There are many PCB surface finish techniques to guarantee optimal performance and increase service lifespan.
Today’s guide focuses on ENIG surface technique – keep reading to learn more.
- What is ENIG in PCB?
- What are the Process Steps in ENIG Plating?
- What are the Benefits of ENIG PCBs?
- What are the Disadvantages of ENIG PCBs?
- What is ENIG RoHS?
- Is ENIG plating Lead-free?
- Which Materials are used in ENIG Plating?
- What is the comparison between ENIG and HASL Finish in PCB?
- What is the Thickness of ENIG Plating?
- What is a Black Pad in ENIG PCB?
- Is there a difference between ENIG and ENEPIG plating in PCB?
- What are the Solderability issues of ENIG and how can you solve them?
- What is the Shelf Life of ENIG PCB?
- What are the Standards of ENIG PCBs?
- How does ENIG Plating and Hard Gold Plating compare?
- What are the Right ways of Handling ENIG PCBs?
- What is ENIG PCB Baking?
- What should you Consider when Determining PCB Surface Finish?
- How can ENIG PCB get Oxidized?
- What should you Watch out for in ENIG Plating?
What is ENIG in PCB?
Electroless Nickel Immersion Gold (ENIG) is a form of PCB surface plating that protects them from corrosion and other defects.
It comprises of a double layer metallic coating where a thin gold layer covers an electroless nickel layer.
The electroless nickel shields the copper pads from oxidation and provides a good soldering surface for the components.
Gold helps in protecting the nickel from oxidation especially during storage.
This facilitates an outstanding and durable electrical performance of the PCB that lasts for many years.
This plating method dominates the market in the PCB industry, thanks to RoHS compliance and its versatility with component assembly.
What are the Process Steps in ENIG Plating?
To achieve an ENIG finish on your PCB, there are several steps that you should follow.
For each of these steps, you need cautious designing, controlling, and monitoring to attain a desirable finish.
The process steps include;
Activation of Copper
- For this step, the objective is to make the copper layer active for the electroless nickel deposition.
The PCB first undergoes a cleaning process.
Cleaning helps to remove dust, fingerprints, oxidation residues, and further wet the surface to remove trapped air in the through-holes.
- Micro-etching follows to strip a layer of copper and modify its topography.
There needs to be careful consideration of the micro-etching base of which sulfuric/peroxide is the best choice.
You should monitor and maintain the dwell-time, operating temperature, bath life, and make-up for this step to be effective.
- Proper rinsing succeeds micro-etching to remove any residues.
This is because they cause oxidation which interferes with the palladium catalysts deposits in the next process.
You may also involve a catalyst pre-dip to remove the rinse water, oxidation traces and to acidify the surface.
- The surface further undergoes a catalyst bath that consists of palladium sulfate in sulfuric acid with low pH.
This bath creates a basis for which the nickel and later gold will deposit.
You should ensure proper rinsing of the surface after the bath to avoid palladium residues into the electroless nickel step.
- This step involves the use of nickel sulfate to generate nickel and sodium hypo-phosphite as the reducing agent.
The hypo-phosphate distributes the necessary electrons to reduce the nickel ion to nickel metal. Also, the specific temperate to facilitate this reaction should range between 175°F and 185°F in a weakly acidic pH solution.
- This eventually deposits a nickel layer on the copper surface. Nickel serves as a preventative layer that protects the copper from reacting with other components including gold.
In this step, the oxidation of nickel-metal to nickel ion produces electrons that facilitate the gold reduction in a solution.
These electrons reduce gold ions to a consistent gold metal layer protecting the nickel until the soldering process.
The gold thickness should match specific criteria to enable the nickel to keep its solderability.
What are the Benefits of ENIG PCBs?
There is an array of benefits that this type of finish offers.
Some of the benefits that make it one of the most dependable plating methods are;
Good Flat Surface
This type of plating achieves an even flat layer that enables the PCB to be compatible with complex surface components.
Some of the components that require such surface planarity are ball grid array (BGA) mounting and flip chips.
The immersion gold layer prevents the nickel surface from any oxidation.
This limits the chances of corrosion occurring on the board. Having this property prolongs the shelf life of the PCB.
Outstanding Electrical Performance
This type of surface finish facilitates good electrical conduction among the components on the board.
This makes it a better choice which manufacturers would opt for comparing to other types of finish.
Resists High Temperatures
ENIG has a good thermal diffusion property enabling it to withstand extreme temperature conditions.
This property enables it to preserve the functionality of the PCB regardless of environment conditions thus making it reliable.
