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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