PCB Surface Finishes

A PCB surface finish is crucial for a printed circuit board as it prevents the copper (exposed traces, pads, holes and ground plains) from oxidation – a critical ingredient to high-level application performance.

A surface finish is essential for making a reliable connection between the PCB and the electronic component. A surface finish has two major functions, to provide a solderable surface for sufficient soldering components to the PCB, and to protect any exposed copper from oxidizing.

When it comes to choosing the right type of final surface finish, it is just as important as selecting the right material for your bare board. Selecting the correct surface finish for your application is critical for performance. For assistance with selecting a surface finish, send our trusted engineers an email or call.

Get your PCBs Built-Fast.

Contact us for a PCB quote
or call us at 1-800-SFC-5143

With increasing demands for thin, high-density, fast speeds, and lightweight applications, the solder finish will make all the difference.

Modern surface finishes are lead-free, in accordance with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, and include:

Choosing the correct PCB surface finish for your application requires taking factors into account like cost, the final application environment (e.g. high-heat/thermal, vibration/stability, RF), component density/selection, lead/lead free requirements, shelf life, shock/drop resistance, production volume, and throughput.

Due to the increased demand for improved performance of electronic applications, surface finishes have also been upgrading. Because of tighter PCB topography the HASL surface finish is quickly being replaced by gold-based surface finishes like ENIG, ENEPIG, and soft/hard gold. The gold-based surface finishes have a lot to offer in terms of features and benefits when used in applications.

Diagram illustrating the difference between ENIG and other surface finishes

Diagram illustrating the difference between ENIG and other surface finishes

 

ENIG (Electroless Nickel Immersion Gold)

Example of an ENIG Surface Finish on PCB

Example of an ENIG Surface Finish on PCB

 

ENIG consists of two layers of metallic coating, which are deposited on the copper surface through chemical process, 2-5μ inches of immersion Gold (Au) over 120-240μ inches of electroless Nickel (Ni).

The nickel layer protects the copper from oxidation, and the gold layer protects the nickel layer. This dramatically increases the resistance to corrosion and also maintains a flat surface which is crucial for assembly of leadless components. ENIG usage has become very common now due to the accountability for lead-free regulations.

 

ENIG (Electroless Nickel Immersion Gold) PROS Flat Surface - Excellent for Assembly of Dense Topography PCBs Long Shelf Life Great for Plated Holes and Fine Pitch / BGA / Small Components Wire Bondable (When Aluminum Wire Is Used) CONS

Expensive

Significant Signal Loss Unable to Rework Black Pad / Black Nickel Not Suitable for Touch Contact Pads MOST COMMON USES High Density PCBs Rigid PCBs Flex PCBs Industries: Consumer, Data/Telecom, Aerospace, Military, High Performance, and Medical

 

Properties of ENIG Properties ENIG

Contact Applications

Yes

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

3 - 10 µin of Gold over
150 - 200 µin of Nickel

Thin Board Finish

Yes

Applications of ENIG Applications ENIG

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

Yes (Al)

 

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

ENEPIG is probably not new to you, but it is a surface finish that has become increasingly popular recently due to the price reductions in palladium. It also has increased in popularity and use because of its many features and benefits. It also has a distinct advantage over the ENIG surface finish, which we’ll get into below.

The ENEPIG surface finish is composed of four metal layers:

• Copper
• Nickel
• Palladium
• Gold

 

Diagram illustrating the difference between ENEPIG and other surface finishes.

Diagram illustrating the difference between ENEPIG and other surface finishes.

 

The difference between ENEPIG and ENIG is the added layer of palladium. The palladium helps protect the nickel layer from corrosion, which helps prevent “black pad” from occurring. This is one of the distinct advantages of ENEPIG over the ENIG surface finish. The immersion gold layer that sits atop the palladium offers nearly-complete board protection by protecting and preserving the palladium underneath.

When compared to other surface finishes, like hard gold and soft gold plating, ENEPIG has become more affordable in recent years. The cost of the ENEPIG surface finish decreased because the added palladium layer reduces the required thickness of the more expensive gold layer.

ENEPIG is Capable of Your Demanding Requirements

The ENEPIG finish is capable of the majority of the ultra-demanding requirements with multiple package types like through-holes, SMT, BGA, wire bonding, and even press fit.

 

The ENEPIG surface finish is capable of ultra-demanding requirements.

The ENEPIG surface finish is capable of ultra-demanding requirements.

 

The PCB surface finish with ENEPIG is very thin (between 0.05μm and 0.1μm), making the assembly and soldering process very simple and definitely more reliable when compared to ENIG. ENEPIG also has a long shelf life due to its durability and resistance from tarnishing.

 

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) PROS

Black Pad Free

Good for Touch Contact / Push Contact

Higher Reliability Wire Bonding than ENIG

Stronger Solder Reliability than ENIG

Cheaper than Electrolytic Gold

Lead Free & RoHS Compliant

Long Shelf Life

Multiple Reflow Soldering Capability

Flat Surface - Excellent for Solderability

Great for Plated Holes and Fine Pitch / BGA / Small Components

Easier Inspections

CONS

More Expensive than ENIG

May Not Be As Reliable for Gold Wire Bonding When Compared to Soft Gold

MOST COMMON USES

Multi-Layer PCBs

High Density Assemblies

Mixed Package Technologies

Industries: Military, Medical, Aerospace, High Performance Devices

 

Properties of ENIG Properties ENEPIG

Contact Applications

Yes

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

1 - 2 µin of Gold over 4 -
8 µin or 8 - 15 µin Palladium
over 100 -150 µin Nickel

Thin Board Finish

Yes

Applications of ENEPIG Applications ENEPIG

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

Yes

 

Electrolytic Nickel/Gold - Ni/Au (Hard/Soft Gold)

Depending on applications, printed circuit board technologies use gold extensively. Gold has good electrical conductivity, tarnish resistance, solderability after storage, and being excellent etch resist.

