Ashok: Black spot [black pad] issues seem fairly well documented in the fine SMTnet Archives. * It has nothing to do with "lead segregation and subsequent oxidation". Lead oxides: - Common PbO is red to reddish-yellow crystals and yellow crystals stable above 489�C. - Less common PbO2 is lustrous black crystals - Fairly rare Pb3O4 is dark brown to red * "Solder paste with 2% silver" will not help. * Odds are your black pad [and solderability] problem is caused by the lack of control that your board fabricator used in making your bare boards. Consider: - Performing metallographic analysis to determine the composition of your black pads. - Involving your board supplier, so they are not surprised when you fine suit and drop them as a supplier.
Information on a specification of gold finish: * I�m surprised there is no ENIG spec in the fine SMTnet Archives, but then again � * Just for the record, a ENIG thickness specification: typical spec is 3-8 uinches of gold over 150-250 uinches of nickel. * It�s probably not a bad thing to own some IPC documents on boards, as you say.
Quick solution => Send the boards back the fabricator and have another set made by someone else.
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