Hi Technet!
The question (for a HASL finish) was:
"Why do I get different Tin content levels on the same board,depending
on the solder thickness I found?
In the range of ~1micrometer I get higher readings,although the alloy
is "recognizable".
Is it a calibration error, the real thing, or me? "
HASL is a very dynamic process and as Tom mentioned a fabricator is
always walking the line between having too much or too little solder left on
the board. Surface tension can be a pretty devious beast to control and with
a variety of pad geometry's and diversity across a board, getting a nice
uniform solder finish is hard to do. That's one reason many folks don't like
HASL finishes if they are not doing JIT manufacturing or have noncontrolled
manufacturing conditions - the changing HASL thickness can lead to a loss of
solderability depending on the oxidation conditions. The copper and tin
metallurgically combined to form copper-tin intermetallic which will shift
the solder alloy tin content levels around depending on the thickness the
HASL process produced. This is a real thing and not magic. One method around
this is to get very precise calibration standards that correspond to the
solder thickness that the HASL process produces. However, getting standards
that mimic HASL solder finish thickness for the purpose of measuring tin
content is mostly an impractical effort because any layer of copper and tin
will continue to change slightly with time as the intermetallic grows making
the "standard" always changing. The good news is you can accurately measure
the solder thickness, the bad news is the tin content is a shifting
target.Just one of those metallurgical items that makes circuit boards fun.
Hope that helps.
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator
_________________________________
Subject: Re: FAB: RE: Question on Pad Leveling
Author: [log in to unmask] at ccmgw1
Date: 6/12/97 3:45 PM
Patrick Ducas wrote:
>=20
> Tom, just a easy one, what type of tool do you use to measure the thick=
ness
> of solder? Is it accurate for that specific application? If you know so=
me
> tools that should be enough accurate, any hint would be appreciated.
>=20
> Thanks in advance
>=20
> >
> > Don:
> > The thickness of solder from a solder level operation
> > is dependent upon pad geometry, amongst other things. Board
> > fabbers walk the tightrope between blowing enough solder out
> > of the holes yet leaving enough remaining on the surface
> > mount pads.
> > I would strongly urge you to specify a horizontal HASL
> > process from your board vendors. Boards should also be
> > cocked at a 45 degree angle when going into the solder to
> > offset a "north-south, east-west" bias on QFPS.
> > Getting back to the pad geometry, the larger the pad,
> > the thinner the solder coat is apt to be, because the solder
> > has a larger surface over which to retreat. Worst case
> > would probably be discrete pads, where we have a stated
> > minimum of 50 microinches at the pad center. Maximum solder
> > thickness can be held to .0015".
> > Where the solder is measured is another issue. We
> > always ask our customers to measure from the pad centroid so
> > that outgoing and incoming measurements are apples and
> > apples. The pad center may or may not be the crest, and it
> > usually isn't. When I see thicknesses called out in specs
> > that say "as measured at the crest", I chuckle because it
> > would take too long just to find the crest of a given pad.
> > In all cases, your board fabricator should guarantee
> > solderability.
> > In summary, the most workable callout would be a solder
> > thickness of .0001" nominal, .00005" minimum, .0015" maximum
> > (all measurements taken at pad center). Again,
> > solderability is to be guaranteed.
> > Regards,
> > Tom Coyle
> > Field Services Engineer
> > HADCO Corporation
> >
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> >
> -------------------------------------------------
> Patrick Ducas
> Process Quality Engineer
> Matrox Electronic Systems Ltd
> E-mail: [log in to unmask]
> T=E9l:(514)969-6000 ext.2971 Fax:(514)685-3415
> -------------------------------------------------
>=20
>=20
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Hi,TechNetters,
Patrick asked Tom how he measures the solder thickness on SMT pads.
I measure the thickness with X-Ray,which gives me the following
opportunities:
1:I can perform several measurements on the same pad.
2:I see WHERE I measure,and usually I find the crest-don't ask me how
much time it takes.
3:I can determine also the composition of the solder.
Tom's suggestion about 45 degrees is very correct, I think, I have seen=20
differences of 40 micrometers or more between N-S and E-W.
Here my experience ends,and because there is always Dave,I will also ask
one question:
Why do I get different Tin content levels on the same board,depending on
the solder thickness I found?
In the range of ~1micrometer I get higher readings,although the alloy is
"recognizable".
Is it a calibration error, the real thing, or me?
Bye
Gaby
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