Electrolytic Ni/Au has a layer of gold plating over a base of electroplated nickel. The purity of the gold plating categorizes this finish as either hard gold (99.6% purity) or soft gold (99.9% purity). The choice of hard gold versus soft gold depends on the type of application you are looking to create

Examples of the Hard/Soft Gold Surface Finish

Examples of the Hard/Soft Gold Surface Finish

 

Hard Gold Surface Finish

Hard gold is a gold alloy with complexes of cobalt, nickel or iron. A low-stress nickel is used between gold over-plating and copper. Hard gold is not suitable for wire bonding.

We recommend using the hard gold surface finish for components and applications with heavy usage and a high likelihood of wear and tear, like:

• Interconnected Carrier Boards
• Edge Connector Fingers
• Keypads
• Contacts

The thickness of the hard gold surface finish will vary depending on the applications. When hard gold is used for compliance in military applications, the minimum thickness shall be 50 -100 micro inch. 

Nonmilitary applications require 25 to 50 micro inches. Here are the recommended minimum and maximum thickness values:

• 17.8 μin is recommended for IPC maximum solderable thickness
• 25 μin gold over 100 μin nickel for IPC Class 1 & Class 2 applications
• 50 μin gold over 100 μin nickel for IPC Class 3 applications

 

Properties of Hard Gold Properties Hard Gold

Contact Applications

Yes

Deposit

Electrolytic

Exposed Copper

Yes

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

98% Pure, 23 Karat 30 -
50 µin over 100 - 150 µin
of Nickel

Thin Board Finish

Yes

Applications of Hard Gold Applications Hard Gold

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium/High

RoHS Compliant

Yes

Solder Joint Integrity

Poor

Wire Bonding

No

 

Soft Gold Surface Finish

A soft gold finish, as the name suggests, contains a higher gold purity on the outer gold plating. Soft gold has a 99.9% purity.

 

An example of the soft gold surface finish on a PCB

An example of the soft gold surface finish on a PCB

 

Soft gold finish is used for boards designed mostly for applications that require wire bonding, high solderability and weldability. Soft gold produces a much stronger welded joint when compared to hard gold.

 

Electrolytic Ni/Au has a layer of gold plating of varying thickness

Electrolytic Ni/Au has a layer of gold plating of varying thickness

 

Generally speaking, and with all other factors being equal, Electrolytic Nickel/Gold is the most expensive PCB surface finish. However, some applications do require the Electrolytic Nickel/Gold surface finish.

 

Properties of Soft Gold Properties Soft Gold

Contact Applications

Yes

Deposit

Electrolytic

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

99.99% Pure, 24 Karat 30
- 50 µin over 100 - 200 µin
of Nickel

Thin Board Finish

Yes

Applications of Soft Gold Applications Soft Gold

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Poor

Wire Bonding

Yes (Au)

 

Immersion Silver Surface Finish

Immersion Silver (also called immersion Ag) is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components with reasonable costs. This surface finish can also replace Immersion Gold over Electroless Nickel (Au/Ni) for most applications. And the thickness ranges from 0.12 to 0.40 μm.

Silver is applied directly to the copper and, unlike ENIG, does not require a diffusion barrier.

It's also an ideal choice for special applications that want to avoid any magnetic materials in the PCB.

It has a decent shelf life of 6 to 12 months, but it can be sensitive to contaminants found in the air and on some surfaces. This makes it critical to have a highly qualified boardhouse to properly control and expedite the board packaging process. This surface finish is also subject to tarnishing, which is something to take into account.

As mentioned above, common applications include flat surface requirements, which may include:

• EMI Shielding
• Aluminum Wire Bonding
• Very Fine Traces
• Membrane Switches

 

Immersion Silver PROS

RoHS Compliant

Planar

Great for Fine Pitch / BGA

Cost Effective

Highly Stable

Simple Processing

Very Flat Surface

Reworkable

Excellent Solderability

CONS

Subject to Tarnishing

Silver Metal Whiskering

High Friction Coefficient

Sensitive to Contaminants

Strict Storage and Handling Requirements

Welding Issues with Micro-Holes

Prone to Electromigration

Difficulty with Electric Testing

 

Properties of Silver Characteristics Silver

Contact Applications

No

Deposit

Immersion

Exposed Copper

No

Handling

Critical

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Medium (9-12 Months)*

SMT

Flat

Thickness

6 - 18 µin

Thin Board Finish

Yes

*Requires Unique Storage Techniques µin = Microinches

Applications of Silver Applications Silver

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium/High

RoHS Compliant

Yes

Solder Joint Integrity

Excellent

Wire Bonding

No

 

Immersion Tin (Also known as White Tin)(ISn)

What is Immersion Tin?

