Thanks, Gary
This will involve changing the PCB manufacturer, new evaluations, but it is
worth , I suppose.
was afraid about black pads, I hope that this problem is of lesser
importance now...
Redesign will involve also TH because we have Press Fit connectors too.
Life is not a Picnic, as we say here.
Thanks,
Gaby

-----Original Message-----
From: Gary Ferrari [mailto:[log in to unmask]] 
Sent: Monday, July 27, 2009 8:45 PM
To: Gabriela
Subject: Re: [TN] OOPS! The copper is gone!

Gabriela,

All of the immersion surface finishes are subject to excessive copper 
dissolution. The surface finishes take electrons from the less noble 
metals (Eg. copper) and you get what you have experienced. The thicker 
boards, and extra copper require longer exposure to soldering 
temperatures, thus exasperating the situation. The reason ENIG works is 
because the nickel is a barrier to prevent the electron migration. 
Another surface finish well worth looking at is ENEPIG. When is an 
electro-less nickel/ electro-less palladium. immersion gold finish. It 
is a robust finish that satisfies lead free soldering requirements, and 
is wire bondable. There is also a more robust nickel/palladium/gold 
finish which is electro-plated.

So, you have additional choices besides the ENIG, which works!

Regards,

Gary Ferrari
FTG Circuits
Director, Technical Support
860-350-9300

Gabriela wrote:
> Right.
> Scrolled the internet for a couple of days, much on copper dissolution
> there.
> Another present from ROHS.
> Luckily we had no delamination because we used high Tg board material, but
> also luckily we have no BGA's on it, because my latest information is that
> High TG board material is bad for pad cratering. (another present from
> ROHS).
> I wonder how many cross sections I will have to do in order to validate
the
> process.
> Gaby
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
> Sent: Monday, July 27, 2009 7:39 PM
> To: [log in to unmask]
> Subject: Re: [TN] OOPS! The copper is gone!
> 
> Hi Gaby - you found the unhappy recipe for lead-free soldering: thick 
> pwbs, lots of thru hole connectors and heavy internal copper layers = 
> danger of copper dissolution. Both the IPC APEX and SMTAI International 
> Conference proceedings for the last 2 years have some excellent papers on 
> the topic of copper dissolution which may be helpful for your issue.  You 
> may need to investigate using a different solder alloy instead of SAC305 
> (?) to control the amount of copper dissolution. Good luck.
> 
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
> 
> 
> 
> Gabriela <[log in to unmask]> 
> Sent by: TechNet <[log in to unmask]>
> 07/27/2009 11:56 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Gabriela <[log in to unmask]>
> 
> 
> To
> [log in to unmask]
> cc
> 
> Subject
> [TN] OOPS! The copper is gone!
> 
> 
> 
> 
> 
> 
> No surprise at all.
> Assembly went through lead free SAC selective soldering. Thick, 20 
> internal
> layers motherboard. Immersion silver plated.
> Lots of connectors (not only), populated on both sides. Conventional 25um
> copper in holes, outer layers over 1/2 oz copper.
> 
> Was soldered before with Sn/Pb.
> 
> No free meals, haha.
> 
> Want to redesign to ENIG - seems to be the only choice.
> 
> I wonder if there is a thumb rule for selecting the possibility of 
> selective
> soldering according to design and platings.
> 
> Also, if ENIG is safe for this process, even if rework with SAC ot Sn100 
> for
> hole fill and shorts must be performed.
> 
> We will try to overcome the problem, in the meantime by using Nihon
> superior, but in order to get good hole fill for such a thick board, the
> parameters will be too harsh , I suppose.
> Cross sections look very beautiful- pads are gone, knee is gone, barrel is
> playing hide and seek, inner layers just end to nowhere.
> How did you wise people validate this process and never mentioned anything
> on the NET?
> 
> 
> 
> Gaby
> 
> 
> 
> 
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