We’ll compare OSP vs. ENIG vs. ENEPIG types of surface finish to see how they perform and their application fields.
Copper is one of the best conductors of electricity. So printed circuit board manufacturers use the metal to create copper pads and traces.
However, copper can oxidize when exposed to air, becoming less conductive. Oxidation is not an issue for copper traces because they have insulating prepregs above them and substrates below.
But the exposed flat surface (pad) is at risk of oxidation. So it needs a PCB surface finish. Read on to learn more!
What Is OSP?
The material is a water-based organic compound that bonds selectively to copper surfaces.
It creates an organic coating that safeguards the copper from oxidation.
Besides being an organic finish, OSP is more environmentally friendly than lead-free surface finish options because it consumes less energy.
And after application, it creates an excellent flat surface. Since it uses water, OSP also creates a smooth surface, which is ideal for soldering components.
OSP Creation Process
You require a conveyorized chemical process or vertical dip tank to create this organometallic layer.
But the general steps include the following, with several rinses in between.
Before application, you must clean the pad to eliminate solder fragments, fingerprints, corrosive chemicals, oily substances, toxins, and other contaminants.
This step involves etching the surface to strengthen the connection between the OSP and the circuit board pad.
Also, it minimizes oxidation. However, you must etch the surface slowly and consistently to get good results.
Blank pads on an unassembled circuit board
The first rinsing step requires using a sulfuric acid solution to prepare the surface for applying the OSP surface finish.
Apply the water-based organic compound to layer the OSP using the absorption process. This compound contains compounds like:
These form a thin layer above the copper surface. And you can adjust the coating thickness by altering the concentration of these compounds and the immersion time.
This process can be a little confusing. It involves infusing the OSP solution with ions to enable easy elimination when soldering.
These ions push other destructive ions out of the solution. It would be ideal to de-ionize the OSP before the preservatives accumulate.
These will avoid surface finish tarnishing due to the presence of these destructive ions.
The last step involves drying the applied OSP layer.
Advantages of OSP
- Lead-free organic finish
- Straightforward application process
- Cost Effective & affordable
- Creates a flat, smooth PCB surface finish
- Repairable surface finish
Disadvantages of OSP
- Not ideal for plated-through holes
- Requires sensitive handling (cannot handle hot temperatures for extended periods)
- Short shelf life
- Can cause in-circuit testing (ICT) issues
- No way to measure surface finish thickness
- Limited to RoHS applications
- Fine-pitch devices: This technology is ideal for fine-pitch devices because it doesn’t need coplanar pads or ridged, uneven surfaces.
- Surface-mount technology: OSP saves time to enable quick assembly when attaching components directly to PCB boards.
Surface mount devices on a PCB
- Server boards: OSP has a wide range of applications (low-end to high-frequency applications).
OSP Issues Post Soldering
The color of OSP finishes can change during soldering, so you must take care of some factors, such as:
- Etching quantity
- Soldering duration
- OSP thickness
And two scenarios can arise after the soldering process. The first is minimal oxidation, and the second is increased oxidation.
In the first scenario, the OSP starts with a uniform color, then becomes darker.
This scenario has no adverse effects, so you can leave the surface finish untouched.
Oxidation occurs if the flux cannot eliminate oxidation, causing the OSP to change from brown to dark brown.
This situation will decrease the soldering effectiveness.
The only way to ensure the surface is ideal for soldering is by verifying the following.
- OSP thickness is within a limited value
- Micro etching is within some range
- Contaminants get eliminated during manufacturing
What Is ENIG?
ENIG stands for Electroless Nickel Immersion Gold. It is costlier than OSP, but manufacturers prefer it due to its durability, reliability, excellent solderability, and suitability for wire bonding.
A circuit board with an ENIG surface finish
The finish creates a double coat of nickel and gold.
Nickel metal acts as a barrier layer between the two metals to keep copper from diffusing into the gold.
On the exposed surface, gold protects the pad from corrosion while preventing nickel from damage.
Applying these layers starts with electroless deposition (nickel) and immersion in gold. The steps include:
- Surface etching
- Activator application
- Post dipping
- Nickel application
- Gold application
Since gold is expensive, the immersion gold layer is thin (2-8 μin) compared to 120-240 μin for nickel.
Advantages of ENIG
- Creates a smooth, flat surface
- More durable than OSP (the surface finish has higher abrasion and corrosion resistance)
- Solidifies with time (ideal for sensitive parts)
- Moisture-resistant (maintains solderability in storage)
- Suitable for plated through holes
Plated through holes in a PCB
Disadvantages of ENIG
- Flammable (can uncover heat-sensitive wires, causing electrical fires)
- Requires professional installation and unique materials
- Creates weak solder joints (thin gold layer does not have enough surface area for solder distribution)
- Can form black pads
- Wire bonding: ENIG allows minimal aluminum wire bonding, even though aluminum is incompatible with gold.
- Complex surface components: ENIG creates a durable flat surface (ideal for chip packages like QFP and BGA).
A BGA pad on a circuit board
- High-reliability applications: Since ENIG is a durable, reliable, common surface finish, it is ideal for boards used in military, aerospace, medical, and high-end consumer products.
What Is ENEPIG?
ENEPIG is an acronym for Electroless Nickel Electroless Palladium Immersion Gold.
So the only difference between this surface finish and ENIG is the electroless palladium layer.
And as you can assume, ENEPIG came after ENIG. So palladium got introduced to solve a problem.
A gold PCB finish
Nickel usually corrodes in ENIG surface finishes, creating black pads. The palladium layer prevents nickel layer corrosion, eliminating the black pad syndrome.
Electronics industry experts usually refer to ENEPIG as the universal finish. Why? Because its qualities make it applicable to a wide range of boards.
The finish creates an excellent flat surface, is easy to process, has no lead, and can withstand several reflow cycles.
Also, the finish is compatible with RoHS directives and several lead-free solders. But most importantly, it has a long shelf life.
The only downside is the cost. A combination of nickel, palladium, and gold makes ENEPIG very expensive.
Advantages of ENEPIG
- RoHS-compliant (Lead-free)
- Produces a flat surface
- Creates solid solder joints
- Long shelf life
- Zero black pad possibility
- Wire bondable
- Handles multiple reflow cycles (multi-cycle assembly)
Disadvantages of ENEPIG
- More expensive than ENIG
- Re-workable but with limitations
- Has processing limits
The applications of ENEPIG resemble those of ENIG. But this surface finish can meet more stringent requirements for mounting different packages.
These include THT, SMT, wire bonding, and BGA.
So the technology creates the ideal surface finish for applications demanding more reliability and high component density.
Think of high-performance electronic products in the military, aerospace, aviation, and medical industries.
A section of a circuit board in military equipment
From the comparison above, ENEPIG is the clear winner.
All three are lead-free and RoHS compliant, but ENEPIG is more durable, reliable, and suitable for high-performance devices. The only downside is its cost.
So you can’t use the gold surface finish in all applications. Therefore, OSP and ENIG still have their respective applications to match consumer budgets.
That’s it for this article. Comment below to let us know if we left something out. We’d love to hear your feedback.