Immersion Tin is a is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components. White Tin (Immersion Tin) is a deposit of a thin layer of Tin on Copper layer of a PCBs. It is not used as often as other surface finishes because not all PCB manufacturers offer it. The flatness of this particular coating makes this an ideal surface finish choice for small geometries and components.

Tin also is the least expensive type of immersion coatings. Although it is an economical choice, it does come with some drawbacks.

It’s Not Without Its Drawbacks

Immersion Tin has its advantages, but there are a few disadvantages to using this surface finish. As you may know, Copper and Tin have a strong affinity for one another. This means, over time, diffusion of one metal into the other will occur. It directly impacts the shelf life of the Immersion Tin deposit and ultimately the performance of the ?nish.

Immersion Tin has a limited shelf life, 3-6 months. For the best results assembly (soldering) shall happen during 30 days.

Tin Whiskering is also another downside to using Immersion Tin as a surface finish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.

The Immersion Tin finish is not very durable, therefore, PCBs with Immersion Tin finish must be handled with caution. It provides a consistently flat surface approximately 20-40 micro inches in thickness.

Why Is It Used?

Immersion Tin has been primarily used as an alternative to a lead-based surface finish. It is also used for its very flat and smooth finish making it ideal for fine geometries and fine pitch surface mount components.

Another reason to use immersion tinimmersion tin is sustainability. Elements that may be difficult to source on a consistent basis are used in other finishes, such as ENIG or HASL. Additionally, it uses less water and chemicals in the application process.

It is also easier to re-work.

As with all of the surface finishes we’ve compared so far, there are a number of advantages and disadvantages. Here are the advantages and disadvantages for Immersion Tin.

Advantages:

• Immersion Tin Finish allows excellent flatness (suitable for SMT), good for fine pitch / BGA / smaller components
• Immersion Tin has mid-range cost for lead-free finish
• Press fit suitable finish
• Retains good solderability after multiple thermal excursions
• Suitable for horizontal production line.
• Suitable for fine geometry processing, lead-free assembly.

Disadvantages:

• Sensitive to handling.
• Short shelf life, tin whiskering can occur after 6 months
• Aggressive to solder masks
• Not recommended to use with peelable masks
• Not a suitable choice for a contact switches.
• Special setting requires (soft probe landing) for electrical test

Here is a full table with some of the pros and cons along with some of the most common uses for the Immersion Tin surface finish.

 

Immersion Tin PROS

RoHS Compliant

Great for Fine Pitch / BGA

Cost Effective

Highly Stable

Simple Processing

Very Flat Surface

Reworkable

Excellent Solderability

CONS

Subject to Tarnishing

Silver Metal Whiskering

High Friction Coefficient

Sensitive to Contaminants

Strict Storage and Handling Requirements

Difficulty with Electric Testing

 

Properties of White Tin Properties White Tin

Contact Applications

No

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Medium (9-12 Months)*

SMT

Flat

Thickness

25 - 60 µin

Thin Board Finish

Yes

*Requires Unique Storage Techniques µin = Microinches

Applications of White Tin Applications White Tin

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

No

PCB surface treatment refers to the coating between components and bare board PCB. It is used for two basic reasons: to ensure solderability and to protect exposed copper circuits. Because there are many types of surface treatment, and each surface treatment has its own advantages and disadvantages, there is no perfect surface treatment. Therefore, choosing the right surface treatment is not easy. The following is a simple analysis of several common PCB surface treatment methods for reference!

Part 1：What is PCB surface treatment?

PCB surface treatment is the metal-to-metal connection between bare copper and components in the weldable area of the printed circuit board. The circuit board has a base copper surface that is easily oxidized without a protective coating, so a surface finish is required.

PCB surface treatment is the most important step in the PCB manufacturing and assembly process. It has two main functions, one is to protect the exposed copper circuit, and the other is to provide weldable surface components to the PCB when welding.

As shown in the figure below, the surface treatment is located on the outermost layer of the PCB, on top of the copper layer, acting as a copper "coating".

Part 2：PCB surface treatment types

1. hot air solder leveling (HASL)

2.Immersion tin (ImSn)

3.electroless nickel immersion gold (ENIG)

4.organic solderability preservatives (OSP)

5. Immersion Silver (ImAg)

6. Electroless nickel electroless palladium electroless immersion gold (ENEPIG)

7.Electrolytic nickel/gold (hard gold/soft gold)

Part 3：PCB surface treatment process

1.Hot air solder leveling (HASL)

HASL is the main surface treatment used in the industry. The process involves dipping the board into a melting tank of tin/lead alloy and then removing excess solder using an "air knife" that blows hot air across the board's surface.

Hot air solder leveling (HASL) is now widely used in SMT processes. There are three main points:

1) PCB should be immersed in molten solder;

2) Before the solder solidifies, the wind knife blows the liquid solder away;

3) The air knife can minimize the meniscus of the copper surface solder and prevent solder bridging.

Advantages:

• Cheap/low cost

• Longer storage time

• After the PCB is finished, the pad is completely covered with tin before soldering

• Lead-free (RoHS compliant) models are widely available

• Mature surface treatment options

• Visual inspection and electrical measurement are possible

Disadvantages:

• Poor flatness due to the natural meniscus of molten solder

• Not applicable to capacitive touch switches

• Cannot be used in high density interconnect (HDI) structures

• Not suitable for fine spacing

2.Immersion tin (ImSn)

Tin immersion (ImSn) is a metal finish deposited by a chemical displacement reaction and applied directly to the base metal (i.e. copper) of the circuit board.

