PCB Surface Finish Types

This guide covers all the critical aspects of PCB surface finishes.

So, before you start fabricating PCBs, read this guide.

What is a PCB Surface Finish?

A PCB surface finish is a coating applied over the conductive circuitry on an unpopulated printed circuit board surface.

You find the PCB surface finish can consist of several elements applied through different processes.

The PCB surface finishing procedure is particularly useful for PCB’s with copper traces and typically precedes the soldering process.

 PCB surface finish

You find the application of a PCB surface finish important for two major reasons.

They include;

• Using a surface finish on a printed circuit board is useful in preventing the corrosion of the conductive board path. Corrosion of the copper traces occurs due to the oxidation process.
• When you apply a PCB surface finish, you find it improves the board’s surface ability when components are soldered.

There are several material options for use when applying the PCB surface finish.

The materials used have different properties and capabilities.

Some of the material elements used in PCB surface finish include gold, nickel, tin, silver, and lead.

These materials also have different application procedures while they provide different solderability levels.

What are the Factors to Consider when Choosing a PCB Surface Finish?

The choice of surface finish to use on a printed circuit board depends on several factors.

As such, you can base your choice of surface finish by considering each of the factors below.

• The cost of applying a particular surface finish as some finishes are costlier than others.
• Consequently, the board size and number of boards produced will also affect your decision.
• The nature and properties of the components to be employed on the board will also guide your decision.
• The expected durability standard from the surface finish used.
• The consequences to the environment resulting from using a particular PCB surface finish.

What are some of the Surface Finishes used on PCBs?

There are many options to select from when picking a surface finish for your printed circuit board.

Some of the common options include:

• The Hot Air Solder Levelling (HASL) that can contain lead or free of lead element.
• Immersion tin and immersion silver.
• Electroless Nickel Immersion Gold (ENIG)

How is Hot Air Solder Leveling applied?

HASL is a surface finish composed of solder material comprising of lead and tin elements.

You find this surface finish as one with the least cost requirements.

Furthermore, the availability of HASL makes its use popular.

HASL Surface finish

In hot air solder leveling, the printed circuit board is bathed in liquefied solder such that the conductive traces are submerged.

A stream of hot air in a line of action perpendicular to the board is used then to level the solder.

The aim is to evenly spread the solder across the board surface in equal thickness.

What are the Advantages of using HASL as a PCB Surface Finish?

There are several benefits to be accrued form employing hit air solder leveling as a finish on your circuit board surface. These benefits include:

• HASL is a low-cost alternative for PCB surface finishing.
• You also find that as a result of its low cost, it makes use of large productions of PCBs affordable.
• HASL provides a dependable soldering surface for both leaded components and surface-attached components.
• The use of HASL has stood the test of time, making it the most implemented finish in the PCB fabrication industry.

In what ways is the use of HASL Limited as a PCB Surface Finish?

While hot air solder leveling is appreciated in many ways, its use is limited in the following instances.

• HASL contains elements of lead in its composition. Lead has been identified to cause both personal and environmental harm and has its use discouraged.

Consequently, the HASL surface finish type is considered not to conform to RoHS recommendations.

• The leveling procedure used in HASL does not create an entirely level surface.

Therefore, you find a thickness disparity on the board especially relating to the solder pad sizes.

Accordingly, attaching components of small sizes, with fine pitch such as the small outline chips, is a problem.

• Additionally, when you attach components with a fine pitch over this surface finish, there is the danger of bridge formation.
• Printed circuit boards employing high density interconnects are ideally not suited for this type of finish.

You find their high connectivity demands and the less impressive finish by HASL incompatible.

Can the Hot Air Solder Level PCB Surface Finish be provided without Lead?

WHILE you find the HASL composition to include tin and lead elements, sometimes lead is replaced with other environmentally friendly elements.

Replacing lead is due to the hazardous nature of the element to the environment and as a stipulation of the RoHS initiative.

It follows that lead can be replaced by elements such as copper, nickel, and germanium.

