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Sunday, 01 June 2008 |
Black Pad Has Multiple Causes
Phil Zarrow’s April column narrowly assumes causes of black pad. And recommending more gold as mitigation is harmful advice (assuming many readers may miss his 4 µin maximum as the reasonable upper limit). Japanese science has proven, in this case, less is better.
Black pad has several causes. The mistake assumes all ENIG chemistries are created equal. We have proven that is not true. Black pad is rare, but can be created by 1) an electroless nickel with less than 5.5% phosphorus content in the plated layer; 2) an overly aggressive gold chemistry (either the chemistry is too acidic, or the gold is permitted to run below recommended levels in the bath, or boards are simply left in the bath too long); 3) the gold layer is too thick (gold deposits by replacing nickel; if this is permitted to continue too long, then the surface is compromised and black pad is guaranteed).
We still fight standards (even military: “But that is what the spec says”) that require a minimum 8 µin of immersion gold. Hopefully, such specs also accept total responsibility for black nickel! We also fight erroneous thinking that the solution is high phosphorus nickel (so much for good solderability).
The key steps are a consistent nickel and gentle gold that give complete coverage (porous gold layers do not protect the nickel surface) with limited thickness. We prefer two or three µin. As a supplier, this reduces our sales but saves customers money and, most important, it is based on years of research and field service.
Another battle fought over the years was the concept of ENIG with a layer of electroless palladium between the nickel and gold (ENEPIG) as the universal finish. However, this does not create as strong a solder joint as regular ENIG, provided leaded solders are used. For non-leaded solders, ENEPIG offers better solderability and eliminates black pad, as the palladium layer acts as a protective barrier between nickel and gold.
Another flexible alternative is adding a layer of electroless (autocatalytic) gold over ENIG (a process called ENAG), for purposes of gold wire bonding.
Don Walsh
Director of Operations
Uyemura International Corp.
Corrections
Due to an editorial error, the April 2008 "Better Manufacturing" column incorrectly stated the IPC standards for gold plating volume in µm. The correct volume is µin. We regret the error.
Send your thoughts to Editor, Circuits Assembly, email: mbuetow@upmediagroup.com. Letters must include the writer’s name, address and company affiliation and may be edited for length and clarity.
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