Since all current solders are tin-based, the tin layer can be matched to any type of solder. From this point of view, tin immersion process has great prospects for development. However, the previous PCB after the tin dipping process appeared tin whiskers, tin whiskers and tin migration during the welding process will bring reliability problems. Therefore, the use of tin leaching process is limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure have a granular structure, overcome the previous problems, and also have good thermal stability and weldability.

Advantages:

• Excellent flatness (for SMT), suitable for fine pitch /BGA/ smaller components

• Lead-free surface treatment technology with medium cost

• Maintain good weldability after multiple thermal shifts

Disadvantages:

• Not suitable for long-term storage

• Tin whiskers can cause short circuits and solder joint problems

• Not a suitable choice for contact switches

• Electrical testing is difficult

3.Electroless nickel immersion gold (ENIG)

ENIG,or Electroless Nickel Immersion Gold finish has historically been the best fine pitch (flat) surface and lead-free option.

ENIG is a two-step process that covers a thin layer of gold on top of a thin layer of nickel. Nickel acts as a barrier to copper and is the surface to which the components are actually welded,while gold protects nickel during storage.

Advantages:

• Suitable for lead-free (RoHS compliant) welding

• Excellent surface smoothness

• Long shelf life, durable surface

Disadvantages:

• Expensive, use gold

• The process is complex and difficult to control

• Black pad/Black nickel

4.Organic solderability preservatives (OSP)

OSP (Organic solderability preservatives) usually uses a conveyor process to coat exposed copper with a very thin material protective layer, thereby protecting the copper surface from oxidation.

This film must have the characteristics of oxidation resistance, thermal shock resistance, moisture resistance, etc., to protect the copper surface from rust (oxidation or vulcanization, etc.) under normal conditions.

However, in subsequent high-temperature welding, this protective film must be easily removed by flux quickly. In this way, the bare clean copper surface can be immediately combined with the melted solder, forming a strong solder joint in a very short time.

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

Advantages:

• Flat surface

• Simple process, suitable for lead-free welding and SMT

• cost-effectiveness

• Environmental protection

Disadvantages:

• Short shelf life

• It is not convenient for visual and electrical measurement

• Not suitable for PTH (plated through holes)

5.Immersion Silver (ImAg)

Immersion silver is a non-electrolytic chemical surface treatment applied by dipping a copper PCB into a silver ion tank. 

Considering modern environmental issues such as RoHS and WEE, immersion silver is more environmentally friendly than HASL and ENIG. It is also popular because it costs less than ENIG.

Even if the PCB processed by the immersion silver process is exposed to high temperature, humidity and pollution, it can still provide good electrical properties and maintain good weldability, even if it will lose its luster. 

Advantages:

• High weldability

• Good surface smoothness

• Low cost and lead-free (RoHS compliant)

Disadvantages:

• High storage requirements

• Easily contaminated

• Electrical testing is difficult

6. Electroless nickel electroless palladium electroless immersion gold (ENEPIG)

Compared to ENIG, ENEPIG has an additional palladium layer between nickel and gold, which further protects the nickel layer from corrosion and prevents the black mat that can appear in ENIG finishes.

Advantages:

• A flat surface is provided Shelf life of 12 months or more

• Lead-free (RoHS compliant) Nickel corrosion - or "black pad

• syndrome" - is not a problem

Disadvantages:

• Complicated process

• High cost

• Immature technology

7.Electrolytic nickel/gold (hard gold/soft gold)

Electrolytic nickel gold is divided into “hard gold” and “soft gold”. Hard gold has a lower purity and is commonly used for gold fingers (PCB edge connectors), PCB contacts or other hard-wear areas. Gold thicknesses may vary according to requirements. Soft gold is purer and is often used in wire bonding applications.

In general, with all other factors being equal, electrolytic nickel/gold is the most expensive PCB surface treatment. 

However, some applications do require electrolytic nickel/gold surface finish.

Part 4：Key Considerations When Choosing Your PCB Finish

PCB surface treatment selection is the most important step in PCB manufacturing, because it directly affects the process yield, the number of reworks, the field failure rate, the testing capacity, the scrap rate and the cost.

1. PCB surface treatment - pad flatness

As mentioned earlier, certain surface treatments can result in uneven surfaces, which can affect performance, weldability, and other factors. If flatness is an important factor, consider a surface finish with a thin, even layer. In this case, appropriate options include ENIG, ENEPIG, and OSP.

2. Weldability and wettability

Solderability is always a key factor when using PCBS. Certain surface treatments such as OSP and ENEPIG have been shown to hinder weldability, while others such as HASL are well suited.

3. Storage conditions

As mentioned earlier, certain surface treatments (such as OSP) can make PCBS brittle when handled, while others improve durability.

4. Welding cycle

How many times does the PCB have to be welded and reworked? Many finishes are ideal for rework. However, other methods such as immersion tin are not suitable for rework.

According to the above introduction to each surface finish, some attributes are the most important elements as selection criteria.All in all, for the choice of surface finish, the key is that all possible factors must be considered to obtain the desired performance.

If you want to know how the surface finish affects the price, you can contact us at sales@tdt-ele.com.

 

A surface finish is essential for making a reliable connection between the PCB and the electronic component. A surface finish has two major functions, to provide a solderable surface for sufficient soldering components to the PCB, and to protect any exposed copper from oxidizing.

