Andy,

This seems simple, but ... question: Won't the pre-heat temperatures be
30-40 C above the current levels? And is that a problem or not. If you
pre-heat to 10 C below the melt point, you are above 200 C ....

Ron Gedney
Director of Operations
2214 Rock Hill Road, Suite 110
Herndon, VA 20170-4214
Tel: 703-834-2084
Fax: 703-834-2735

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Andy Mackie
Sent: Thursday, July 29, 1999 5:00 PM
To: [log in to unmask]
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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