Oxidized ?  Are you sure, or do you not have ample Alpha Sn phase in the
plating... You mentioned that you heatcycled these PCB's, did you cycle
them all twice,,, You may have a Cu Migration issue, by not having
enough Alpha phase Sn present from your PCB house.  I'm a big fan of
White Sn, but you need to specify the thickness of final pure tin,
verified by XRF testing you may find you're constitution is suspect, not
from oxidation but rather Cu migration.

The PCB's could be refinished, but, alas, you lose the parts to do this
on the side that is populated.... An tangled web of destruction may lay
ahead for you.

My .02

-----Original Message-----
From: Rich Wolbert [mailto:[log in to unmask]]
Sent: Wednesday, October 18, 2006 9:18 AM
Subject: Re: Oxidized PCBs


Wayne,

Thanks for the input. I didn't release industrial N2 had enough
contaminants to form a reducing atmosphere. Years ago we had an oven
that could employ hydrogen but we have since sold it. Something about
the legal department didn't like the idea of us having propane, oxygen
and hydrogen all in the same facility. Bunch of babies!

Sorry to hear you had the same experience with the counterfeit parts. It
seems like you have to buy direct from the factory to get the correct
parts.

Rich
----- Original Message -----
From: "Wayne Thayer" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, October 18, 2006 9:59 AM
Subject: Re: [TN] Oxidized PCBs


> Sounds like you need to either find a flux aggressive enough to do the

> job, or to place the boards in a reducing environment for an
> "appropriate" period of time.
>
> Creating a reducing environment may not be that difficult.
>
> I have used a retort (sealed chamber in an oven) at 100C with
> industrial nitrogen flood until an O2 meter tells me its below 10ppm
> O2. Apparently there are enough trace contaminant reducing agents in
> the industrial nitrogen to create the reducing environment.  I suppose

> you could also dope the environment with a tiny bit of H2.  I'd try a
> 4 hour 100C bake in such an environment to start with.
>
> Of course, if you don't have a retort, you will need to find an
> alternate reducing technique.  Chemicals can do it, but then we have
> to worry about the parts on the other side.
>
> We've been nailed by counterfeit parts too, so I feel your pain!
>
> Wayne Thayer
>
>>>> [log in to unmask] 10/18/2006 9:47 am >>>
> Mornin' Rich,
>
> Just curious, what kind of flux are you using in your paste?
>
> -Steve Gregory-
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Rich Wolbert
> Sent: Wednesday, October 18, 2006 8:32 AM
> To: [log in to unmask]
> Subject: [TN] Oxidized PCBs
>
> OK, I know you guys are probably going to shake your head on this one,

> but I have to ask. We have a double sided fine pitch SMT assembly and
> we decided to go to white tin for the pad material. Here's where the
> problem comes in, we ran one side and when we started process the
> other
> side we found counterfeit chips. We stopped production for two weeks
> until the correct parts came in. When we tried to run the boards
> again,
> the PCB's oxidized so badly over the two weeks that we can not get the
> solder to wet. Any suggestions, other than scrapping the 170
> assemblies
> at $600 a piece?
>
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