When it comes to choosing the right type of final surface finish, it is just as important as selecting the right material for your bare board. Selecting the correct surface finish for your application is critical for performance. For assistance with selecting a surface finish, send our trusted engineers an email or call.

Get your PCBs Built-Fast.

Contact us for a PCB quote
or call us at 1-800-SFC-5143

With increasing demands for thin, high-density, fast speeds, and lightweight applications, the solder finish will make all the difference.

Modern surface finishes are lead-free, in accordance with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, and include:

Choosing the correct PCB surface finish for your application requires taking factors into account like cost, the final application environment (e.g. high-heat/thermal, vibration/stability, RF), component density/selection, lead/lead free requirements, shelf life, shock/drop resistance, production volume, and throughput.

Due to the increased demand for improved performance of electronic applications, surface finishes have also been upgrading. Because of tighter PCB topography the HASL surface finish is quickly being replaced by gold-based surface finishes like ENIG, ENEPIG, and soft/hard gold. The gold-based surface finishes have a lot to offer in terms of features and benefits when used in applications.

Diagram illustrating the difference between ENIG and other surface finishes

Diagram illustrating the difference between ENIG and other surface finishes

 

ENIG (Electroless Nickel Immersion Gold)

Example of an ENIG Surface Finish on PCB

Example of an ENIG Surface Finish on PCB

 

ENIG consists of two layers of metallic coating, which are deposited on the copper surface through chemical process, 2-5μ inches of immersion Gold (Au) over 120-240μ inches of electroless Nickel (Ni).

The nickel layer protects the copper from oxidation, and the gold layer protects the nickel layer. This dramatically increases the resistance to corrosion and also maintains a flat surface which is crucial for assembly of leadless components. ENIG usage has become very common now due to the accountability for lead-free regulations.

 

ENIG (Electroless Nickel Immersion Gold) PROS Flat Surface - Excellent for Assembly of Dense Topography PCBs Long Shelf Life Great for Plated Holes and Fine Pitch / BGA / Small Components Wire Bondable (When Aluminum Wire Is Used) CONS

Expensive

Significant Signal Loss Unable to Rework Black Pad / Black Nickel Not Suitable for Touch Contact Pads MOST COMMON USES High Density PCBs Rigid PCBs Flex PCBs Industries: Consumer, Data/Telecom, Aerospace, Military, High Performance, and Medical

 

Properties of ENIG Properties ENIG

Contact Applications

Yes

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

3 - 10 µin of Gold over
150 - 200 µin of Nickel

Thin Board Finish

Yes

Applications of ENIG Applications ENIG

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

Yes (Al)

 

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

ENEPIG is probably not new to you, but it is a surface finish that has become increasingly popular recently due to the price reductions in palladium. It also has increased in popularity and use because of its many features and benefits. It also has a distinct advantage over the ENIG surface finish, which we’ll get into below.

The ENEPIG surface finish is composed of four metal layers:

• Copper
• Nickel
• Palladium
• Gold

 

Diagram illustrating the difference between ENEPIG and other surface finishes.

Diagram illustrating the difference between ENEPIG and other surface finishes.

 

The difference between ENEPIG and ENIG is the added layer of palladium. The palladium helps protect the nickel layer from corrosion, which helps prevent “black pad” from occurring. This is one of the distinct advantages of ENEPIG over the ENIG surface finish. The immersion gold layer that sits atop the palladium offers nearly-complete board protection by protecting and preserving the palladium underneath.

When compared to other surface finishes, like hard gold and soft gold plating, ENEPIG has become more affordable in recent years. The cost of the ENEPIG surface finish decreased because the added palladium layer reduces the required thickness of the more expensive gold layer.

ENEPIG is Capable of Your Demanding Requirements

The ENEPIG finish is capable of the majority of the ultra-demanding requirements with multiple package types like through-holes, SMT, BGA, wire bonding, and even press fit.

 

The ENEPIG surface finish is capable of ultra-demanding requirements.

The ENEPIG surface finish is capable of ultra-demanding requirements.

 

The PCB surface finish with ENEPIG is very thin (between 0.05μm and 0.1μm), making the assembly and soldering process very simple and definitely more reliable when compared to ENIG. ENEPIG also has a long shelf life due to its durability and resistance from tarnishing.

 

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) PROS

Black Pad Free

Good for Touch Contact / Push Contact

Higher Reliability Wire Bonding than ENIG

Stronger Solder Reliability than ENIG

Cheaper than Electrolytic Gold

Lead Free & RoHS Compliant

Long Shelf Life

Multiple Reflow Soldering Capability

Flat Surface - Excellent for Solderability

Great for Plated Holes and Fine Pitch / BGA / Small Components

Easier Inspections

CONS

More Expensive than ENIG

May Not Be As Reliable for Gold Wire Bonding When Compared to Soft Gold

MOST COMMON USES

Multi-Layer PCBs

High Density Assemblies

Mixed Package Technologies

Industries: Military, Medical, Aerospace, High Performance Devices

 

Properties of ENIG Properties ENEPIG

Contact Applications

Yes

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

1 - 2 µin of Gold over 4 -
8 µin or 8 - 15 µin Palladium
over 100 -150 µin Nickel

Thin Board Finish

Yes

Applications of ENEPIG Applications ENEPIG

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

Yes

 

Electrolytic Nickel/Gold - Ni/Au (Hard/Soft Gold)

Depending on applications, printed circuit board technologies use gold extensively. Gold has good electrical conductivity, tarnish resistance, solderability after storage, and being excellent etch resist.

