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The Technical Wire |
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Latest on Black Pad
Initial Beliefs
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
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 7-10%. 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.
HyperCorrosion Diagnosis
HyperCorrosion is characterized by noting the following on the affected product:
1. Excessive Gold deposition
2. Indication of excessive nickel corrosion in cross section analysis
3. PCB is affected in localized areas
Causes and Prevention
HyperCorrosion from Assembly
Often, an assembler can have an issue whereby a small group of pads have a dewetting problem.
In this instance, the solder has come into contact with the gold plating layer, causing the gold to dissolve. This leaves the nickel plating layer exposed to flux and heat, which in combination form a highly corrosive atmosphere for the nickel. This situation will cause the nickel to corrode. The knee-jerk reaction is to blame Black Pad as the culprit, which is an incorrect assumption. The most typical cause is an improper condition in the soldering process, including, but not limited to low solder temperature, cold spots on the panel, and improper fluxing.
HyperCorrosion from Fabrication
HyperCorrosion is primarily caused by excessive gold thickness, which is due to either a higher-than-recommended gold plating rate or dwell times in the gold plating bath in excess of recommended times by the chemical supplier.
Both the IPC and major chemistry suppliers recommend a minimum of 2 microinches of gold, with an appropriate range being 2-4 microinches for plaiting variation. 2 microinches should ensure one year of shelf life. Typically, plating lines are installed so that the panels dwell in each bath for a set amount of time. If a gold plating bath becomes overly aggressive via an increased gold plating rate, the subjected panels will achieve a much higher plating thickness.
Conversely to nickel, areas subjected to strong solution movement will have increased gold plating. Since gold plating itself is a corrosive process, in which nickel atoms are exchanged for gold atoms, there can be more corrosion at the knee and in hole walls since these are the same areas where there is, typically, a lower nickel plating process. Essentially this is a phenomenon called "nickel attack".
Conclusion
While the initial assumptions regarding the cause of Black Pad may not have been as comprehensive as we had all hoped, the keys to prevent receiving printed circuit boards with Black Pad as end user remain the same: AUDIT YOUR SUPPLIER.
A thorough audit will result in confidence that the supplier has the process installed in their facility, is maintaining chemistry per supplier guidelines, and is verifying plating rates as well as plating thickness on the actual product.
Further / Researched Information
If you're interested in further researching this and other ENIG-related topics, here are a few articles we recommend:
Houghton, F.D. Bruce. "ITRI Project on Electroless Nickel / Immersion Gold Joint Cracking".
Biunno, Nick. "A Root Cause Failure Mechanism for Solder Joint Integrity of Electroless Nickel / Immersion Gold Surface Finishes".
F.D. Bruce Houghton, K. Johal, D. Cullen, E. Hueger, M. Toben. "A study on interfacial fracture phenomena of solder joints formed using the electroless nickel / immersion gold surface finish".
Kuldip Johal. "Study of the Mechanism Responsible for "black pad".
Mike Walsh. "Electroless Nickel Immersion Gold and Black Pad".
This article was written with extensive contribution from Mike Fancher and Jim Gilbert, Jim Trainor, and Hank Lajoie from OMG Electronic Chemicals.
Saturn Electronics Corporation, bare printed circuit board manufacturer, is a Top 30 domestic PWB Fabricator managing the nuances between Quick-Turn Prototype Boards and High-Volume PCB Production while offering industry-leading Lead Free circuit boards that meet RoHS compliance. |