Tin is retained in this composition.

Additionally, a mixture of more than three elements can be employed in this solder type.

The HASL without lead conforms to the ideals of the RoHS initiative.

What Benefits are Derived from using Lead-free HASL as a PCB Surface Finish?

You find using lead-free solder in hot air leveling achieves the following:

• The response of the surface to the soldering process is exceptional allowing for optimum adherence.
• You also find that lead-free HASL is much affordable compared to other techniques used in PCB surface finish.
• Additionally, you can easily and cheaply employ lead-free HASL to a large number of boards.
• The response of a lead-free surface finish to temperature changes is remarkable.

Are there any Drawbacks Associated with Lead-free HASL as a PCB Surface Finish?

Yes, there are.

You find lead-free HASL is limited as a PCB surface finish in the following ways:

• Like HASL containing lead, the surface achieved is not entirely smooth. You find varying thicknesses exhibited with different solder lands.
• Additionally, eliminating lead results in increased temperature requirement for solder preparation. You find temperatures of over 250 ^oC required to process lead-free HASL.
• The lead-free HASL offers little support for small surface mounted components. Some of these components include those with fine pitches for attachments such as the ball grid array.
• When the lead-free HASL is used for components with fine pitches such as the small outline, bridging tends to occur. Bridging creates undesired paths of electric charge flow on the board surface.

What are Immersion Coatings in the PCB Surface Finish?

Immersion coatings are surface finishes employed on printed circuit boards by submerging them in molten states of particular elements.

The formation of the coating is dependent on a chemical process that occurs during submersion.

In this process, the metal ions of the molten element are attached to the board surface.

Common elements used in the immersion process include silver, tin, and gold.

You find immersion coatings to cost more than ordinary finishing techniques such as HASL.

Why are Immersion Coatings Preferred over other PCB Surface Finish Techniques?

While immersion surface finishes are pricier than other PCB surface finishes, they find popular use for a variety of reasons.

Use of immersion coatings is influenced by the following reasons:

• You achieve a high level of smoothness from employing immersion coatings.
• Immersion surface finishes are suitable for small surface mounted components with fine profiles such as small outline and ball grid arrays.
• Using immersion techniques for applying a surface finish is compliant with the RoHS initiative. You find compounds used in the immersion process do not contain lead.
• The surface resulting from immersion coatings responds favorably to the soldering process even when carried out multiple times.
• A finish resulting from the immersion technique can tolerate temperature fluctuations of a wide range without deforming.
• You can apply a new surface finish over an immersion coating over time without hampering the board functionality.

What Immersion Coatings are used in PCB Surface finish?

There are two common immersion coatings used for printed circuit boards.

You find immersion tin and immersion silver applied as popular circuit board surface finishes.

· Immersion Tin

Immersion tin entails the chemical layering of the copper trace on a PCB surface with tin ions.

The result is the formation of a single coat of tin over the conductive pattern.

Also, the surface is even allowing for the use of this surface finish for small board attachments.

You find that tin provides the least cost requirement when implemented for the immersion surface finish.

Additionally, while it can support small board attachments, it responds remarkably well to the soldering process.

The surface finish achieved through immersion tin is unaffected by temperature fluctuations.

However, you find immersion tin coating delicate to touch and consequently requiring the use of protective gloves.

Also, the formation of whiskers is a concern when employing immersion tin surface finish.

Moreover, tins react harshly to solder mask applications requiring a wide dam.

It follows that its use with removable masks is highly ineffective.

· Immersion Silver

In immersion silver, a coating of silver is chemically deposited over the copper pattern to form a uniform layer.

While the copper-silver compatibility is better than copper-tin compatibility, silver is tarnished on exposure to air.

Packaging of PCBs with immersion silver coating, therefore, has to be air-tight.

When securely packaged, you can take up to a year before carrying out the soldering process on the PCB.

Otherwise, the soldering process has to be carried out within twenty-four hours.