Electrolytic Ni/Au has a layer of gold plating over a base of electroplated nickel. The purity of the gold plating categorizes this finish as either hard gold (99.6% purity) or soft gold (99.9% purity). The choice of hard gold versus soft gold depends on the type of application you are looking to create

Examples of the Hard/Soft Gold Surface Finish

Examples of the Hard/Soft Gold Surface Finish

 

Hard Gold Surface Finish

Hard gold is a gold alloy with complexes of cobalt, nickel or iron. A low-stress nickel is used between gold over-plating and copper. Hard gold is not suitable for wire bonding.

We recommend using the hard gold surface finish for components and applications with heavy usage and a high likelihood of wear and tear, like:

• Interconnected Carrier Boards
• Edge Connector Fingers
• Keypads
• Contacts

The thickness of the hard gold surface finish will vary depending on the applications. When hard gold is used for compliance in military applications, the minimum thickness shall be 50 -100 micro inch. 

Nonmilitary applications require 25 to 50 micro inches. Here are the recommended minimum and maximum thickness values:

• 17.8 μin is recommended for IPC maximum solderable thickness
• 25 μin gold over 100 μin nickel for IPC Class 1 & Class 2 applications
• 50 μin gold over 100 μin nickel for IPC Class 3 applications

 

Properties of Hard Gold Properties Hard Gold

Contact Applications

Yes

Deposit

Electrolytic

Exposed Copper

Yes

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

98% Pure, 23 Karat 30 -
50 µin over 100 - 150 µin
of Nickel

Thin Board Finish

Yes

Applications of Hard Gold Applications Hard Gold

BGA & μBGA

Yes

Contact/Connector

Yes

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium/High

RoHS Compliant

Yes

Solder Joint Integrity

Poor

Wire Bonding

No

 

Soft Gold Surface Finish

A soft gold finish, as the name suggests, contains a higher gold purity on the outer gold plating. Soft gold has a 99.9% purity.

 

An example of the soft gold surface finish on a PCB

An example of the soft gold surface finish on a PCB

 

Soft gold finish is used for boards designed mostly for applications that require wire bonding, high solderability and weldability. Soft gold produces a much stronger welded joint when compared to hard gold.

 

Electrolytic Ni/Au has a layer of gold plating of varying thickness

Electrolytic Ni/Au has a layer of gold plating of varying thickness

 

Generally speaking, and with all other factors being equal, Electrolytic Nickel/Gold is the most expensive PCB surface finish. However, some applications do require the Electrolytic Nickel/Gold surface finish.

 

Properties of Soft Gold Properties Soft Gold

Contact Applications

Yes

Deposit

Electrolytic

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

High

Required Thermal Cycles

>2

Shelf Life

Long (1+ Years)

SMT

Flat

Thickness

99.99% Pure, 24 Karat 30
- 50 µin over 100 - 200 µin
of Nickel

Thin Board Finish

Yes

Applications of Soft Gold Applications Soft Gold

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

High

RoHS Compliant

Yes

Solder Joint Integrity

Poor

Wire Bonding

Yes (Au)

 

Immersion Silver Surface Finish

Immersion Silver (also called immersion Ag) is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components with reasonable costs. This surface finish can also replace Immersion Gold over Electroless Nickel (Au/Ni) for most applications. And the thickness ranges from 0.12 to 0.40 μm.

Silver is applied directly to the copper and, unlike ENIG, does not require a diffusion barrier.

It's also an ideal choice for special applications that want to avoid any magnetic materials in the PCB.

It has a decent shelf life of 6 to 12 months, but it can be sensitive to contaminants found in the air and on some surfaces. This makes it critical to have a highly qualified boardhouse to properly control and expedite the board packaging process. This surface finish is also subject to tarnishing, which is something to take into account.

As mentioned above, common applications include flat surface requirements, which may include:

• EMI Shielding
• Aluminum Wire Bonding
• Very Fine Traces
• Membrane Switches

 

Immersion Silver PROS

RoHS Compliant

Planar

Great for Fine Pitch / BGA

Cost Effective

Highly Stable

Simple Processing

Very Flat Surface

Reworkable

Excellent Solderability

CONS

Subject to Tarnishing

Silver Metal Whiskering

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High Friction Coefficient

Sensitive to Contaminants

Strict Storage and Handling Requirements

Welding Issues with Micro-Holes

Prone to Electromigration

Difficulty with Electric Testing

 

Properties of Silver Characteristics Silver

Contact Applications

No

Deposit

Immersion

Exposed Copper

No

Handling

Critical

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Medium (9-12 Months)*

SMT

Flat

Thickness

6 - 18 µin

Thin Board Finish

Yes

*Requires Unique Storage Techniques µin = Microinches

Applications of Silver Applications Silver

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium/High

RoHS Compliant

Yes

Solder Joint Integrity

Excellent

Wire Bonding

No

 

Immersion Tin (Also known as White Tin)(ISn)

What is Immersion Tin?

Immersion Tin is a is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components. White Tin (Immersion Tin) is a deposit of a thin layer of Tin on Copper layer of a PCBs. It is not used as often as other surface finishes because not all PCB manufacturers offer it. The flatness of this particular coating makes this an ideal surface finish choice for small geometries and components.

