PCB surface finish – how to choose the best surface finish

Types of finish

Lead-free HASL

HASL is the common finish on the market. Circuit boards are fully immersed in a molten tin/lead alloy pot. It blows hot air across the board while using air knives to remove excess solder. This procedure is advantageous because it prevents potential board damage by exposing the PCB to temperatures of 265°C.

The Lead-free HASL procedure is simple. A panel of boards placed on a frame is lowered vertically into the solder bath, held there for two to three seconds, and then lifted out using two air knives that have been adjusted to pass through the holes while leaving enough solder on all areas that must have a solderable finish. The panel goes through the dryer to remove any remaining flux after it cools.

Advantages

• It is affordable
• Easy availability in the market
• Reusable
• It is more long-lasting than other finishes

Disadvantages

• It can cause rough surfaces
• Not ideal for fine pitching

Immersion Tin

It is one of the most economical types of PCB surface finish coating. The flatness of the tin coating makes it good for small components. It is often used for fine-pitch products. Although it prevents oxidation of the copper underneath, one metal may diffuse into another, affecting the finish’s shelf life.

The application of immersion coating is carried out chemically. A thin layer of tin is used to coat copper to prevent damage. Immersion Tin has lot of uses in BGA & μBGA, Fine Pitch SMT and Flip Chip.

Advantages

• It works best for fine pitch.
• The lead-free finish is available at an affordable price.
• It is good for press fit.

Disadvantages

• Its shelf lifespan is too short.
• It cannot be used with peelable masks. Difficult to handle.
• It needs special tools to handle electrical tests.

Immersion Silver

This type is well in-demand as an environmentally friendly finish. It is a non-electrolytic chemical finish that costs lesser than ENIG.

The immersion tin finish is much more stable and resists tarnishing as it contains OSP. However, because of its sensitivity to a few compounds, it needs immediate packaging.

The copper printed circuit board is submerged in a tank of silver ions for the immersion silver finish. A tank full of silver ions engulfs the copper with a silver finish. It is perfect for circuit boards with EMI shielding. The immersion silver finish has various thickness ranges, from 5 to 18 microinches. It has very useful applications in BGA & μBGA, Fine pitch SMT and flip chip.

Advantages

• Environment friendly than other types of finishes.
• Less expensive.
• Moderate shelf life.
• Due to its flat-pack coating, it is ideal for fine pitch.

Disadvantages

• It can tarnish easily if not handled properly.
• It is not durable.
• It needs to be soldered at once.

Organic solderability preservative (OSP)

A water-based finish layered copper is called OSP. It is getting popular as the types of equipment used for maintenance is much more economical than its competitors. This is why OSP is the pocket-friendly PCB surface finish.

An anti-tarnish coating called OSP protects the copper surface from oxidation. It uses a conveyorized process to apply a thin protective coating over copper.
The water-based compound is attached to copper and forms a layer to protect it before electroplating.

Advantages

• It is environmentally friendly.
• Needs low maintenance.
• Lead-free finish.
• It can be repaired.

Disadvantages

• It is not durable.
• Delicate to handle.
• Not much strong.
• It has a relatively shorter shelf-life.

ENIG

Electroless Nickel Immersion Gold is known for its dual coating of metallic layers. Copper surface becomes more resistant to tarnish due to this double coating.The nickel finish is the first and the top layer to prevent the oxidation process in copper. This layer also has a protective gold layer finish, making it safer against damage.

A chemical process is used to apply ENIG, a double-layer finish. During the procedure, 120–240μ inches of electroless nickel are covered with 2–5μ inches of immersion gold (Ni).

Advantages

• Strong due to its coverage of two metallic layers.
• A lead-free finish to meet new industry trends.
• It can stay much longer on the shelf.

Disadvantages

• It is much more expensive and relatively HASL.
• It can not rework.
• A black pad

ENEPIG

This is a new form of PCB finish. It has been warmly welcomed in the field due to its three bonding layers. It increases the reliability of the finish and makes it the right choice for high-frequency applications.

ENEPIG has better solderability for PCB due to its three layers finish. In ENIG, nickel and gold layers prevent the copper from tarnishing, but these layers sometimes cause corrosion and damage the copper. It has a layer of palladium that prevents diffusion of nickel to the copper.

Advantages

• It is lead-free content.
•  Better shelf life.
• No risk of corrosion.

Disadvantages

• Highly expensive.

Electrolytic Nickel/Gold

Electrolytic Nickel has an added layer of gold. The layer of gold, along with the nickel, makes it best for high-density areas due to its high resistance to tarnishing.

Here a layer of gold covered the electroplated nickel. One can choose the thickness of the gold layer as required.

Advantages

• Highly resistant to tarnish
• Highly durable
• Shelf life is longer

Disadvantages

• Expensive
• Fewer solderability