Extended Shelf Life
This plating safeguards your PCB from any external defects including corrosions enabling it to last longer before its use.
You can keep them on storage for an average of 12 months or more.
This finishing method does not use lead in any of its process steps.
Therefore, you eliminate the exposure of lead, a harmful and fatal substance to human beings.
Ideal for Plated Through Holes (PTH)
ENIG plating provides a perfect solder surface making it easier for manufacturers to effectively work on the plated through holes.
Versatile with Electronic Products
This surface finish is widely suitable for use across electronic products.
Its ease of precise fitting and reliable performance make it desirable for computers, smartphones, medical appliances, etc.
This form of surface finish satisfies all the Restrictions of Hazardous Substances (RoHS) requirements for manufacturing PCBs.
Manufacturers, therefore, widely utilize this plating method when fabricating printed circuit boards.
What are the Disadvantages of ENIG PCBs?
This type of PCB plating has its share of shortfalls. Some of its disadvantages are:
- It is an expensive process in comparison to other types of surface finish. This is because it follows a complex process to come up with the result.
- It is not re-workable once you experience defects making it difficult to repair.
- It also experiences signal loss when you use it for a long time. This can affect the general performance of your device, especially for signal integrity applications.
- It is susceptible to experiencing a black pad which is an accumulation of phosphorus between the gold and nickel layers.
This can further cause surface fracturing and the connections to malfunction.
What is ENIG RoHS?
The Restrictions of Hazardous Substances (RoHS) is a regulation restricting the use of hazardous substances in electric and electronic devices.
Ten poisonous substances including Lead, Mercury, Cadmium, Hexavalent chromium, etc. fall under this restriction.
Using these substances to finish your PCB poses a high health risk to the manufacturers and consumers.
ENIG surface finish is RoHS compliant meaning that they are safe for use and handling without great risk.
Is ENIG plating Lead-free?
Yes, this type of plating is Lead-free.
It conforms to the RoHS directive for manufacturing PCBs making it environmentally friendly and offering a wide scope for use.
This surface finish also enjoys a large dominance in the PCB industry due to this characteristic.
Which Materials are used in ENIG Plating?
There are two main materials manufacturers use in this type of PCB plating, that is Nickel and Gold.
The nickel goes through an auto-catalytic chemical reaction in a liquid bath forming a nickel layer on the copper surface.
This electroless nickel layer prevents the copper surface from getting into contact with compounds that may cause oxidation.
An additional gold layer protects the nickel layer from oxidation while on storage and also provides a low contact resistance.
What is the comparison between ENIG and HASL Finish in PCB?
These are two different surface plating methods that you can use to finish your PCB. They compare in the following ways;
- ENIG is quite a complex method to plate your PCB making it more expensive. On the other hand, HASL is an uncomplex and a pocket-friendly method.
- ENIG is an ecofriendly method because it doesn’t use hazardous compounds such as Lead thus it is RoHS compliant. On the contrary, HASL coating contains Lead demanding special handling and you cannot use it for RoHS compliant products.
- When it comes to re-working, you can easily rework a HASL finish PCB without any difficulties. ENIG is quite difficult to undergo any re-works in case of damages made on it.
- ENIG has a good thermal diffusion and can withstand high temperatures. HASL on the other side is not able to withstand high temperatures and may lead to the boards being defective.
- ENIG is more suitable when working with plated through holes on your board while HASL is unsuitable for this.
- ENIG provides a flat surface making it ideal for Surface Mount Technology (SMT) components placement. The HASL surface is inconsistent causing it to be unsuitable for SMT.
What is the Thickness of ENIG Plating?
Achieving a consistent thin flat layer for both nickel and gold is vital when manufacturing PCBs with ENIG finish.
You need to constantly monitor the process, especially when bathing in a liquid solution to achieve this.
The two layers should also be different in their thickness dimension.
The electroless nickel layer needs to achieve a thickness ranging between 4 – 7 µm.
While the gold layer should vary from 0.05 – 0.23 µm.
What is a Black Pad in ENIG PCB?
This is one of the fabrication issues that occur when plating with ENIG finish.
This condition arises when the nickel bath contains excessive phosphorus leading to corrosion of the nickel layer.
The black pad can also emerge when there is an excessive gold deposit on nickel in the immersion process.
It is difficult to detect or see the black pad on the board when the plating process is complete.
It also leads to faulty solder joints which crack when there is an occurrence of stress.
This ends up being an irreparable and costly outcome for manufacturers.
Is there a difference between ENIG and ENEPIG plating in PCB?
