July 31, 2018

The finish of your printed circuit board will be one of the last steps you complete. It’s meant to prevent oxidation of any exposed copper. The soldermask is there to cover the majority of the circuitry. If the copper is oxidized, it cannot be soldered. When a PCB oxidizes, it eventually becomes unusable.

That final step of the surface finish is one of the most critical steps in the PCB process. Beforehand, the Hot Air Solder Leveling (HASL) was the method relied on for the finish. However, as the circuit complexity increased, new finishing methods were needed.

Below is a list of some of the most common finishes used today.

HASL/ Lead-Free HASL

What It Is

A leaded solder is the most common type of finish. It’s a combination of tin and lead (63/37) that is applied to the board. It provides a shiny silver finish to the board. The lead-free HASL is similar to the leaded solder, but it’s a mixture of tin and copper (99.3%/0.6%).

What It’s Used For

Typically, you would use a leaded solder as a default process and for boards with thru-hole technology. On the other hand, you would use a lead-free HASL Solder as a replacement to the leaded solder.

Pros

The lead-free HASL provides a uniform and flat mount pad surface. For the leaded solder, it is the most affordable option compared to other finishes. It also has an excellent shelf life and can be re-worked.

Cons

The leaded solder does not provide a flat, smooth finish. It is not reliable for fine pitch and creates solder bridging along the board.

Immersion Tin

What It Is

An immersion tin solder is a metallic, white finish that occurs due to a chemical displacement reaction, which is applied directly to the basis metal of the PCB, or the copper. It is there to protect the copper from oxidation. However, copper and tin are highly attracted to each other, and they inevitably will diffuse.

What It’s Used For

Immersion tin is an alternative to lead-free solders. It provides a flat surface and typically has a shelf life up to a year.

Pros

This finish can be re-worked and is a popular choice of solder. It is affordable and has a flat surface finish.

Cons

The downfall to this finish is it can cause handling damage and involves a carcinogen (Thiourea). Exposed tin can corrode, and it is difficult to measure the thickness.

OSP/Entek

What It Is

The OSP (Organic Solderability Preservative), or anti-tarnish, provides a thin protective layer of material onto the exposed copper to prevent oxidation. It is an organic, water-based chemical that bonds to copper.

What It’s Used For

This finished is used when you need a very flat surface from a lead-free alternative.

Pros

Very environmentally friendly compared to other lead-free finishes. Provides a flat surface and is a relatively simple process to go through. It is cost effective and can be re-worked later on.

Cons

An OSP finish, there is no way to measure the thickness and is not suitable for plated through holes (PTH). There is also a short shelf life and might leave exposed copper during final assembly.

Electroless Nickel Immersion Gold (ENIG)

What It Is

An ENIG finish is a two-layer metallic coating of nickel. It is the barrier to the copper and is what the components of the PCB are actually soldered to. The gold is there to protect the nickel during storage.

The Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG PCB) has been making a comeback for PCB finishes. The price of palladium is more affordable and has been noted as the best option for wire bonding.

What It’s Used For

An ENIG finish is most likely the most common finish in the PCB industry due to increasing regulations. It is good for surface mount devices (SMDs) and ball grid arrays (BGAs).

Pros

Gold is an excellent electrical conductor. This finish provides a flat surface with no Pb. It is also good for PTH and provides a long shelf life.

Cons

An ENIG finish is quite expensive and cannot be re-worked compared to other finishes. It is a complicated process and is vulnerable to damage and signal loss.

Hard Gold

What It Is

Hard electrolytic gold is a gold plated layer over a barrier coat of nickel. It can also be called a deep gold finish. Typically, a hard gold is not used on solderable areas due to having poor solderability.

What It’s Used For

Because of its durability, a hard golf finish is used when the PCB is for high-wear areas like connector fingers and keypads.

Pros

Extremely durable and hard, and a long shelf life.

Cons

This finish is very expensive and has an extras labor-intensive process. It also requires additional materials. It has a poor solderability, which makes it difficult to use with other surface finishes.

Soft Gold

What It Is

A soft gold finish, also called a wire bondable gold, is softer compared to other golf finishes. This allows it to be easier to bond to create a strong and more conductive connection.

What It’s Used For

Typically, a soft gold finish is used when you need to do gold wire bonding.

Pros

It produces a stronger welded joint (compared to a hard gold finish) because it does not disappear at the point of soldering or wire bond. Easily bonded to other finishes.

Cons

Does not have the durability and hardness of other gold finishes. Also, quite an expensive finish.

Immersion Silver

What It Is

An immersion silver finish is a silver plating to protect the copper from corroding and maintaining its solderability.

What It’s Used For

An immersion silver finish would be used for high-speed signal PCBs.

Pros

Creates strong solder joints and can be re-worked. It is environmentally friendly and a lead-free finish. Silver is highly electrically conductive and forms strong interconnect and very flat surfaces.

Cons

It has a shorter shelf life compared to other finishes, and it needs to be shipped with separator sheets to prevent tarnishing.

Electroless Palladium Immersion Gold (EPIG)

OSP (Organic Surface Protectant)

What It Is

An OSP finish is an organic chemical finish that you would apply to the copper.

What It’s Used For

This finish is an organic alternative for lead-free finishes.

Pros

It is an easy process that is cost effective and can be re-worked. It is environmentally friendly as it is a water-based compound.

Cons

This finish has a very short shelf life and needs to be used immediately after you apply the coating. The color can change from brown to a dark brown and can end up with oxidation spots.

Conclusion

There are many factors to consider before deciding which finish is best suited for your PCB. You will want to consider all of the circuit board materials used, what the board is used for, and what kind of price range you are looking at. Each solder will affect the performance and end result of your board, so consult with your PCB manufacturer to help.