A Short Guide on Different Types of PCB Surface Finishes

Feb 19, 2021

Surface finishing is one of the crucial steps in the printed circuit board (PCB) assembly. Unshielded circuit boards are susceptible to oxidation, severely affecting their structural integrity. Being a vital interface between the circuit boards and its components, a surface finish provides a solderable surface for board components and prevents copper from oxidizing. As the demand for thin, lightweight, fast speeds and high-density applications increases, the solder finishes will make a significant difference. There are different types of PCB surface finishes available, from options that are easy-to-apply and affordable to difficult-to-manage or costly finishes used in most advanced applications. Would you like to know what they are?

5 Types of PCB Surface Finishes Analyzed

Here are some prominent types of PCB surface finishes:

Hot Air Solder Level (HASL): HASL is one of the least expensive types of surface finishing performed on PCBs. This surface finishing option utilizes lead/tin for creating a protective covering on the circuit board. Basically, HASL boards are dipped in molten solder and then taken out. The excess lead or tin is removed from the board surface using hot air bursts. During HASL surface finishing, the parameters like welding temperature, air knife pressure, air knife air temperature, dip soldering time need to be measured. This surface finishing offers several advantages, such as solderability, long shelf life, excellent pad wetting and copper coverage, and many more.
Electroless Nickel Immersion Gold (ENIG): It is a widely used PCB surface finishing treatment with two metallic layers. In this method, nickel is put onto the copper layer that is coated in gold atoms through a displacement reaction. Nickel acts as a barrier to copper, while gold prevents nickel from corrosion. Besides offering protection to nickel against corrosion, gold has a low and stable contact resistance. This PCB surface finish method is gaining immense popularity owing to its high yield-rate and compliance with RoHS standards. ENIG plating thickness for electroless nickel should be from 2.5 to 5.0 µ and from 0.05 to 0.23 µm for the immersion gold layer.
Immersion Silver (IAg): Like HASL and ENIG, immersion silver is also RoHS compliant. It is an affordable surface finishing method than ENIG. This method is directly applied to the base metal of the circuit board through chemical displacement. Immersion silver provides a reliable solder joint, and thus makes the board ideal for prototyping. This is an eco-friendly method and can withstand several reflows. There are many advantages of this surface finishing method such as high solderability, excellent surface planarity, cost effectiveness, and suitability for aluminum wire bonding.
Immersion Tin (ISn): Immersion tin is RoHS compliant, and more affordable option than immersion silver and ENIG. Similar to immersion silver, this surface finish is directly applied to the base metal of the circuit board through chemical displacement. This surface finish forms a flat copper-tin intermetallic compound, which means it has good solderability without intermetallic diffusion problems and flatness problems. It is a popular choice for press-fit applications.
Organic Solderability Preservative (OSP): Popularly known as an anti-tarnish surface finish, this method prevents the copper surface from oxidation. This is done by applying a thin, protective layer of material using a conveyorized method. This surface finish uses a water-based compound that produces an organometallic layer for protecting the copper beneath. OSP is an environment-friendly and cost-effective surface finishing process.

It is important to choose the right surface finish for your PCB assembly by considering factors – component types, production volume, environmental impact, overall cost, and more. Not sure of what you will need? In such scenarios, you must employ a reliable printed circuit board fabricator before making a selection. This will ensure a perfect combination of material and surface finish for performance-driven and cost-effective design. Being a trusted PCB assembly company, Accelerated Assemblies helps their client in the selection of surface finishing and is an end-to-end PCB solutions provider.