Tin also is the least expensive type of immersion coatings. Although it is an economical choice, it does come with some drawbacks.

It’s Not Without Its Drawbacks

Immersion Tin has its advantages, but there are a few disadvantages to using this surface finish. As you may know, Copper and Tin have a strong affinity for one another. This means, over time, diffusion of one metal into the other will occur. It directly impacts the shelf life of the Immersion Tin deposit and ultimately the performance of the ?nish.

Immersion Tin has a limited shelf life, 3-6 months. For the best results assembly (soldering) shall happen during 30 days.

Tin Whiskering is also another downside to using Immersion Tin as a surface finish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.

The Immersion Tin finish is not very durable, therefore, PCBs with Immersion Tin finish must be handled with caution. It provides a consistently flat surface approximately 20-40 micro inches in thickness.

Why Is It Used?

Immersion Tin has been primarily used as an alternative to a lead-based surface finish. It is also used for its very flat and smooth finish making it ideal for fine geometries and fine pitch surface mount components.

Another reason to use immersion tin is sustainability. Elements that may be difficult to source on a consistent basis are used in other finishes, such as ENIG or HASL. Additionally, it uses less water and chemicals in the application process.

It is also easier to re-work.

As with all of the surface finishes we’ve compared so far, there are a number of advantages and disadvantages. Here are the advantages and disadvantages for Immersion Tin.

Advantages:

• Immersion Tin Finish allows excellent flatness (suitable for SMT), good for fine pitch / BGA / smaller components
• Immersion Tin has mid-range cost for lead-free finish
• Press fit suitable finish
• Retains good solderability after multiple thermal excursions
• Suitable for horizontal production line.
• Suitable for fine geometry processing, lead-free assembly.

Disadvantages:

• Sensitive to handling.
• Short shelf life, tin whiskering can occur after 6 months
• Aggressive to solder masks
• Not recommended to use with peelable masks
• Not a suitable choice for a contact switches.
• Special setting requires (soft probe landing) for electrical test

Here is a full table with some of the pros and cons along with some of the most common uses for the Immersion Tin surface finish.

 

Immersion Tin PROS

RoHS Compliant

Great for Fine Pitch / BGA

Cost Effective

Highly Stable

Simple Processing

Very Flat Surface

Reworkable

Excellent Solderability

CONS

Subject to Tarnishing

Silver Metal Whiskering

High Friction Coefficient

Sensitive to Contaminants

Strict Storage and Handling Requirements

Difficulty with Electric Testing

 

Properties of White Tin Properties White Tin

Contact Applications

No

Deposit

Immersion

Exposed Copper

No

Handling

Normal

Process Control

Fair

Process Cost

Medium

Required Thermal Cycles

>2

Shelf Life

Medium (9-12 Months)*

SMT

Flat

Thickness

25 - 60 µin

Thin Board Finish

Yes

*Requires Unique Storage Techniques µin = Microinches

Applications of White Tin Applications White Tin

BGA & μBGA

Yes

Contact/Connector

No

Fine Pitch SMT

Yes

Flip Chip

Yes

High Reliability

Medium

RoHS Compliant

Yes

Solder Joint Integrity

Good

Wire Bonding

No

PCB surface treatment refers to the coating between components and bare board PCB. It is used for two basic reasons: to ensure solderability and to protect exposed copper circuits. Because there are many types of surface treatment, and each surface treatment has its own advantages and disadvantages, there is no perfect surface treatment. Therefore, choosing the right surface treatment is not easy. The following is a simple analysis of several common PCB surface treatment methods for reference!

Part 1：What is PCB surface treatment?

PCB surface treatment is the metal-to-metal connection between bare copper and components in the weldable area of the printed circuit board. The circuit board has a base copper surface that is easily oxidized without a protective coating, so a surface finish is required.

PCB surface treatment is the most important step in the PCB manufacturing and assembly process. It has two main functions, one is to protect the exposed copper circuit, and the other is to provide weldable surface components to the PCB when welding.

As shown in the figure below, the surface treatment is located on the outermost layer of the PCB, on top of the copper layer, acting as a copper "coating".

Part 2：PCB surface treatment types

1. hot air solder leveling (HASL)

2.Immersion tin (ImSn)

3.electroless nickel immersion gold (ENIG)

4.organic solderability preservatives (OSP)

5. Immersion Silver (ImAg)

6. Electroless nickel electroless palladium electroless immersion gold (ENEPIG)

7.Electrolytic nickel/gold (hard gold/soft gold)

Part 3：PCB surface treatment process

1.Hot air solder leveling (HASL)

HASL is the main surface treatment used in the industry. The process involves dipping the board into a melting tank of tin/lead alloy and then removing excess solder using an "air knife" that blows hot air across the board's surface.

Hot air solder leveling (HASL) is now widely used in SMT processes. There are three main points:

1) PCB should be immersed in molten solder;

2) Before the solder solidifies, the wind knife blows the liquid solder away;

3) The air knife can minimize the meniscus of the copper surface solder and prevent solder bridging.

Advantages:

• Cheap/low cost

• Longer storage time

• After the PCB is finished, the pad is completely covered with tin before soldering

• Lead-free (RoHS compliant) models are widely available

• Mature surface treatment options

• Visual inspection and electrical measurement are possible

Disadvantages:

• Poor flatness due to the natural meniscus of molten solder

• Not applicable to capacitive touch switches

• Cannot be used in high density interconnect (HDI) structures

• Not suitable for fine spacing

2.Immersion tin (ImSn)

Tin immersion (ImSn) is a metal finish deposited by a chemical displacement reaction and applied directly to the base metal (i.e. copper) of the circuit board.

