How to Work Around “Black Pad”
“Black pad” is a term that usually refers to nickel corrosion on electroless nickel and gold immersion, or ENIG, surface finishes on PCBs. The defect is not seen as part of normal QC procedures as the immersion gold masks the appearance of the nickel under the gold. The “black pad” represents the corrosion and oxidation of the electroless nickel, which primarily initiates at the nickel nodule boundaries and then spreads as the corrosion increases. As the deposited gold thickness increases, more oxidation of the nickel is possible. With proper process control, black nickel occurrence is rare. This particular issue results in reduced solderability and weakly formed joints. When those joints come under pressure, they break easily and leave behind exposed corroded nickel, hence the “black pad” name.
ENIG finish, which suffers from the black pad error, have been around since the 1990s. They originally had several things working in their favor:
- It had good shelf life.
- It was easy to planar inspect.
- It could stand up to testing better than some alternatives since the nickel protected the through-holes on the PCB.
The ENIG process has become increasingly popular over time for several reasons, including the need to comply with the Restriction of Hazardous Substance Directive as well as rising demand for lead-free soldering. PCB manufacturers have gotten a lot better at understanding and working around black pad along the way, but it’s still a problem worth exploring.
What really causes black pad?
There are several possible causes of black pad:
High phosphorus content
In a 2009 article for IPC, John Buchanan explained that the reflows and repairs of the ENIG process can increase the amount of phosphorus and ultimately weaken the board. As nickel dissolves, phosphorus is left behind. Overall, a high percentage of phosphorus is correlated with black pad errors. However, as Werner Engelmaier of Engelmaier Associates noted in the IPC piece, this explanation for black pad is not universally accepted.
Corrosion during gold deposition
When gold is deposited during the ENIG process, it relies on a corrosion reaction to place itself (the gold) on top of the nickel substrate. “Aggressive” gold baths as well as high gold thickness can increase the corrosion to a level approaching black pad. It is generally accepted to keep gold thickness to 2-4 ins, as per the IPC-4552 ENIG Specification, to steer clear of black pad.
Sometimes what manufacturers label “black pad” is actually brittle fracture. Black pad is similar to brittle fracture because it creates a cracked structure within the nickel after the gold has been deposited and then removed. Brittle fracture itself can occur when tin dissolves a portion of the nickel while also creating a thin layer of phosphorus that does not dissolve. Thermal stress, vibrations, and shocks can then weaken the metallurgic bonds.
What are the practical solutions to black pad?
One of the main problems with black pad is that the error is often not caught until late in the PCB manufacturing process when the board is already populated. Catching it earlier requires a mix of looking out for certain flaws and considering alternatives finishes when appropriate.
It may be possible to rule out black pad after the fact by inspecting the surface of a finished PCB. For example, if there are no spikes near the nickel boundaries, and no dark bands near the nickel, then black pad may not be an issue.
Before that, it is a sensible idea to control the gold bath by achieving a good exchange of nickel and gold; stoichiometry is a good guide here. Keeping tabs on the pH level is also important for knowing how much phosphorus will ultimately be deposited.
“A well-controlled nickel bath is one of the keys to the elimination of black pad,” George Milad of Uyemura International Corp., an ENIG supplier, told IPC.
Verify any potential ENIG provider’s baths and processes. There are many requirements that have to be met across the ENIG line to minimize the risk of black pad, including pretreatment (removing oils and other residues to prepare for etching) and use of chelating agents and stabilizers to prevent the nickel in the bath from plating out into the tanks.
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