PCB surface finishing: Their Types, Advantages and Disadvantages

A surface finish on a circuit board is a fine coating on a bare PCB before mounting the electronics over it. PCB surface finishing ensures the reliability of a PCB by offering two good benefits.

  • It provides a better solderable surface while assembling the electronics on the PCB.
  • It prevents the exposed copper on the PCB from oxidation and acts as a shield for the entire circuit board.

Choosing the correct surface finish method for your PCB board is not an easy task. Not only for PCBs, but a surface finish must also be safe for the environment. That’s why PCBWay uses the materials listed under RoHS compliance on all its circuit boards. To know the best surface finish method for your PCB needs, stick with this article till the end.

Hot Air Solder Leveling (HASL) / HASL lead-free

PCB surface finishing: A PCB board with HASL surface finish
A PCB board with HASL surface finish
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It is a simple and low-cost method to protect PCB boards. In this process, the PCB gets immersed in a molten tin-lead / lead-free solder alloy. An air knife then removes the excess solder from the board surface by blowing a stream of hot air. The HASL process imparts a layer between 70 micro-inches (0.07 mils) to 200 micro-inches (0.2 mils) over the surface of PCB.

HASL process can either be done horizontally or vertically on a PCB board. But, for a uniform coating, PCBWay prefers the horizontal process.

Advantages

  • The cost involved in the process is low.
  • It provides an excellent shelf life to the PCB.

Disadvantages

  • This process sometimes leaves an uneven surface over the board.
  • The HASL process is not satisfactory for fine-pitch components and Plated Through-hole.
  • It leads to thermal shock.

Organic Solderability Preservative (OSP)

PCB surface finishing: A PCB board with OSP surface finish
A PCB board with OSP surface finish

It is a chemical process to apply a thin anti-tarnish protective layer of organic material over the surface of PCB. The thin film protects the PCB from oxidation and vulcanization. This method is simple, cheap, and most environment-friendly as compared to other PCB surface finishing processes. The OSP bonds to copper and protects the copper pad before soldering. After coating multiple layers of OSP, the OSP layer thickness varies from 4 micro-inches (0.004 mils) to 24 micro-inches (0.024 mils) over the PCB.

Advantages

  • This method ensures a flat surface over the PCB.
  • The entire process is lead-free, which is safe for the environment.

Disadvantages

  • It provides a shorter shelf life to the PCB.
  • This method is not satisfactory for Plated Through-hole.

Electroless Nickel Immersion Gold (ENIG)

PCB surface finishing: A PCB board with ENIG surface finish
A PCB board with ENIG surface finish

After the introduction of RoHS regulation, ENIG has become a popular choice for PCB surface finishing. The process starts with plating a nickel layer over the bare circuit board. A thin gold layer over the nickel protests it during storage. The two-layer metallic coating ensures better electrical performance in the long run of PCB. The thickness of both layers is different. While the thickness of nickel varies between 100 micro-inches (0.1 mils) to 200 micro-inches (0.2 mils), the thickness of the gold layer is only 2-4 micro-inches (0.002-0.004 mils).

Advantages

  • The ENIG provides a better shelf life to the PCB.
  • It ensures a flat surface and is optimum for Plated Through-hole.

Disadvantages

  • It is expensive as compared to other PCB surface finishing methods.
  • The Phosphorous buildup between Gold and Nickel sometimes lead to black pads and fractured surface.

Immersion Silver (IAg)

A PCB board with a surface finish of Immersion Silver
A PCB board with a surface finish of Immersion Silver

IAg is a better alternative to ENIG in fine pitch and flat surface requirements. It gives a moderate shelf life (around one year) to the PCB. The silver layer is sensitive and tarnishes when exposed to the environment. So, an additional coating of gold or organic matter like OSP over Silver improves the PCB’s shelf life. The PCB surface finishing using silver gives excellent electrical properties and maintains good weldability even at high temperatures.

Advantages

  • This method is economical and environment-friendly.
  • It ensures a flat surface and provides better stability to the PCB.

Disadvantages

  • It requires extra care in packaging and storing.
  • It is not compatible with masks that require peeling off.

Immersion Tin (ISn)

A PCB board with a surface finish of Immersion Tin
A PCB board with a surface finish of Immersion Tin

The tin protects the PCB from oxidation. But due to the strong affinity between Copper and Tin, the two metals diffuse into each other. This diffusion affects the shelf life of the PCB. The addition of organic matter eliminates the above problem and offers a flat surface on the PCB.

Advantages

  • The method provides a smooth and flat lead-free surface.
  • It provides higher reliability to the PCB at a low cost.

Disadvantages

  • This process is not perfect for Plated Through-hole.
  • It is not suitable for multiple reflow or assembly processes.

So, depending upon your application, you can easily select any of the above PCB surface finishing methods. PCBWay provides all the solutions for any of your project requirements at an economical price. For more details, visit PCBWay.com.

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