Black pad is notorious for causing many PCB failures. But most people don’t suspect it when such situations occur. In truth, a defective component or inadequate assembly is the first thing that comes to mind during such mishaps.
But that’s not always the case. Undoubtedly, many issues can cause problems on a PCB, but black pads are one to always prevent before assembly.
So, what is a PCB black pad? And how can you prevent them? In this article, we’ll discuss the details of the problem and more.
What is a PCB Black Pad?
A man working on a PCB
A black pad occurs when a layer of dark nickel settles on the exposed areas of a PCB. Unfortunately, this nickel layer forms during fabrication, especially when manufacturers overuse phosphorus in the gold deposition stage.
Also, black pads lead to oxidation and corrosion from the electroless nickel. And this situation becomes an issue during the assembly process. As it worsens and corrosion spreads, it becomes harder to join various metal components.
As a result, manufacturers will experience low solderability and solder joints with weak integrity. Sadly, those solder joints will break when strained and expose the rusted nickel underneath.
Why does Black Pad Happen?
Several factors can cause black pads on PCBs. Let’s explore some of them.
Gold Deposition Corrosion
The gold deposition is a crucial aspect of creating ENIG finishes in the PCB manufacturing process. However, this process is one of the culprits for black pads.
Using aggressive gold baths on the PCB can kickstart speedy nickel corrosion, which causes the notorious black pad. Also, a heavy gold thickness may cause this problem.
A PCB with excess gold on its surface finishes will lead to galvanic hyper-corrosion and the creation of black pads.
Excessive Phosphorus Content
High phosphorus content in the gold deposition process, especially during reflows and soldering, can cause black pads.
In addition, excess phosphorus surface tends to permeate the nickel, causing oxidation. Eventually, the process will activate delamination and crack solder joints, causing electrical shorts.
A brittle fracture occurs when certain materials suffer from high stress, particularly when they can’t withstand pressure. Stressed materials snap without warning and can cause several problems on a PCB. Generally, a brittle fracture appears as a black pad.
Usually, brittle fracture occurs when tin transmutes into nickel. This process creates a thin phosphorus layer that won’t dissolve, affecting the board’s metallurgical bonds.
In addition, vibrations, shocks, and thermal stress can also create brittle fractures. Such situations will cause cracked structures inside the nickel, causing electrical shorts.
How to Prevent Black Pads
Sadly, it’s almost impossible to detect black pads during the manufacturing process–and that’s where the problem starts. It becomes noticeable during assembly or when stressed joints and open circuits create a catastrophic issue.
But that’s not all. You may not notice black pads until you attach components to the PCB unless you perform non-destructive tests on the board. Therefore, preventing black pads are critical responsibility of suppliers and manufacturers.
However, knowing that high phosphorus causes black pads is not the same as controlling these levels during manufacturing. Here are some solutions manufacturers can use to prevent black pads:
- Verification of potential ENIG providers.
- Using chelating agents.
- Careful inspection of pH levels to ensure plating the adequate amount of phosphorous.
- The removal of oil and residues before the PCB etching process.
- Applying a well-controlled immersion gold process with a proper nickel-to-gold ratio.
- Avoid nickel plating out into tanks.
- Cleaning the suspicious black pad areas regularly.
A black pad on a PCB can spell doom for the board. Unfortunately, it’s almost impossible to detect during manufacturing and may become evident after mounting components.
However, partnering with a good manufacturer will help prevent black pads from plaguing your PCB. Quality manufacturing services know how to avoid nickel corrosion during immersion gold plating. Also, you can follow the solutions above to get specific instructions–for your designs.
Do you have any questions? Feel free to contact us, and we’ll be happy to help.