mind while choosing a surface finish are as follows:
- Operating environment
- Board aesthetics
- Shock and drop issues
- Corrosion
- Reliability requirements
- Presence of fine pitch devices/components
- The flatness of SMT pads for BGA applications
The types of surface finishes available at Sierra Circuits are listed below:
Having a suitable surface finish can prolong the shelf-life of the PCB, and enhance solderability. The selection of the surface finish thus becomes an important aspect during the design of the circuit board.
