In a message dated 6/23/98 12:09:38 PM Pacific Daylight Time,
[log in to unmask] writes:

> Dear "Techies":
>
>  I have a "GOLDEN" opportunity for the experts to shine.  I have in my
> possession
>  some SMT assemblies built on gold flash fabs that exhibit gold residing on
> the
>  surface of the fillet, rather than being dissolved in the solder.  The
> fillet is
>  fully formed with no graininess or other visual anomilies.  The phenomenon
>  occurs on ONE end of a MELF diode, not on both ends.  To answer the obvious
>  question, it is NOT dependant on the diode polarity; it happens on either
> end
>  but on one end only.
>
>  I have digitally captured a couple of photos of the phenomenon.  If you
need
>  pictorial representation, please e-mail me directly and I can attach it to
a
>  return e-mail.
>
>  As you will see,  the gold remains on the surface of the fillet, rather
than
>  being dissolved in the fillet.   A slight scraping of the fillet surface
> will
>  uncover the base solder with NO visual evidence of gold inside.  Other
>  terminations on other components do NOT exhibit this phenomenon.  The
> assembly
>  subcontractor is using standard NO-CLEAN solder paste (OMG Americas 500ADV
>  NO-CLEAN 63SN). Unsoldered parts show a tin/lead termination, indicating
> that
>  the gold has possibly migrated over the entire termination.
>
>  My question:  What mechanism is at work to keep the gold from totally
> dissolving
>  into the Sn/Pb paste after it leaves the fab during reflow?
>
>  Thanks for the help.
>
>  Bill Fabry, Sr. Manufacturing Engineer
>  Plantronics, Inc.
>  345 Encinal Street
>  Santa Cruz, Ca. 95060
>  (408) 458-7555

Hi ya' Bill!!

     You got the boards like that from a subcontractor huh? (Hmmmm, I wonder
if the same guy I know is working at your subcontractor...could be, I'm just
over the hill from ya' in Sunnyvale...)

     What I was referring to in the previous sentence, was this production
operator I used to know. But I don't need to get into that...lemme ask you
this, is the end of the diode that has the gold on it anywhere close to the
gold fingers of the board? The reason I ask is because I used to know an
operator that got a little too crazy with the replating unit we had to fix the
solder splashes on the gold fingers...wound-up plating some solder fillets on
components that were close to the fingers...BOY! was that ever fun explaining
to the customer!

     Anyways, kinda' sounds like what may have happened with your boards...

                            -Steve Gregory-

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