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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Black spot on Gold Pads

Ashok Dhawan

#2381

Black spot on Gold Pads | 25 November, 2000

I am getting black spots / stuff while I run gold finish PCB through reflow soldering process using NC solder paste.The affected spots have intermetalic as black in color. Does not solder very well. I did look at the archieve but could not find info on specification of gold fininsh. I guess IPC-2221 has spec on gold finish - it will help if someone can provide me IPC recommeded spec on gold finish. Can this black spot be due to lead segregation and subsequent oxidation as well ? Will it help if I use Solder paste with 2% silver ?

Your inputs will be appreciated as I have this problem on my table and am looking for a quick solution to the problem.

Ashok Dhawan adhawan@wpg.cmac.com

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#2382

Re: Black spot on Gold Pads | 27 November, 2000

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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Alvin K

#2383

Re: Black spot on Gold Pads | 28 November, 2000

Gold is porous and spots on gold plated PCBs is one of the drawbacks of using gold as a solderability protector. Base Nickel exposure seems to be the cause of this and is aggravated when the gold thickness is less than what it should be. I agree with Dave. The PCB fab house is the culprit in all this.

I would say 3-4 microinch (8-10 micron) is more the norm. Higher thicknesses will cause gold embrittlement of the solder joint.

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