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Black Pad: Everything You Need to Know

If you work with electronics, you've probably heard of the black pad. When your PCBs form a layer of dark nickel hyper-corrosion on the exposed areas of the board, we call it a black pad.

Black pads can cause all sorts of problems with your electronics. For example, they may result in shorts between traces or stop vias from attaching to the proper planes.

In this article, I'll explain what black pads are, how they form, and what you can do to prevent them from causing problems with your electronics.

 

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What Is the Meaning of ENIG Black Pads?

 

Although ENIG surface finishes have a long shelf life, they have a significant flaw known as the black pad.

This flaw occurs due to corrosion of the electroless nickel layer, and it increases as you deposit more gold above it.

Nickel corrosion begins around the edges and then spreads to the middle to form an end-to-end black pad.

Black pads reduce the surface area’s solderability, resulting in weak solder joints that can break under pressure, exposing the black surface.

An optoelectronic image sensor for a computer’s mouse with gold-plated contacts

An optoelectronic image sensor for a computer’s mouse with gold-plated contacts

 

Why does Black Pad Occur?

 

High Phosphorus Content

 

Phosphorus element in spherical form

Phosphorus element in spherical form

 

Due to the reflows and soldering process, the high pass phosphorus content in the gold deposition process generally causes black pads.

When there's too much phosphorus present, it diffuses into the nickel and causes it to oxidize. This process creates a barrier between the gold and the nickel, which prevents the formation of an adherent bond.

So, this can cause delamination and cracking of the reliable solder joints, leading to circuit boards' electrical shorts.

 

Gold Deposition Corrosion

 

The gold deposition is a necessary process to create the ENIG (electroless nickel immersion gold) surface finishes that are popular in the PCB manufacturing process.

Yet, this process can also lead to the formation of black pads if you don't do it correctly.

One of the reasons why black pads form during gold deposition is the use of aggressive gold baths.

This nickel bath process can cause nickel corrosion at a rapid rate, leading to the formation of black pads.

 

Corrosion of austenitic stainless steel

Corrosion of austenitic stainless steel

 

The high gold thickness can also contribute to the formation of black pads. Too much gold on the popular surface finishes of the nickel substrate can lead to excessive galvanic hyper-corrosion and the eventual formation of black pads.

To prevent black pad formation, use gold with a thickness of 2-4 μin, as specified by the IPC-4552 ENIG specification.

 

Galvanic hyper-corrosionGalvanic hyper-corrosion

 

Brittle fracture

 

Low carbon steel that has been applied fracture test

Low carbon steel that has been applied fracture test

 

Brittle fracture is a type of failure that occurs in materials under high stress, but they don't have the flexibility to withstand the stress and fracture suddenly without warning.

This failure generally appears as a black pad on the PCB's surface.

The most prevalent cause of brittle fracture is the transmutation of tin into nickel.

This transformation results in a thin layer of phosphorus that does not dissolve, compromising the metallurgical bonds.

Also, thermal stress, vibrations, and shocks can cause brittle fractures.

When this happens, it creates a cracked structure within the nickel leading to electrical shorts.

 

The Formation and Harm of Black Pad

 

The Nickel layer's quality primarily depends on the plating solution's formulation and temperature during the chemical displacement reaction. The key factor is how you treat it with acid gold water.

During the electroless plating process, the plating layer is obtained through an autocatalytic reaction of hypophosphite and nickel salt on the pad's surface.

This processing aid in determining how much phosphorus (P) is present in the final product.

Several studies determined that the industry standard phosphorus (P) ratio in chemical nickel deposition is 7-10%.

However, if the temperature fluctuates or the solution's composition isn't constantly regulated, this percentage will fall outside the desired range.

If the ratio of phosphorus content is low, the coating will be prone to hyper-corrosion from acidic gold water erosion.

This hyper-corrosion happens because the low phosphorous content in the metallic nickel ions plating causes the chemical substitution reaction to not go properly during the gold immersion process.

Removing acidic residue will be difficult if a significant number of cracks appear in the gold layer.

