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July 26, 2016

What Causes Black Pad?

What is the primary cause of Black Pad and/or pad contamination during the assembly process?

E.T.

Experts Comments

Black pad occurs during the immersion gold deposit and not during assembly which is responsible only for revealing the defect. The primary factors are a very aggressive I Au reaction with the electroless nickel whereby there is severe hyper corrosion of the nickel surface that leaves an elevated phosphorus content at the interface of the gold to the nickel.

Phosphorus is non solderable and once the Au is dissolved by the assembly solder, this underlying non wettable phos rich surface is exposed resulting in black pad. Gold concentration and copper contamination in the gold bath have a direct impact on the aggressiveness of the reaction and have been found to contribute to black pad. This is obviously a simple explanation of a very complex problem
Gerard O'Brien
President
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
Black pad is caused by a high phosphorous content in the nickel layer of an ENIG finish due to an improper balance in the plating tank, although we have begun to see a trend that any problem with ENIG solder joints is being referred to as black pad.
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Renee Michalkiewicz
General Manager
Trace Laboratories
Renee has been with Trace and an IPC member for 16 years. She has managed all military and commercial PB qualification and conformance testing and training, as well as product qualification and testing in the areas of solder pastes, fluxes, solder masks, and conformal coat. She is the chairman of the IPC Testing and the IPC-J-STD-004 Flux Specification Committees and the Vice Chairman of the Assembly and Joining Committee. She has published more than a dozen papers and presented at numerous electronics conferences.
Gold thickness results above the specified range (Thickness > 2 to 5 micro-inches) shall result in an attack on the nickel itself. The nickel may corrode and ultimately result in black pad if aggressive enough. The thicker the gold, the greater the risk of black pad.
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Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
Black pad is basically a reaction of the chemistry in Immersion Gold with the Electroless Nickel. This can be caused by a number of factors including phosphorous concentration out of spec (either above or below the 7-10% range).

Excessive time in the gold bath due to low gold concentration or desiring an excessive gold thickness (>5 micro-inches).

Build up products in the Nickel bath (this is why the MTO should be less than 5). There are other factors and in general black pad is still not 100% understood. Hope this helps.
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Mike Scimeca
President
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.
Black pads are not created in the assembly process, they are revealed. They actually come from your board vendor in the form of poor plating due to improper control of the plating process.

The assembly defect refereed to as "black pad" is often found following soldering operations when the resulting poor solder joints are mechanically stressed and a part pops off revealing a black pads.  The smaller pads are more likely to be affected.  Assemblies with "black pad" can be reliability nightmares.
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Steve Stach
President
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
Reader Comment
We are the largest ENIG supplier for over ten years and have proven thru testing at IPC that gold over 5 (we suggest under three) is number one cause; but also an erratic nickel bath (esp phos level under four) can cause black nickel frequently; the third cause is very acidic gold baths (which we have never sold) - all that said, actual black nickel is a very rare event, especially in the years since the word got out that thick gold was destroying the nickel.
Don Walsh, Uyemura International Corp., US
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