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.
What Is the Meaning of Black Pads?
A black pad is a layer of dark nickel surface deposited on the exposed areas of your PCB. This layer forms during the fabrication process due to the overuse of phosphorus in the gold deposition procedure.
The metal's black pad reveals oxidation and corrosion of the electroless nickel. This assembly process occurs at the seams where various metal components join together, but as the corrosion worsens, it gradually spreads.
During manufacturing, the deposited gold thickness and nickel oxidation increase, causing insufficient solderability and poorly formed soldering joints.
When those solder joints come under strain, they break easily and expose rusted nickel beneath, earning the name "black pad."
Why does Black Pad Occur?
High Phosphorus Content
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
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-corrosion
Brittle fracture
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
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
How Can We Solve the Issue of the Black Pads?
Sadly, until the operation is over and the result evaluates, 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.