Acidic water will cause the electroless nickel surface to corrode and turn black.

On the other hand, if the phosphorus content is too high, the hardness of the formed coating will increase significantly, reducing weldability and affecting the batch solder joints’ reliability.

 

pH value scale chart for acid-alkaline solution

pH value scale chart for acid-alkaline solution

 

When you heat this solder pad to a high temperature, the gold that acts as a protective layer against hyper-corrosion will quickly dissolve into the solder paste.

Thus, the nickel will rust and oxidize, not forming an IMC (intermetallic compound) with molten tin.

The process results in a severe decline in the solder joint's reliability- even slight external forces can cause cracking.

 

Pickling paste

Pickling paste

 

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Performance Specification for ENIG

 

IPC released the ENIG IPC-4552 specification in 2002 when Leaded solder was the dominant material for component-to-PCB attachment.

This specification requires the electroless nickel thickness to be 3-6 μm, while the immersion gold part should be 0.075-0.125 μm.

However, with the increasing price of gold, IPC revised this specification in 2012, decreasing the immersion gold layer’s lower thickness limit to 0.04 μm.

 

Gold contact pads on an LCD TV’s circuit board

Gold contact pads on an LCD TV’s circuit board

 

This IPC-4552A specification also includes a section for measuring the phosphorus levels in the electroless nickel layer and explains how to evaluate the ENIG deposit’s potential to cause corrosion.

The revised standard focuses on the nickel layer’s resistance to corrosion, as well, due to a poorly controlled plating process when depositing the gold above it.

An exchange reaction occurs during immersion, where gold atoms displace nickel atoms, causing corrosion.

Therefore, corrosion is gold removing nickel ions from the surface, and these free ions build up in the gold.

 

A printed circuit board with an ENIG surface finish

A printed circuit board with an ENIG surface finish

 

Hyper Corrosion of Nickel

 

Corrosion will always occur, but end-to-end black pads form when hyper-corrosion of nickel occurs when immersing the pads in gold.

This galvanic hyper-corrosion reaction depletes the nickel metal layer and enriches phosphorus.

You won’t be able to tell if the pad has a corroded surface underneath because the gold layer will cover the damage.

But when you solder components on it, the black pad prevents intermetallic bonding and surface wetting using the solder.

Therefore, the solder joints will be weak and fracture when stressed.

Once broken, the component will peel the gold layer, exposing the dark corroded nickel, which is the black pad.

Failed solder joints on a printed circuit board

Failed solder joints on a printed circuit board

The IPC-4552A specification requires the average gold thickness to be three standard deviations below 3.94 μin (0.1 μm) and three above 1.58 μin (0.04 μm) to ensure the board is reliable while reducing manufacturing costs.

 

How Can We Solve the Issue of the Black Pads?

 

Sadly, until the operation is over and the result is evaluated, there is no way to detect any black pad creation.

To identify if a black pad is present, you can look for specific flaws (cracked structure, non-planar surface, etc.) and then plan your next steps accordingly.

Some solutions to take control of black nickel and its consequences:

  • Verify potential ENIG providers
  • Remove oils and residues for PCB etching preparation
  • Use chelating agents
  • Prevent nickel plating out into tanks
  • Regularly clean the area where the black pad occurred
  • Make sure the immersion gold process is well-controlled and has a suitable nickel and gold ratio
  • Keep tabs on the pH level to ensure that you plate the correct amount of phosphorus.

 

Are Black Pads Currently a Problem for ENIG?

 

Even though the ENIG finishing process uses nickel and gold, black pads are a big issue.

To avoid any black pads, verifying your potential ENIG providers is essential. Make sure they use the latest technologies and understand how to control their process correctly.

 

Summary

 

Black pads can be a severe issue in the ENIG finishing process. However, you can take steps to avoid them by working with a reputable provider and keeping tabs on the process.

We hope this article helped clear things up for you. If you have any questions, please feel free to contact us.

 

 

Special Offer: $1 for 5 PCB Assemblies!

One requirement only: Order must be placed using a company account.
Please email [email protected] for details.
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