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When dealing with a printed circuit board (PCB) you must protect the copper finish, which will oxidize and depreciate if left untreated. You also need a surface to which components can be directly soldered. Superficially, this concept seems simple: add a surface treatment. Viola! You’re all set. Unfortunately, it is never that easy.

There are assorted options for PCB surface finishes and each one has advantages and disadvantages. Further, regulations such as RoHS and WEEE have changed industry standards.

This is a guide to the most popular types of PCB surface treatments including their pros and cons and is a great place to get started with your decision making.

HASL and Lead-Free HASL

“Hot Air Solder Leveling” is the least expensive type of PCB surface finish. It is widely available and very economical.

The board is dipped in molten solder and then leveled off with a hot air knife. If you are using through-hole or larger SMT components, HASL can work well; if your board will have fine pitch SMT components or smaller than 0402 components, it is not ideal. The surface is not completely level, so this can cause an issue with small components.

Lead-free HASL is similar to standard HASL, but with an obvious difference, it does not use tin-lead solder. Instead, tin-copper, tin-nickel or tin-copper-nickel germanium may be used. This makes lead-free HASL an economical and RoHS compliant choice. However, like standard HASL, it is not ideal for smaller and fine-pitch components.

Electroless Nickel Immersion Gold (ENIG)

Thanks to a double layer metallic coating of nickel and gold, where nickel both protects the copper and provides a surface to which components are soldered, and gold protects the nickel, ENIG is becoming the most popular surface finish in the industry. This treatment has also seen an increase in popularity due to the RoHS regulations and provides a level exterior for complex surface components such as BGAs and flip chips.

Of course, just having gold involved means it is not always cost-effective.

OSP or Entek

A water-based, organic surface finish, organic solderability preservative (OSP) is an environmentally friendly option that requires low equipment maintenance. The finish places selective bonds to the exposed copper, usually using a conveyorized process, and provides an organometallic layer that protects the copper before soldering.

The downside is OSP is not as robust and it can be sensitive to handling, so multiple soldering can destroy the film.

Immersion Tin (ISn)

With immersion coatings, a chemical process is used. A flat layer of metal gets deposited on the copper traces. The flatness of coating makes it ideal for small components.

Although tin is an economical choice, it comes with some drawbacks. The main drawback is that after it is deposited onto copper it begins to tarnish. That means that if you want to avoid lower quality solder joints, you need to do your soldering within 30 days.

If you are expecting a high volume of production this may not be an issue. And if you are using large batches of boards quickly, you can avoid tarnishing.

Immersion Silver

Immersion silver does not react with copper the way that tin does. However, it does tarnish when exposed to air. That means it needs to be stored in anti-tarnish packaging. When stored in proper packaging it will still be solderable for 6-12 months.

However, once the PCB is removed from its packaging, it will need to go through solder reflow within a day. A higher shelf life can be achieved with gold plating (ENIG).

Hard Gold

Among the most expensive PCB surface finish types, hard gold applications are extremely durable and have a long shelf life. It is not often used for soldering points, due to poor solderability.

Hard gold is typically used for edge connectors, battery contacts, and some test pads.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG technology developed based on ENIG technology by adding a palladium layer to improve performance. The palladium layer totally covers the nickel layer to avoid degeneration of nickel, commonly referred to as black pad. Palladium has a higher hardness level than gold, which results in the improvement of solder reliability, wire bonding capacity and antifriction.

There is no surface finish that is suitable for all applications and environments in the industry. Here are some of the considerations for surface finishes:

• Lead- or lead-free assembly
• Cost
• Shelf life
• Fine pitch components
• End environment
• Volume and throughput
• RF applications

One advantage to working with a full-service electronics manufacturing services partner is that we can help you determine the best finish for your printed circuit board.  We can help you make the best decisions around even the smallest details such as PCB surface treatments. Contact ACDi today for your printed circuit board project.