Below are the responses I received to my question about the mixing of
hi-temp and eutectic PbSn solders.
Thanks to Dave, Bill, Paul, and Andy for their responses, numbered [1]
through [4] below; my original question is included at the end of this
message.
After reading the replies, I'm a bit uneasy about allowing these 2
alloys to mix and reflow in an "uncontrolled" manner. We want to use the
hi-temp alloy on this particular joint, so I'll recommend removal of the
eutectic preform, and complete the subsequent clip lead attachment using
the hi-temp solder (probably Sn10/Pb90, AND NOT 90-94% Sn, as stated in
the original posting).
If anyone else wants to take a stab at explaining what happens when the
two alloys mix, please send a posting to TechNet. The 3 days of mid-80F
temps we are now experiencing are purely coincidental.
Best Regards,
Michael Alderete
[1] Dave Hillman Wrote...
-----Original Message-----
From: David D Hillman [mailto:[log in to unmask]]
Sent: Friday, December 04, 1998 9:12 AM
To: TechNet E-Mail Forum.; Alderete, Michael
Subject: Re: [TN] ASSY:mixing of high and low-temp PbSn alloys
Hi Michael! Turkeyday in Iowa was only 70F but we didn't' complain! The
answer to your question is pretty simple: you will get a solder alloy
which
lies somewhere between the two alloy compositions you start with. The
tin
content will be in the 63% - 90% range and you Pb content will be in
the
10%-37% range provided the soldering operation allows for the normal
diffusion/mixing mechanisms to take place. Some folks consider this a
problem because finding or understanding the reliability characteristics
of
this "new" solder alloy can be difficult. Other folks just evaluate the
two
alloys (the Sn63 and Sn90) for their reliability characteristics/merits
and
if both alloy data sets fit their use environment then they assume the
"new" alloy won't cause them any issues. The use/combination of
different
solder alloys for solder joints can be a pretty involved topic - I don't
know of a lot of published information on the subject. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
==============================================================
[2] Bill Kenyon Wrote...
To: MICHAEL ALDERETE <[log in to unmask]>
--------------------
I asked a similar question of the person who is head of all
soldering operations at Siemens at an ISO soldering materials
standards meeting. The answer I got was "we don't know what sort
of material would result, could be really excellent alloy, could
be very brittle, properties could not vary much over a range or
could optimize at a very sharp peak (in the phase diagrams)." So
they were not into mixing different alloys on the PWB, lead
finish and the joining alloy.
Too bad some excellent work done about ten years ago was never
published.
Experimenter ran designed experiments with alloy compositions, then
using a
mathematical model, generated an Excel spreadsheet where you plugged in
the
basics and got a phase diagram output. Really slick predictive tool.
Just
heard one presentation on it, so may not have all the details just
right, but
the essence is there:
Run some basic experiments and you can predict the phase diagram for any
combination of the alloy components!!
Experimenter's mgt. never understood the value and put him on "work with
a
real payoff potential" which of course killed it.
--Bill
==============================================================
[3] Paul Gill Wrote....
Michael,
I am a bit behind on my reading so you may already have the
answer
that you are looking for. But I will give you what I can just the same.
During reflow at equillibrium the low temperature solder will
melt
first. Given sufficient time and temperature and the high temperature
solder will be dissolved into the liquid.
Grab yourself a binairy phase diagram for the alloys that you
are
looking into. Assuming that your system is only lead and tin then you
have a simple eutectic situation. This means that at the ideal
composition (ie the eutectic) a phase transition will occurr at a single
temperature where by the solid will go completly liquid.
Armed with your phase diagram you will also note that the
eutectic
point is formed by the intersection of three lines. The upper two line
are the liquidus lines and the bottom horizontal line is the solidus
line.
So lets put this in perspective. If you have a eutectic tin
lead
mixture and you place it on top of a bar of lead and you heat everything
up past the eutectic point but below the melting point of pure lead.
Then
your reflowing eutectic ball will dissolve lead from the bar into
solution
until the composition reaches the liquidus line at which time the system
will essentially be at equillibrium and it will no longer continue to
dissolve. (This is a bit of an aproximation but it is valid for typical
reflow times)
If this doesn't help pick up a reference book on metalurgy and
check out the section on brazing or soldering.
I guess I didn't answer your question. The answer is yes the
two
will alloy as long as sufficient time and temperature are used.
Remember
if you go above the liquidus of both alloys they will mix readily.
However if you are somewhere in between the two liquidus temperatures
then
the rate of dissolution will be much slower. And lastly if the max
temperature isn't above the liquidus for the final composition of the
two
alloys then complete mixing will not occurr.
Some thing are easier to explain in person.
Good Luck
Paul Gill
Metalurgist at large
==============================================================
[4] Andy Magee Wrote...
Michael,
QUESTION: at "normal" soldering iron temperatures, what happens when you
melt a eutectic solder preform (p/o clip lead) together with hi-temp, 90
to 94% Sn (bal=Pb) solder?
Will the two solders alloy, or just form a mixture? Please provide as
metallurgically correct an answer as possible.
Generally speaking, they will alloy. Issues that may impact the result
are things like:
melt (mixing) time
thermal (convection) gradients
flux activity
presence of oxides
presence of copper (intermetallics) from previous reflow cycles
Andy Magee
Flex Guru - Consulting
[log in to unmask]
(937) 435-3629
==============================================================
==============================================================
==============================================================
My Original Question was posted as follows....
On Wed, 25 Nov 1998, Alderete, Michael wrote:
> Dave Hillman, et al...
> 1st, Happy Thanksgiving to all on TechNet.
>
> QUESTION: at "normal" soldering iron temperatures, what happens when
you
> melt a eutectic solder preform (p/o clip lead) together with hi-temp,
90
> to 94% Sn (bal=Pb) solder?
> Will the two solders alloy, or just form a mixture? Please provide as
> metallurgically correct an answer as possible.
>
> Thanks in advance
>
> Michael Alderete
> Aerojet, Azusa (So.) CA
>
> ...it's going to be about 80F here tomorrow. Hope you'll still answer
> this question on solder mixtures.
>
> ################################################################
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