What is PCB Surface Finish? Complete Guide to 7 PCB Surface Finishes.

What is PCB Surface Treatment?

PCB surface treatment refers to the metal interconnection between the bare copper of the printed circuit board and the components’ solderable area. A circuit board has a base copper surface that, without a protective coating, is prone to oxidation, hence the need for surface cleanliness.

PCB surface treatment is a crucial step in the PCB manufacturing and assembly process, serving two main functions: protecting the exposed copper circuits and providing a solderable surface for components to attach to the PCB. Surface treatment is located on the outermost layer of the PCB, above the copper layers, acting as a “coating” for the copper.

Types of PCB Surface Treatment

Hot Air Solder Leveling (HASL)
Tin Plating (Immersion Tin)
Electroless Nickel Immersion Gold (ENIG)
Organic Solderability Preservatives (OSP)
Immersion Silver (ImAg)
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
Hard Gold (Electrolytic Hard Gold)

PCB Surface Treatment Processes

Hot Air Solder Leveling (HASL)

HASL is one of the most commonly used surface treatment methods in the industry. It comes in two types: one with lead-tin and one without. HASL is also one of the cheapest available PCB surface treatment types.

To achieve a smooth HASL surface, the circuit board is immersed in molten solder (tin/lead), covering all exposed copper surfaces on the board. After leaving the molten solder, high-pressure hot air is blown over the surface through an air knife, which smoothens the solder deposits and removes excess solder from the circuit board surface.

Important parameters to control during this process include soldering temperature, air knife temperature, air knife pressure, immersion time, lift-off speed, and more.

Advantages of HASL:

• Abundant supply
• Reworkable
• Excellent shelf life
• Outstanding solderability
• Inexpensive/low cost
• Allows for a larger processing window
• Longer storage time
• After PCB completion, the pads are fully covered with tin before soldering
• Suitable for lead-free soldering
• Mature surface treatment choice
• Allows for visual inspection and electrical measurements

Disadvantages of HASL:

• Uneven surface
• Not suitable for fine pitch
• Contains lead (HASL)
• Thermal shock
• Solder bridging
• Clogged or reduced PTH (plated through-hole)
• Thickness/appearance differences between large and small pads
• Not suitable for SMD and BGA smaller than 2000 millionths of an inch
• Not suitable for HDI products
• Not suitable for wire bonding

Tin Plating (Immersion Tin)

Immersion tin (ImSn) is a metal finish deposited by a chemical displacement reaction directly onto the base metal of a circuit board, which is copper.ISn protects the underlying copper from oxidation within its expected shelf life. Since all solders are tin-based, the tin layer can match any type of solder.

By adding organic additives to the tin immersion solution, the tin layer structure becomes granular, overcoming the problems of tin whiskers and tin migration, while also possessing good thermal stability and solderability.The tin immersion process can form a flat copper-tin intermetallic compound, giving it good solderability, without flatness and intermetallic compound diffusion problems.

Advantages of Tin Plating:

• Excellent flatness (suitable for SMT), suitable for fine pitch/BGA/small components
• Medium-cost lead-free surface treatment technology
• Suitable pressing cleanliness
• Maintains good solderability after multiple thermal excursions
• Suitable for horizontal production lines
• Suitable for fine geometry machining, lead-free assembly

Disadvantages of Tin Plating:

• Sensitive to handling
• Short shelf life, tin whiskers after 6 months
• Corrosive to solder mask layers
• Not recommended for use with peelable masks
• Not a suitable choice for touch switches
• Electrical testing requires special setup (soft probe landing)

Electroless Nickel Immersion Gold (ENIG)

ENIG (Electroless Nickel Immersion Gold) surface treatment has always been the best option for fine pitch (flat) surfaces and lead-free options.
ENIG is a two-step process, covering a thin layer of nickel over the copper layer and then covering it with a thin layer of gold. Nickel acts as a barrier for copper and is the surface to which components are actually soldered, while gold protects nickel during storage.

Electroless Nickel Immersion Gold (ENIG)

The inner layer thickness of Ni is generally 3-6μm, and the outer layer thickness of Au is generally 0.05-0.1μm.

Ni forms a barrier layer between the solder and copper.

The role of Au is to prevent oxidation of Ni during storage, thus extending the shelf life, but the gold plating process also produces excellent surface flatness.

The processing flow of ENIG is as follows: Cleaning -> Etching -> Catalyst -> Electroless Nickel -> Gold Plating -> Residue Cleaning

While this coating process has a long shelf life and is advantageous for electroplating through-holes, it is a complex and expensive process that is not reworkable and is known to cause loss in signal RF circuits.

Advantages of ENIG:

Flat surface
Lead-free
Suitable for PTH (plated through-hole)
Long shelf life

Disadvantages of ENIG:

Expensive
Not reworkable
Black pad/black nickel
Damage from aliens
Signal loss (RF)
Complex process

Organic Solderability Preservatives (OSP)

OSP (Organic Solderability Preservatives) or anti-corrosion agents are typically applied to exposed copper using a conveyor belt process, protecting the copper surface from oxidation with a very thin layer of material.

This film must have properties such as anti-oxidation, thermal shock resistance, and moisture resistance to protect the copper surface from rusting (oxidation or sulfidation, etc.) under normal conditions.

However, during subsequent high-temperature soldering, this protective film must be easily and quickly removed by the flux to allow the clean copper surface to immediately bond with the molten solder, forming a strong joint in a very short time.

In other words, OSP acts as a barrier between copper and air.

The general process of OSP is: degreasing -> micro-etching -> acid washing -> pure water rinsing -> organic coating -> cleaning.

