Hi TechNet - I just wanted to add some additional info to Peter's reply.
The NCMS has published their final report on the Lead Free Solder Project
which investigated a multitude of lead free solder alloys. The report has
some excellent data concerning bismuth containing alloys and the pro/cons
associated with trying to use such an alloy in today's electronics material
mix. The report can be obtained through the NCMS. The concern of impacting
the solder joint reliability due to the addition of bismuth is real and has
been documented in "Solder Mechanics - A State of the Art Assessment" ISBN
# 0-87339-166-7 also. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
Peter Swanson <[log in to unmask]> on 06/25/98 02:19:37 PM
Please respond to [log in to unmask]
To: [log in to unmask]
cc: (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject: Re: [TN] Low-melt solder alloys for desoldering
In message <[log in to unmask]> [log in to unmask]
writes:
> I would like your opinions and experiences using low-melt solder alloys
for
> rework of smd. The mixture contains tin, lead, and bismuth. What
> potential problems are associated with this combination?
It's an interesting concept, one which we have been considering.
The process involves reflowing a LMP (low melting point) solder into the
existing solder joints, using a soldering iron or hot air pencil (and
optionally using some under board heating). The LMP solder mixes with the
existing stuff to form a mixture which stays molten at a much lower
temperature
than standard 63/37, and therefore stays molten much longer, even when you
remove the heat source; so, you can take off even a large QFP device
having just used a soldering iron or hot air pencil.
I see pros:
1) Very inexpensive on the equipment side, no capital investment needed
2) Can be done anywhere (in the field)
I see cons:
1) Material (which is a tin/lead/bismuth/indium type combination) is
expensive,
so not really a process for people doing lots of rework
2) It is vital that the LMP solder and mixtures be removed from the pads
before a new component is placed and soldered, otherwise your joint could
reflow at a low (maybe operating!) temperature
3) Won't work for BGAs and other array devices
Para 2 is what worries me; no matter how well you wick off the LMP
solder from the pads, I think you will still have a (thin) layer of
bismuth-
bearing intermetallic left on the pad. When a new joint is made with
conventional solder, the bismuth will stay at that interface (not being
very
mobile). Bismuth being brittle, I reckon the new joint is much more likely
to
fatigue crack at the joint/pad interface.
So, reliability suspect? I dunno, no data, just theory. Maybe the approach
is
valid for the hobbyist or the occasional job in a low-tech consumer product
(Class 1). Other inputs appreciated.
Some info from a company who have commercialised this process can be
found at http://chipquikinc.com (I offer this for your interest, not as an
advertisement - sheesh, some people are sensitive!).
Peter
--
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Peter Swanson Oxfordshire,
England
INTERTRONICS
[log in to unmask]
http://www.cygnetuk.demon.co.uk
Suppliers of materials and consumables to the electronics & related
industries
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