ENIG PCB surface finish is one of the most popular types of PCB plating.
With PCB surface finish we mean the coating applied on a bare Printed Circuit Board (PCB). The main goal of a HASL finish or ENIG PCB finish is to protect the exposed copper circuits from corrosion. The challenge is to pick a PCB coating material that still allows for the electronic components to be easy and firmly soldered onto the circuit board.
Since there are different types of surface finishes available for Circuit Boards, selecting the right one can be challenging.
Especially now that surface mounts have become more complex, and regulations such as RoHS and WEEE which enforce environmental protection have changed the industry standards.
- Top 5 PCB Surface Finishes List
- 1- ENIG PCB Finish
- 2- HASL PCB Finish
- 3- OSP PCB Finish
- 4- ISn PCB Finish
- 5- IAg PCB Finish
- PCB Surface Finish Comparison Table
- PCB Surface Finish: the Definitive FAQ
Top 5 PCB Surface Finishes List
- ENIG finish (Electroless Nickel Immersion Gold)
- HASL finish (Hot Air Solder Leveling)
- OSP Finish (Organic Solderability Preservative)
- ISn finish (Immersion Tin)
- IAg finish (Immersion Silver Plating)
Now we’ll explain the top 5 PCB coatings in detail, giving you the pros & cons for each type of PCB surface finish.
1- ENIG PCB Finish
ENIG PCB finish is a double layer metallic coating with nickel that acts both as a protective barrier to the copper circuits and a surface on which the components are welded.
A layer of gold protects nickel during storage.
The ENIG finish was developed as a response to changing industry trends that focuses on lead-free requirements and the increasing complexity of surface mounted components such as BGA, which require very flat surfaces.
ENIG coating is hard and durable. It also has a long shelf life, lasting for years. However, it is also more expensive than any of the other finishes on this list.
Sometimes, it can have a failure mode known as “black pad syndrome,” which is the accumulation of phosphor between the gold and nickel layers. This can cause broken surfaces and faulty connections.
The ENIG PCB plating process
The Electroless Nickel step, standing for the EN in ENIG, is an auto-catalytic process that involves depositing nickel on the palladium-catalyzed copper surface.
The reducing agent, containing nickel ions, must be replenished to provide the proper concentration, temperature, and pH levels necessary to create a consistent coating.
During the Immersion Gold (IG) step, the gold adheres to the nickel-plated areas through a molecular exchange, which will protect the nickel until the soldering process.
The gold thickness needs to meet very precise tolerances to ensure that the nickel maintains its solderability.
ENIG PCB Finish Advantages
- Flat surfaces
- Good for PTH (Plated Through-Hole)
ENIG PCB Finish Disadvantages
- Black pad syndrome
- Not good for rework
When Should You Use ENIG PCB Surface Finish?
An ENIG finish has excellent corrosion resistance; it’s suitable for aluminum wire bonding, is excellent for fine-pitch technology, has excellent solder-ability, and remarkable shelf-life; an ENIG finish is RoHS compliant as well.
If your budget allows it, this may be the best option for any requirement you have.
2- HASL PCB Finish
This used to be the most popular surface finish choice, in part because of its low cost and robust structure, but as technology has progressed over time, manufacturers now realize its limitations.
The rise of more complex surface mount technology has made it evident that a HASL finish has limited functionality.
A significant downside of HASL finish is that it creates an uneven surface, making it unsuitable for PCB designs that include fine pitch components.
There are generally two different categories of HASL finish: lead and lead-free. Lead-free boards are commonly plated in solder, containing mostly tin with small amounts of silver and copper.
Lead-based boards generally are coated in eutectic tin-lead solder with possible filler metals.
Unfortunately, the vast majority of HASL finish boards contain lead and very harsh flux chemicals, which can potentially cause a wide range of health and safety issues for individuals working with HASL in an improperly controlled environment.
The HASL PCB Plating Process
The process involves submerging the circuit boards in a bath of molten tin/lead alloy and then removing excess solder using ‘air blades,’ which blow hot air through the plate surface.
The HASL coating process will expose the PCB to temperatures up to 265 ° C; this is good as it will identify any potential delamination problems long before expensive components are connected to the board.
HASL PCB Finish Advantages
- Low Cost
- Widely Available
- Good Shelf Life
HASL PCB Finish Disadvantages
- Uneven Surfaces
- Not Good for Fine Pitch
- Usually Contains Lead
- Thermal Shock to the PCB
- Solder Bridging
- Plugged or Reduced PTH’s (Plated Through Holes)
When Should You use HASL PCB Surface Finish?
HASL finish is preferred for hand soldering as joints are easy to be formed. If the welding alloy is close to the HASL alloy, adhesion will be smooth and robust as the metals will interact at a molecular level.
This strong bond also makes HASL an excellent finish for high-reliability applications.
3- OSP PCB Finish
OSP surface finish is a water-based organic surface finish utilized in copper pads. It bonds to copper and protects the copper pad before welding.
OSP surface finish is an environmentally friendly finish, it provides a flat surface, is lead-free, and requires little maintenance, should be noted that it’s not as strong as a HASL finish and can be sensitive when handled.
This organic finish can keep copper from being oxide, thermal shock or moisture
The OSP PCB Plating Process
This process maintains the copper surface from oxidation by applying a thin protective layer of a water-based organic compound over the exposed copper usually using a conveyorized method.
