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.
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.
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.
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.
Gold thickness results above the specified range(Thickness > 2 to 5micro-inches) shall result in an attack on the nickelitself. The nickelmay corrode and ultimately result in black pad ifaggressive enough. Thethicker the gold, the greater the risk of blackpad.
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".
Blackpad is caused by a high phosphorous content in the nickel layer of an ENIGfinish due to an improper balance in the plating tank, although we have begunto see a trend that any problem with ENIG solder joints is being referred to asblack pad.
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.
Blackpad occurs during the immersion gold deposit and not during assembly which isresponsible only for revealing the defect. The primary factors are a veryaggressive I Au reaction with the electroless nickel whereby there is severehyper corrosion of the nickel surface that leaves an elevated phosphoruscontent at the interface of the gold to the nickel. Phosphorus is nonsolderable and once the Au is dissolved by the assembly solder, thisunderlying non wettable phos rich surface is exposed resulting in black pad.Gold concentration and copper contamination in the gold bath have a directimpact on the aggressiveness of the reaction and have been found to contributeto 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.
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