At the onset of the conversion to Lead-Free/RoHS compliance, we believed that Black Pad was a condition exclusive to improper maintenance of the nickel plating bath. This suggested that Black Pad was an entirely avoidable condition that an end user could prevent by auditing their supplier(s) to ensure they were following best maintenance practices.
Later, industry testing suggested that the immersion gold plating process also plays a significant role in this peculiarity. A heavy attack on the nickel during the gold plating process could initiate excessive nickel corrosion, leading to a condition similar to Black Pad.
Now, five years later, more information has surfaced regarding the ENIG finish, Black Pad, and its causes.
What is Black Pad?
Black Pad is an acute corrosion of the nickel plating layer, causing it to appear in black in color. This abnormality has several known causes. Often there is an improper diagnosis when there is a solderability/dewetting issue that traditional Black Pad is the cause.
Traditional Black Pad Diagnosis
Black Pad is characterized by noting the following on the affected product:
1. PCB is affected uniformly;
2. Thinner nickel deposition measured by XRF/cross-section analysis;
3. No intermetallic layer formed during final soldering process;
4. Higher phosphorus levels detected during the final soldering process; and,
5. Improper balance of nickel bath stabilizers.
Causes and Prevention
There are three main parameters to control that will prevent Black Pad from occurring.
Nickel Plating Rate
Maintaining a nickel plating rate of 7 to 10 microinches per minute will keep the phosphorus percentage in the nickel between 7 to 10%. A plating deposit below 5% and above 13% is suspect and cause for alarm and subsequent analysis of the entire operation.
Nickel Plating Thickness
Maintaining a minimum nickel thickness also contributes to preventing Black Pad. The IPC requires a minimum of 118 microinches for nickel plating thickness. Most chemistry suppliers recommend a slightly higher thickness on the surface to ensure that the minimum thickness is maintained at the knee and in the walls of through-holes. There are areas within the tank in which there is a strong solution movement that may contribute to increased absorption of stabilizers, thereby reducing the nickel thickness.
Nickel Bath Components
Proper control of the nickel bath components ensures the proper stabilizer ratios are maintained.