Gabriela,
o  Best practice on QFP solder thickness is 90% of all pads 100-2000
microinches.
o  It is normal to see changes in alloy composition as a result of solder
thickness.  Since the tin copper intermetallic changes very little during HASL,
there is a high tin concentration (low lead) near the interface.  As the solder
thickness increases the tin rich surface has less effect on the XRF readings so
the lead concentration appears to increases.  The fact is that the tin and lead
occur in a dynamic equilibrium in the bulk solder.  Tin is rich at the solder
surface and near the copper while lead is at a maximum just above the IMC.

Gabriela Bogdan wrote:
>
> etgar wrote:
>
> > Fellow Technetters:
> >
> > I had been measuring solder thickness on QFPs and found out that the
> > thickness on horizontal pads is slightly thicker than the vertical
> > pads. I
> > know this is normal specially for a Vertical Hot Air solder leveler.
> > But
> > what should be the limit? Also along with test, using an x-ray coating
> >
> > thickness measuring machine, is it normal that the tin % composition
> > on the
> > vertical pads lower than horizontal pads? I tried lowering the air
> > knife
> > pressure thinking that high temperature depletes Sn, but I got similar
> > data
> > on 200, 210, 220, 230 and 240 C air knife spray pressure.
> >
> > Garret Dimaculangan
> >
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>
>   Hi!
> I experienced the same with solder thicknesses under 3 um.
> More experienced Netters could explain us maybe if it is related to
> intermetallic formation .
> Gaby
>
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--
Regards,
Jim Reed
Raytheon Systems Company
512-250-7172 (Voice)
512-250-7967 (Fax)
[log in to unmask] (email address)

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