Since all current solders are tin-based, the tin layer can be matched to any type of solder. From this point of view, tin immersion process has great prospects for development. However, the previous PCB after the tin dipping process appeared tin whiskers, tin whiskers and tin migration during the welding process will bring reliability problems. Therefore, the use of tin leaching process is limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure have a granular structure, overcome the previous problems, and also have good thermal stability and weldability.

Advantages:

• Excellent flatness (for SMT), suitable for fine pitch /BGA/ smaller components

• Lead-free surface treatment technology with medium cost

• Maintain good weldability after multiple thermal shifts

Disadvantages:

• Not suitable for long-term storage

• Tin whiskers can cause short circuits and solder joint problems

• Not a suitable choice for contact switches

• Electrical testing is difficult

3.Electroless nickel immersion gold (ENIG)

ENIG,or Electroless Nickel Immersion Gold finish has historically been the best fine pitch (flat) surface and lead-free option.

ENIG is a two-step process that covers a thin layer of gold on top of a thin layer of nickel. Nickel acts as a barrier to copper and is the surface to which the components are actually welded,while gold protects nickel during storage.

Advantages:

• Suitable for lead-free (RoHS compliant) welding

• Excellent surface smoothness

• Long shelf life, durable surface

Disadvantages:

• Expensive, use gold

• The process is complex and difficult to control

• Black pad/Black nickel

4.Organic solderability preservatives (OSP)

OSP (Organic solderability preservatives) usually uses a conveyor process to coat exposed copper with a very thin material protective layer, thereby protecting the copper surface from oxidation.

This film must have the characteristics of oxidation resistance, thermal shock resistance, moisture resistance, etc., to protect the copper surface from rust (oxidation or vulcanization, etc.) under normal conditions.

However, in subsequent high-temperature welding, this protective film must be easily removed by flux quickly. In this way, the bare clean copper surface can be immediately combined with the melted solder, forming a strong solder joint in a very short time.

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

Advantages:

• Flat surface

• Simple process, suitable for lead-free welding and SMT

• cost-effectiveness

• Environmental protection

Disadvantages:

• Short shelf life

• It is not convenient for visual and electrical measurement

• Not suitable for PTH (plated through holes)

5.Immersion Silver (ImAg)

Immersion silver is a non-electrolytic chemical surface treatment applied by dipping a copper PCB into a silver ion tank. 

Considering modern environmental issues such as RoHS and WEE, immersion silver is more environmentally friendly than HASL and ENIG. It is also popular because it costs less than ENIG.

Even if the PCB processed by the immersion silver process is exposed to high temperature, humidity and pollution, it can still provide good electrical properties and maintain good weldability, even if it will lose its luster. 

Advantages:

• High weldability

• Good surface smoothness

• Low cost and lead-free (RoHS compliant)

Disadvantages:

• High storage requirements

• Easily contaminated

• Electrical testing is difficult

6. Electroless nickel electroless palladium electroless immersion gold (ENEPIG)

Compared to ENIG, ENEPIG has an additional palladium layer between nickel and gold, which further protects the nickel layer from corrosion and prevents the black mat that can appear in ENIG finishes.

Advantages:

• A flat surface is provided Shelf life of 12 months or more

• Lead-free (RoHS compliant) Nickel corrosion - or "black pad

• syndrome" - is not a problem

Disadvantages:

• Complicated process

• High cost

• Immature technology

7.Electrolytic nickel/gold (hard gold/soft gold)

Electrolytic nickel gold is divided into “hard gold” and “soft gold”. Hard gold has a lower purity and is commonly used for gold fingers (PCB edge connectors), PCB contacts or other hard-wear areas. Gold thicknesses may vary according to requirements. Soft gold is purer and is often used in wire bonding applications.

In general, with all other factors being equal, electrolytic nickel/gold is the most expensive PCB surface treatment. 

However, some applications do require electrolytic nickel/gold surface finish.

Part 4：Key Considerations When Choosing Your PCB Finish

PCB surface treatment selection is the most important step in PCB manufacturing, because it directly affects the process yield, the number of reworks, the field failure rate, the testing capacity, the scrap rate and the cost.

1. PCB surface treatment - pad flatness

As mentioned earlier, certain surface treatments can result in uneven surfaces, which can affect performance, weldability, and other factors. If flatness is an important factor, consider a surface finish with a thin, even layer. In this case, appropriate options include ENIG, ENEPIG, and OSP.

2. Weldability and wettability

Solderability is always a key factor when using PCBS. Certain surface treatments such as OSP and ENEPIG have been shown to hinder weldability, while others such as HASL are well suited.

3. Storage conditions

As mentioned earlier, certain surface treatments (such as OSP) can make PCBS brittle when handled, while others improve durability.

4. Welding cycle

How many times does the PCB have to be welded and reworked? Many finishes are ideal for rework. However, other methods such as immersion tin are not suitable for rework.

According to the above introduction to each surface finish, some attributes are the most important elements as selection criteria.All in all, for the choice of surface finish, the key is that all possible factors must be considered to obtain the desired performance.

If you want to know how the surface finish affects the price, you can contact us at sales@tdt-ele.com.

 

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