The water-based chemical compound belongs to the azole family, such as benzotriazoles, imidazoles, and benzimidazoles, all of which are adsorbed on the surface of the copper with coordination formed between them and the copper atoms, this leads to the production of a protective film.
The optimal storage conditions are relative humidity (30-70% RH), moderate temperature (15-30%), not sunlight exposure.
These are the steps to follow for OSP surface finish
- Clean the copper
- Topography Enhancement
- Acid Rinse
- OSP application
- Deionization Rinse
OSP PCB Finish Advantages
- The manufacturing process is simple and reworkable
- OSP coated boards perform better in terms of solder wetting.
- Low Cost
- Good for SMT assembly
- Flat Surface
OSP PCB Finish Disadvantages
- Thickness is difficult to measure
- Not suitable for plated through-hole
- Highly sensitive
When Should You use OSP PCB Surface Finish?
Consider this option when you require your PCB to be RoHS compliant, with a flat surface and good wettability for best solderability, not to say that other types of finish don’t offer this as well.
4- ISn PCB Finish
Immersion Tin is a chemical process that applies a thin layer of tin on the copper, and it provides an excellent flat surface for fine pitch components use. Immersion Tin has been used mainly as an alternative to a lead-based surface finish.
Apart from being ideal for fine pitch, surface mount, components thanks to its flat and smooth surface, immersion tin is also used for a sustainability issue as elements that are somewhat difficult to source continually are used in other finishes, like ENIG finish or HASL.
Additionally, it uses fewer water and chemicals in the application process.
It is also easy to do any re-work in the event of an error in the application of components on the board.
The Immersion Tin Plating Process
Immersion tin is applied using an electroless chemical bath that applies approximately 0.7 to 1.0 microns (or about 44 microinches) of tin to the copper.
Copper and tin have a strong mutual affinity making the diffusion of one metal into the other inevitably easy; this directly impacts the shelf life of the deposit and the performance of the ﬁnish.
ISn PCB Finish Advantages
- High Reliability
- Can substitute for reflowed solder
ISn PCB Finish Disadvantages
- Exposed Tin on Final Assembly can Corrode
- Process Uses a Carcinogen (Thiourea)
When should you use ISn PCB Surface Finish?
Immersion tin is RoHS compliant, it provides a flat surface and has a right solderability level, however, the shelf life s quite short, on average is only six months.
5- IAg PCB Finish
This is a surface finish that gained popularity after the RoHS and WEEE directives took effect.
Immersion Silver applies a Lead-Free layer of silver-plated onto a PCB, to protect copper traces from corrosion.
Silver surface finishes have excellent solderability, comparable to solder plating, accommodating a straightforward and risk-free PCB Assembly Process.
Immersion Silver Plating also boasts a moderately long shelf life, though still less than other ones, such as an ENIG finish.
Silver is sensitive to contaminants, both in the air and on the board, and thus it must be packaged as soon as possible to prevent tarnishing.
IAg PCB Finish Advantages
- Fine pitch
- A good alternative to ENIG finish
- High stability
IAg PCB Finish Disadvantages
- Silver Whiskering
- Some Systems Cannot Throw into Micro-Via Aspect Ratios of > 1:1
- High Friction Coefficient/Not Suited to Compliant-Pin Insertion (Ni-Au Pins)
When Should You Use IAg PCB Surface Finish?
Immersion Silver plating is a popular choice because silver is the most electrically conductive metal available and thus ideal for High-Speed PCB applications.
Silver surface finishes can be applied to copper traces with an electroless immersion reaction, displacing the copper layer.
While we did attempt to give you a general idea of when should you be more inclined to use a specific type of PCB finish, it all comes down to the intended functionality of the end product.
Here are some questions that will guide you in your search for the most suitable type of PCB finish for your product requirements.
- Will my circuit board be exposed to many solder cycles?
- Is flatness critical?
- What’s the minimum shelf life required?
- Is budget a constraint?
- How wettable/solderable do I need te PCB to be?
- Does my PCB need to be RoHS compliant?
- What kind of storage conditions will I have to keep the PCBs?
- Does the PCB need gold or aluminum wire bonding?
Once you answer all of those questions, you’ll have a better understanding. The following table will help you decide.
PCB Surface Finish Comparison Table
PCB Surface Finish: the Definitive FAQ
What is a PCB surface finish?
Why is PCB Surface Finish Needed?
What are the different types of PCB surface finish?
PCB finishes can be classified into different categories based on different technologies and involved chemical substances: HASL (hot air soldering leveling), Immersion Tin/Silver, OSP, and ENIG finish.
What is OSP PCB surface finish?
OSP stands for Organic Solderability Preservative. This is a type of PCB surface finish. Its main distinguishing characteristic is that it is water-based, therefore making it environmentally friendly. It is also lead-free and provides a low planar surface.
What is OSP surface finish in PCB ?
OSP in PCB refers to a type of PCB surface finish. OSP means Organic Solderability Preservative. A surface finish on a PCB is needed for 2 reasons to ensure solderability, and to protect exposed copper circuitry.
Is OSP Conductive?
OSP is conductive. OSP coated PCBs also performs better when it comes to solder wetting when flux meets vias and pads.
What is Enig in PCB?
ENIG in PCB refers to a coating method for printed circuit boards. ENIG means Electroless Nickel Immersion. Its distinguishing characteristic is that it uses an electroless nickel and a thin layer of immersion gold to protect the PCB from oxidation.