Daan,
After dewetting, are you sure it is gold that is visible and not
copper? Gold diffuses so fast into tin, that it is hard to imagine any
gold being left on a pad after soldering. If the board shop is having a
problem with hyperactive corrosion during the gold plating process,
nickel can be removed from the pad nearly down to the copper; leaving a
very porous nickel layer. The gold will then diffuse into the copper
and the pad becomes unsolderable. That would be some pretty severe
corrosion, but you would likely find other pads that looked either black
or grey.
If the pads are truly still gold plated after soldering, then something
must certainly be preventing the tin from touching the gold, i.e.
surface contamination such as oil, plastic, whatever.
Thanks
Ryan Grant
Process Development Engineer
MCMS
(208) 898-1145
FAX (208) 898-2789
[log in to unmask]
>-----Original Message-----
>From: d. terstegge [SMTP:[log in to unmask]]
>Sent: Friday, September 10, 1999 7:27 AM
>To: [log in to unmask]
>Subject: Re: [TN] soldering to immersion gold boards
>
>Hi Roger,
>
>According to our suppier the boards have 4-5uM of nickel and 0.1uM Au, which
>fits very nice within the immersion gold specification that was recently
>discussed on Technet.
>Our solder paste contains 2% silver (62/36/2), with a RMA flux.
>Problems occur during the second soldering step (we use nitrogen inerted
>convection reflow) on about 50 % of the boards. We see a serious degree of
>dewetting (gold still visible) especially at the fine-pitch components.
>Strange thing is that this happens on boards of two suppliers (both new for
>us), where with other suppliers we usually have excellent results. It's
>difficult to imagine that both our new suppliers provide bad boards, but on
>the other hand I can't imagine what's wrong with the assembly process since
>we normally solder gold boards (with similar specs) without any difficulty.
>I'll try to get the pads analysed somewhere. Thanks for your help !
>
>Daan Terstegge
>Unclassified mail
>
>>>> Roger Massey-G14195 <[log in to unmask]> 09/10 3:07 pm >>>
> Daan,
>
> Can you define exactly what the platings are, in terms of thickness
> and purity if any, ie Ni is 4-6uM with 0.5uM Au etc!! and what the
> solders etc are Is the problem effect the whole batch, or just some of
> the boards?
>
> Regarding your specific questions:-
>
> Prebake of boards will enhance any diffusion of Cu through the Ni
> and into the Au, I cant see why this would effect solderabilty, but
> may effect reliability The degree of diffusion etc will be related to
> time and temp of prebake and the thickness and quality of the Ni
> plating.
>
> Effect of previous soldering operations would all effect the
> diffusion as above
>
> I assume that as long as the solder paste is relatively active it
> should work fine on Ni-Au as HASL
>
> Have you done any surface analysis of the pads? sounds like they
> are contaminated with something, either from the plating, storage
> handling, or even within your own facility and process.
>
>
> I think we need more info, and you should get some basic surface
> analysis of the pads themselves.
>
> Roger
> Motorola AIEG
>
>
>______________________________ Reply Separator
>_________________________________
>Subject: [TN] soldering to immersion gold boards
>Author: "TechNet E-Mail Forum." <[log in to unmask]> at #email
>Date: 10/09/99 14:17
>
>
>Hi Technet,
>
>I'm having some problems lately with soldering of pcb's with an electroless
>nickel/ immersion gold conductor finishing, which brings me (after some
>discussions with our board supplier) to the following questions :
>Is solderability influenced by pre-baking the boards ? Does the first
>soldering
>step have an effect on solderability for the second soldering step (double
>sided
>reflow) ? How about the time between all these thermal excursions ?
>Is it possible that a solder paste that performs excellent with HASL-boards
>(and
>normally also with immersion gold boards) is a bad choice for soldering to
>immersion gold ?
>
>Greetings,
>
>Daan Terstegge
>Unclassified mail
>
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