Actually, Andy did and did not misinterpret what I meant- sorry for being a
bit ambiguous. By "reflow window", I meant the varying peak temperatures
that the different users of solder paste will require for all of their
different parts/applications. However, delta T, quality of heat transfer,
etc, etc all will have an obvious effect on the peak temps required for
proper soldering results.
I wholeheartedly agree with Andy that if all users of paste adopted his
"simple solution", there would be no problem in jumping to lead-free
soldering operations. However, it seems that this will never occur, at least
not on a wide-scale basis. Just yesterday I was speaking to a relatively
large OEM, and they were having solder balling problems. Turns out they're
using a 5 zone infrared oven. Soldering Sn/Ag/Cu in that might require a
peak temp of 245-250C! I'm sure all of you can relate to this. And the use
of nitrogen seems a step that only the largest (or best?) companies will
take. How many companies are investing in new capital equipment nowadays?
I'm afraid that it's unrealistic to put such tight operating constraints on
production floors.
I do not want to come across as cynical, but I feel that if Pb-Free
soldering operations are to come about (or be mandated), then some worst
case scenarios need to be looked at in order to develop parts and processes
that can handle all of these.
Thanks.
Dave Suraski
-----Original Message-----
From: Andy Mackie <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, July 29, 1999 4:56 PM
Subject: [LF] Reflow Windows 99
>Hey Gang: Dave Suraski quite justifiably mentions the worry about reflow
windows
>to meet the 218-230°C target....
>
>I'm always troubled by the term 'reflow window' - it seems so nebulous;
although
>I must confess to having resorted to it a few times myself. I think, in
this
>instance, Dave is using the term to mean "the spread of peak
reflow-temperatures
>seen across an assembly as a result of different thermal masses and
>heat-transfer coefficients seen across the board". [I hope I'm not
>misinterpreting you here, Dave!]
>
>Actually there is a simple solution:
>
> - Used forced-convection to optimise heat transfer rates
>
> - Preheat assemblies to high temperatures (maybe only 5-10°C below the
melting
>point of the alloy)
>
> - Use much lower spike temperatures to keep the front corners at or below
the
>230°C
>
> - Use an inert atmosphere to retain solderability and paste reflow
>
>It is possible to get delta T down to a few degrees C using this technique,
even
>with a large assembly. So it can be done, it just needs a bit of getting
away
>from old technomyth that "the profile is dictated by the solder paste
>activator": not under nitrogen, it isn't!
>
>Andy Mackie, PhD
>Development Associate
>Praxair Inc.
>765 Old Saw Mill River Road
>Tarrytown
>NY 10591
>Ph: 914-345-6402
>Fx: 914-345-6405
>
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IPCWorks -October 25-28 featuring an International Summit on Lead-Free Electronic
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Please visit IPC's Center for Lead-Free Electronics Assembly
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