Hi Bruce,
We had some boards reported with epoxy (uncured) bleeding from vias, disturbing soldering. Made FTIR, found amounts of bensoates, which is said to be added into epoxies for limiting the molecular weight. Addition of bensoates are moreover said to make board material smoother and more flexible. Summary of investigation was not so specific as I wished, but there were two possibilities for bleeding: locally uncured epoxy or dito solder mask. Words: bensoates and bifenols, look for them.
As soldering takes place at nanometer level on the pads, a further knowledge of the bleeding would have to pay attention to superthin films, an area few want to sniff at. So we have not gone further in a scientific point of view, the remedy has to found by the PWB makers themselves, adjustment of what process step, I don't insist to know exactly, it's mainly their headache.
Anyway, vacuum bake, plasma cleaning, acids or any other cure does not seem attractive if you make products for MIL users. If the contamination is on a nanometer level in the plating's microstructure, e.g. creeping epoxy groups, such cleaning is said to be extremly questionable. Better to get new boards which you can rely on.....
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: Misner, Bruce [mailto:[log in to unmask]]
Sent: den 27 september 2000 14:56
To: [log in to unmask]
Subject: Re: [TN] ENIG SMD PWBs Aluminum wire bonding
Dave,
I had a similar problem a few years back on an automotive application and
did an extensive failure analysis and resulting DOE. Contamination and/or
epoxy bleed were suspected but were not evident in SEM, Auger or ESCA. It
will take me a couple days to dig through my archives for the results and
refresh my memory, which I will do.
In either regard, I implemented an evaluation at incoming inspection which
completely eliminated bad lots from getting into production and also
highlighted a superior vendor. Contact me off line.
Bruce Misner
> ----------
> From: Dave Gardiner[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.;Dave Gardiner
> Sent: Tuesday, September 26, 2000 9:47 PM
> To: [log in to unmask]
> Subject: [TN] ENIG SMD PWBs Aluminum wire bonding
>
> Thanks all for your suggestions on epoxy bleed. Plasma clean and vacuum
> bake are being investigated. Has anyone seen any specifications that
> cover epoxy bleed or bondability. We have one part number where we have
> twice had trouble with no stick to 15 grams pull on the same panels. We
> had tests done for surface contamination but there was little difference
> in the findings between good and bad boards.
>
> Dave Gardiner
>
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