Advantages of OSP:

• Flat surface
• Simple process, very smooth surface, suitable for lead-free soldering and SMT
• Reworkable, suitable for horizontal production lines
• Cost-effective
• Environmentally friendly

Disadvantages of OSP:

• Thickness cannot be measured
• Not suitable for PTH (plated through-hole)
• Short shelf life
• May cause ICT issues
• Exposed copper at final assembly
• Sensitive processing
• Cannot be soldered (reworked) more than twice
• Not suitable for press-fit technology and wire binding
• Inconvenient for visual and electrical tests

Immersion Silver (ImAg)

Immersion silver is applied to copper PCBs by immersing them in a silver ion tank using a non-electrolytic chemical surface treatment. It is an ideal choice for circuit boards with EMI shielding and is also used in communication equipment.

This method is especially suitable for circuits with microwave frequencies and transmission lines.

Silver immersion provides a flat, solderable surface, and excellent electrical properties. The silver immersion process can be used on a large variety of circuit boards, including rigid and flexible circuit boards.

The general process of immersion silver is: degreasing -> micro-etching -> silver deposition -> rinsing.

Advantages of Immersion Silver:

• Suitable for fine pitch
• Suitable for horizontal production lines
• Good for lead-free assembly, good solderability, and suitable for SMT
• Flatness
• Long shelf life, good for double-sided boards, wire binding, suitable for ICT
• Good at retaining signal quality in high-frequency circuits

Disadvantages of Immersion Silver:

• Short shelf life, difficult to store (black spots, requires vacuum packaging)
• Chemical disposal
• Oxidation during storage
• Sensitive to handling
• Ag3Sn formation
• Expensive
• Not suitable for touch switches and peelable masks
• Not suitable for gold wire bonding
• ICT testing may cause a malfunction
• Process control difficulties

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is a complex process involving three layers: nickel, palladium, and gold, providing excellent conductivity and protection.

The processing flow of ENEPIG is: Cleaning -> Etching -> Catalyst -> Electroless Nickel -> Palladium -> Gold Plating -> Residue Cleaning

ENEPIG is the most suitable surface treatment for high-reliability and high-speed products, where its flatness, high copper resistance, and good anti-corrosion properties are especially important.

Advantages of ENEPIG:

Excellent for wire bonding
Suitable for fine pitch and small parts
Suitable for gold wire bonding
Good for PTH
Good for press-fit technology and wire bonding
Suitable for ICT and electrical tests
Long shelf life
Lead-free

Disadvantages of ENEPIG:

High cost
Not suitable for touch switches
ICT testing may cause a malfunction
Requires stable production environment
Not suitable for wave soldering

Hard Gold (Electrolytic Hard Gold)

Hard gold is a type of PCB surface treatment that uses a layer of gold alloyed with a small amount of other metals to increase its durability. This process involves electroplating a layer of gold onto the surface of the PCB, which is then coated with a layer of electrolytic nickel to increase its hardness and wear resistance.

Hard gold is commonly used in applications where the PCB will be subject to repeated insertions and removals, such as in connectors and switches. The hard gold coating helps prevent the gold from wearing away over time, ensuring reliable electrical connections.

Advantages of Hard Gold:

Excellent durability and wear resistance
Suitable for applications requiring repeated insertions and removals
Good for high-reliability applications
Suitable for ICT and electrical tests
Suitable for press-fit technology and wire bonding
Suitable for fine pitch and small parts

Disadvantages of Hard Gold:

High cost
Not suitable for wave soldering
Not suitable for touch switches
Limited thickness range

How to Choose PCB Surface Finish?

The selection of surface finish for PCBs is one of the most critical steps in PCB manufacturing, as it directly impacts process yield, rework quantity, on-site failure rate, testability, scrap rate, and cost. To ensure high quality and performance of the final product, all important considerations about assembly must be taken into account in the selection of surface smoothness.

PCB Surface Finish

Solder Pad Flatness As mentioned earlier, some surface finishes can lead to uneven surfaces, which can affect performance, solderability, and other factors. If flatness is a critical factor, consider surface finishes with thin and uniform layers. In this case, suitable options include ENIG, ENEPIG, and OSP.

Solderability and Wetting

Solderability is always a key factor when using PCBs. Specific surface finishes like OSP and ENEPIG have been shown to hinder solderability, while others like HASL are more suitable.

Gold or Aluminum Wire Bonding

If your PCB requires gold or aluminum wire bonding, you may be limited to ENIG and ENEPIG.

Storage Conditions

As mentioned earlier, some surface finishes (such as OSP) can make the PCB brittle during processing, while others enhance durability. Storage and handling requirements should be considered in advance, and surface finishes that make the PCB brittle should only be used when storage and handling requirements can be met without risk.

Soldering Cycles

How many times will the PCB be soldered and reworked? Many surface finishes are ideal for rework. However, other methods like tin immersion are not suitable for rework.

PCB Surface Finish-RoHS Compliance

RoHS compliance is crucial in determining the surface finish to be used. Typically, all surface finishes that use lead are not suitable for RoHS compliance and should be avoided.

Depending on the specific requirements and characteristics of your PCB product, you can use this table to select the perfect surface finish option.

Conclusion

The selection of surface finish types for surface smoothness must be optimal to fulfill various functions. Each type of surface treatment has its own advantages and disadvantages. There are some engineering techniques to address issues caused by the shortcomings of surface smoothness. For example, for the low wetting force of OSP, there are solutions such as changing the plating of the board or wave soldering alloy, increasing top-side preheating, and so on. The key is to consider all possible factors to achieve the desired performance.