Immersion silver can be plated with a gold layer to protect from tarnishing.

Accordingly, the coating can last longer than when unplated.

Why is Immersion Tin not used as a PCB Surface Finish for Small Production Batches?

While immersion tin is commonly engaged when working with large PCB volumes, its use is limited in small board numbers.

A contributing factor to this limitation is the tarnishing of the tin coating over the conductive copper trace.

Consequently, this affects solderability over time.

Green surface finish on PCB

Therefore, to assuage the impact of tarnishing on the circuit board surface, the soldering process has to quickly follow.

When soldering is done without delay, you can achieve better solder connections.

Since large volumes of PCBs are carried out on successive step processes, the time taken between processes is reduced.

As a result, when the coating process is finalized, the soldering process will follow.

Small productions are approached differently, as they may be stalled to allow for volume achievement to mitigate costs.

What are the Characteristics of Immersion Silver as a PCB Surface Finish?

Immersion silver has several features that work in its favor and disfavor.

Some of the pros attributed to immersion silver include:

• A brilliant surface finish that is smooth and flat.
• Immersion silver provides a stable attachment surface for small board attachments such as small outline chips and ball grid arrays.
• You can work on a silver-coated conductive surface multiple times without affecting board functions.
• The high electrical charge transfer capacity of silver makes this coating type well suited for applications with high-speed requirements.

On the other hand, the misgivings of immersion silver as a PCB surface finish include:

• Its easy irritability when handled thus requiring care to prevent surface infractions as a result.
• Furnishing PCBs with immersion silver as a surface coating requires air-tight packing which adds to the cost. Similarly, costs are increased when a gold layer is deposited over the silver layer to prevent tarnishing.
• To eliminate additional costs caused by tarnishing, the soldering process has to be carried out not long after. As such, there is little time to work with for checking other board aspects after coating and before soldering.
• You find compatibility issues when you use PCBs with immersion surface finish with removable masks.

What is ENIG in the PCB Surface Finish?

ENIG is an acronym for Electroless Nickel Immersion Gold.

ENIG is a surface finish that is composed of a nickel-gold combination achieved through electroless plating and immersion respectively.

You find an excellent surface outcome with this surface finish attributing to the high cost incurred in its achievement.

The ENIG process consists of two steps, the nickel application, and the gold immersion.

Gold is applied over the nickel to prevent it from succumbing to a reaction with atmospheric oxygen.

Before that, the copper surface is catalyzed with palladium before the application of nickel.

A suitably concentrated solution composed of nickel ions is used to electrolytically apply a layer of nickel over the conductive pattern.

To ensure the formation of a uniform coat, the solution temperature and nickel concentration are controlled.

The laying of a gold coat involves the use of an immersion technique.

In this case, the nickel-coated board is submerged in a solution heavily concentrated with gold ions.

A reduction process occurs where the gold ions in the solution are reduced forming a metal deposit on the nickel surface.

The gold coating deposited over the nickel layer is made to a thickness that allows nickel to retain its solderability.

However, it also needs to prevent interaction with oxygen molecules to prevent oxidation.

Consequently, you find the need to keep close tolerance.

Why is ENIG Preferred over other PCB Surface Finishes?

While executing an ENIG PCB surface finish is a costly process, the benefits derived from the finish justify the cost.

 ENIG surface finish

You find the preferment of ENIG to other surface finishes for PCBs to be based on the following grounds:

• The ENIG coating is rigid and resilient with the capability of lasting a long time. Consequently, you find no need to rework with this finish.
• The level of smoothness and evenness achieved via ENIG is exemplary providing a good surface for small components attachment.
• With ENIG circuit board finishes, you can employ wire attachments without interfering with the quality.
• You find ENIG finish has impressive anti-oxidation and solderability performance strengthened by the dual use of gold and nickel materials.
• Furthermore, you can effectively use board attachments with adjustable contacts on a board surface with ENIG finish.
• Elements used in ENIG surface finish pose no known harm to the environment making the finish compliant with RoHS regulation.

