We have developed an automatic inline screening system for the detection of the "Black Pad" phenomenon on electroless Ni/Au printed circuit boards: THESSY BP (thermal screening system for Black Pad).

Composite coatings of electroless nickel-phosphorous and immersion gold (ENIG) have become established as preferred solderable surface finishes for high reliability applications involving complex PCB designs. Over the last three years, more and more interconnection failures occurred after final assembly of the PCB's due to a poorly formed joint at the solder/nickel interface. When the suspect joint is stressed, the connection is easily broken, leaving an open circuit. In other cases, the pads are dewetting during the soldering process. The dewetted pads are discolored black, which lead to the nomenclature "Black Pad".
Investigations on Black Pad show that it is caused by a corrosive interlayer between the gold and nickel layer. It is an insidious problem, since this interlayer is covered by gold and therefore, it has been not able until now to detect Black Pad by non-destructive methods. The symptoms of Black Pad only occur after soldering which causes high costs.

THESSY BP is based on thermographic measurements on a PCB whichever before shipping or assembling. It works automatically as an inline system which is able to sort out "infected" PCB's. The prototype setup yielded a detection rate of 95%.
For further information, please contact Fritz Bergmann.
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