About Surface finish & Conformal Coating in pcb boards
Surface finish is an important process to protect the board from copper oxidation. It provides protection while in the stage of rework, testing etc..,
Let’s explore the types of surface finishes:
1)Hot air solder leveling(HASL)
-Leaded solder and Leadless solder.
2)Immersion Gold Plating
-Electroless Nickel Immersion Gold(ENIG).
-Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
-Immersion Tin
-Immersion Silver
3)Organic Surface Protectant
4)Hard Gold
Image by PCB unlimited
- Leaded solder (HASL)
Leaded solder is the most commonly used low-cost method where the boards is dipped in a mixture of tin and lead. Excess mixture is removed by air blow. However, the use of lead poses a threat to the environment. It is used for the boards without ROHS (Restriction of Hazardous Substance) compliance.
2. Lead-free solder
In this methos, the board is immersed in a mixture of molten tin and copper. The excess mixture is removed by air blow, results in a shiny silver-colored finish.
3.Electroless Nickel Immersion Gold(ENIG)
This is the best lead free finish. First, a layer of Nickel (about 150-200 microinches) is applied to the cu surface, followed by a thin layer of gold over the Nickel. This coating process is laborious and expensive. It is preferable for BGAs (Ball Grid Array) and PTH (Plated Through Hole)components. However, ENIG can cause black pad issue due to Nickel corrosion. To address this issue, ENEPIG was developed.
4. Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
This surface finish consists of 3 layers: Nickel, Palladium and immersion gold. The thickness will be in micro inches. It is commonly used for edge connectors.
5. Immersion Silver
In this process, a one-micron of thick layer of silver is coated over cu surface through electroless plating process. The life time is around 12 months.
6. Immersion tin
A thin layer of tin is applied to the surface, resulting in a whitish finish. It is cost-effective but shorter lifetime of 6 to 12 months.
7. Organic surface protectant :
It uses organic compounds such as carbon, to form a protective layer.
- OSP ( organic solderability preser ative): This is a water based organic compound is through a conveyor process.
- Carbon ink surface finish: The carbon ink is applied as a protective layer, commly used in RF shielding, keypads, keyboards, smartphones,remote controllers and automotive vhicles.
7) Hard Gold finish:
This one is a best option when PCB is to be inserted into another board. It is used to create gold edge fingers, it uses gold which is a high corrosion resistance.
Which type of surface finish we will choose?
The choice of surface finish depends on the factors such as environment in which circuit board will be used, operational hours, temperature and resistance to shock and drop.
CONFORMAL COATING
Conformal Coating is a thin polymeric flim applied to the PCB boards after assembly to protect from corrosion and environment. It involves the thin layer of chemicals like Acrylics, Silicones, Epoxes,Parylene and Urethane.
Image by everything RF
This can be done through methods such as brushing, aerosol sparying, atomized spraying, dispensing and dipping. Conformal coating helps resist dust and moisture and also improves electrical properties.
Thank you,
Sathya Priya N.
