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October 2005

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Subject:
From:
John Maxwell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Maxwell <[log in to unmask]>
Date:
Fri, 28 Oct 2005 15:39:21 -0700
Content-Type:
text/plain
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text/plain (59 lines)
David,
Salt or sugar for that matter in water is a solid to liquid diffusion
process. It like all diffusion processes is affected by time,
temperature and quantity of materials in the diffusion source. Soldering
is nothing more than liquid to solid diffusion with tin being the main
diffusion species into a base metal, typically copper or nickel. Before
this diffusion takes place, surface coating being solder (most common in
past), tin (probably most common in future), gold (i.e. an ENIG surface)
and PdAu over nickel on some semi lead frames must be dissolved. Then we
have solder joint formation. But if we just solder to the surface finish
we have not achieved tin diffusion into the base metal and that
diffusion results is solid crystalline structures called intermetallic
layers creating the solid bond between solder and component lead or pad
surface..

Also I have observed in the past if a reflow process is marginal then we
see ENIG "brittle fractures" and what has happened is the the gold has
not been completely dissolved restricting tin diffusion into the nickel
layer and there is soldering to the finish. There are two stable tin
gold eutectic alloys than can form and freeze out during solder joint
formation. One alloy is 80/20 Au/Sn with a eutectic temperature of 280C.
The other is Au/Sn 20/80 and has an eutectic temp of 240C. These frozen
zones are readily dissolved into the solder but if the time and temp are
not adequate the there are solder joint fractures in these zones. The
resulting fractured surface can be very smooth and frustrating. In the
last couple of year there was an excellent paper by some lads from
Taiwan who were able to capture solder joints with these zones frozen
and had some excellent photo in their paper. I believe it is available
thru the SMTA web site if one is a member.

John Maxwell

David Tremmel wrote:

> Hello,
>
> When someone equated soldering to that of salt dissoving in water, it
> threw
> me for a loop.  Now I am wondering if someone willing to explain how an
> inter-metallic bond is formed when one of the metals (Ni, in the example
> below) is not in a liquid state?
>
>
> Thank you in advance,
>
> David
>
>

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