What is a Black Pad on PCB Surface Finish?

A black pad is an unwanted formation that sometimes manifests when ENIG is used as a surface finish.

A black pad results from leakage of sulfur-based substance contained in the solder mask through the gold coating.

It follows that from this seepage stems the formation of a layer of phosphorus sandwiched by the nickel and gold coats.

You find this formation cannot transfer electrical charge and can cause electrical isolation.

Additionally, it can occasion cracks on the surface disrupting the evenness and subsequently the solderability.

How is OSP Applied as a PCB Surface Finish?

OSP refers to organic solderability preservative.

As the name suggests, this finish type is organic and water-based.

You find the OSP finish adheres well with the copper surface offering effective cover from corrosion.

Furthermore, it offers a good soldering surface.

To kick start the application of OSP, the copper surface is cleaned to ensure it is free of contaminants.

Chemical agents are used in the cleaning process before being rinsed off.

You can also improve the board’s surface appearance by removing bumps and nodules.

Another rinsing process follows the surface enhancement step only this time an acidic solution is employed.

Thereafter, the organic solder preservative is applied by a conveyor mechanism ensuring a thin layer is applied.

You find the resulting coating has a thickness in the nanometer range.

Upon layering, a specialized rinsing procedure that deionizes the coat is undertaken.

Finally, the surface is dried ready for storage.

Storing a circuit board with OSP finish requires ideal temperature and humidity conditions.

Extreme temperature and humidity values will affect the board surface by causing breakage and soaking.

Also, boards with this finish type need to be kept away from direct sunlight.

Does OSP Finish Support Small Component Attachment?

Yes, it does.

You can use OSP finish when looking to attach small profile PCB components such as ball grids.

Additionally, the planarity achieved by the OSP finish is remarkable.

Also, its wettability impresses on the ability to carry out the soldering procedure on the coating.

PCB components

Other notable benefits derived from using OSP include the low cost incurred in implementing this finish and its simplicity.

Besides, it does not pose any environmental threat due to its zero lead content.

You also find this finish tolerant to thermal cycles within certain ranges and can be applied several times over.

What are some of the Limitations of OSP as a PCB Surface Finish?

The use of the OSP finish has been on the rise due to its low cost and environmentally friendly features.

However, this PCB surface finish has a few drawbacks related to its use.

• You find that since the OSP coating process is not done chemically, determining the layer thickness is challenging.
• Additionally, using the organic solder preservative is difficult on circuit boards with metalized through-holes.
• Comparing the OSP finish strength with other surface finishes, it posts a lower score. You find it highly responsive to touch requiring specialized handling.
• The ability of the OSP finish to withstand repeated temperature tests is low. Therefore, soldering cannot be carried out several times.
• The OSP finish has a limited life span and durability discouraging long haul productions and stocking.
• Scrutinizing a board with OSP finish for defects is a taxing process.
• When applying solder paste on a PCB with OSP finish, correcting application errors can hamper the board functionality.

Can a PCB Surface Finish have Multiple Layers?

While many surface finishes containing a single or dual coating, the ENEPIG finish implements three layers of material.

ENEPIG is an acronym for Electroless Nickel Electroless Palladium Immersion Gold which is an ENIG finish with an extra palladium layer.

The PCB finish poses no imminent harm to the environment adhering to RoHS standards.

In an ENEPIG finish, the palladium layer is furnished over the nickel coat.

You find this prevents the corrosion of the copper surface due to leakage that causes black pads formation.

A gold coat is then applied over the palladium layer.

Gold offers resistance to oxidation preserving the chemical state of the elements underneath.

Using palladium in ENEPIG finish allows for the use of a thinner gold coating saving on cost.

Additionally, you find this finish to be highly resilient with an extended life span.

You can also use ENEPIG in HDI PCBs with large speed requirements.

Equally splendid about the ENEPIG finish is its planarity and solderability.

You can contact Venture Electronics today for the best PCB surface finishes.