Yes, these two plating methods differ in several ways. Some of their differences include;
- ENIG introduces an immersion gold layer directly on top of its electroless nickel layer. Instead, ENEPIG has an additional layer of electroless palladium between the nickel and the final gold layer.
- ENIG is subject to experiencing a black pad due to boundary corrosion between nickel and immersion gold. On the other side, ENEPIG is free from this defect as it introduces palladium between the gold and nickel layers.
- ENIG is a less costly contrast to ENIG due to the introduction of palladium as an extra ingredient.
- For wire bonding such as gold, ENEPIG is more and highly reliable in comparison to ENIG.
What are the Solderability issues of ENIG and how can you solve them?
Some problems arise with the soldering of ENIG solder joints.
Among these issues include; Non-wetting and solder joint cracks that cause an electrical continuity failure.
These issues may arise during the surface mount assembly process or when a customer is using the product.
The main cause of such issues is the corrosion of the nickel layer that is commonly called a black pad.
This affects the solder reflow process.
To prevent black pad, you need to take the following measures;
- Control the pH level of the electroless nickel bath.
- Analyze the stabilizer content that you use in the electroless nickel bath.
- During immersion gold, closely monitor the process to keep its thickness as ideal as possible.
What is the Shelf Life of ENIG PCB?
When using this type of surface finish, you can keep your PCB on storage for an average of 12 months.
This type of platting has a longer shelf life in comparison to other finish methods in the PCB industry.
What are the Standards of ENIG PCBs?
Several standards relate to this type of surface finish.
Some of them are;
IPC Standard 4552A
This set of standards address the performance specifications for ENIG plating thicknesses.
It specifies the acceptance criteria for its applications including soldering, wire bonding, and contact finish for PCBs.
This standard covers the design and fabrication for ball grid array (BGA) and fine-pitch BGA (FBGA) technology.
It centers on the inspection, repair, and reliability issues that printed circuit boards utilizing these packages may face.
How does ENIG Plating and Hard Gold Plating compare?
These two forms of PCB finish have varying characteristics.
They compare in the following ways;
- In ENIG, it takes a chemical reaction to enable the attachment of the gold layer to the nickel layer. While as in hard gold plating, the gold attachment happens through plating.
- The thickness of ENIG and hard gold plating vary. The hard gold layer is thicker in comparison with ENIG which is thinner.
- The cost of ENIG surface finish is relatively lower in comparison to that of hard gold plating.
- ENIG is easier for soldering, while hard gold plating is not suitable for soldering as it causes soldering defects.
- You are likely to experience short-circuiting with hard gold because of its dense layout. On the other hand, ENIG limits this as it only has nickel-gold on the pads.
What are the Right ways of Handling ENIG PCBs?
This finish needs proper handling as it is prone to oxidation and corrosion from exposure to moisture and humidity.
Its oxidation leads to poor wetting after soldering, which causes defects such as faulty joints bringing about board failure.
Oil and sweat from human hands can also cause tarnishing.
Proper handling of would therefore entail; wearing gloves and storing the circuit board in an airtight package to prevent oxidation.
What is ENIG PCB Baking?
This is a process of heating ENIG finish PCBs in an oven, intending to drive out moisture.
This process is undertaken before the surface mounting of the boards.
It is important to note that baking can also be damaging to your board if you overlook the right conditions.
You should always consult with your manufacturer to guide you on the appropriate conditions to observe before baking your board.
What should you Consider when Determining PCB Surface Finish?
Selecting the right finish is an important decision that you have to make before fabricating your board.
Some of the key factors to consider are;
- Application environment for your final product.
- Components you will install.
- RoHS compliance
- Shelf life
- Heat resistance
- Shock/drop resistance
- Production volume
- Volume and throughput
How can ENIG PCB get Oxidized?
Oxidation of such PCBs can happen under certain conditions including;
- If the gold and nickel density fall below the requirements of ENIG finish, they become porous and cause oxidation.
- Improper handling with bare hands, unclean countertops, and poor storage can also lead to oxidation.
What should you Watch out for in ENIG Plating?
There are several factors that you should pay keen attention to with this type of surface finish.
Some of them include;
This finishing method uses materials such as gold in its process which is expensive.
On the other hand, this coating method is also not reworkable once defects exist on them.
Therefore, the cost of production becomes sensitive as it can escalate tremendously.
This can occur in some ways if you do not closely monitor the plating process.
It is also not easily detectable and visible and can lead to solderability issues.
Therefore, pay close attention to avoiding black pad existence in your PCB.
Depending on your specific requirements, Venture Electronics offers a range of PCB surface finishes.
Contact us today for all your PCB fabrication needs.