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December 2006

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Mon, 11 Dec 2006 17:00:07 -0000
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Advantages
lead free, relatively cheap for a non tin lead, non SAC material, much
cheaper than other low temp materials based on indium.
Compatible with most common finishes.
Thermal fatigue resistance of this alloy is quite good for its low melting
point.
It is not ductile but HP have published work with a modified version with a
small percent of Ag added, this makes the material much more like Sn/Pb. I
have a reference somewhere.

Disadvantages
Do not mix with lead containing solders probably including finishes (but not
the pure Sn with limited amount of anti whisker lead)
This is a slow to solder alloy as well as a low temperature material. You
will need to choose your flux to make sure that it will work and wet OK,
particularly to nickel (as in ENIG / IMAuNi). For ENIG if you consider a
normal profile to be tent shaped, for Bi/Sn Bi/Sn/1Ag your profile will be a
marquee to get good intermetallic formation. If you have a choice avoid ENIG
if you can.
No clean formulations are available subject to seeing a min peak (according
to supplier) otherwise clean.
A few companies do a useable and genuine electronic grade Bi/Sn as paste,
(fewer still the preferable Bi/Sn/1Ag version)
You may find a flux cored wire supplier, but solid is readily doable with
separate gel flux as described above. If you stick with the Bi/Sn you will
have some restrictions on smaller diameters but the Bi/Sn/Ag is much more
manufacturable, as you would expect from the increased ductility.
Be aware that alloys with >50% Bi expand slightly on freezing. This is not
necessarily a disadvantage, its just a thing to note.
Bi/Sn and especially the Bi/Sn/Ag are used in electronics and have  a
relatively large application on non Kapton flexible circuits where the lower
MP is a considerable benefit.
If you are looking for a rework or odd form/missing components etc solder
this has merit, but I would think hard before adding it to BOM without
exhausting suppliers of regular alloys. This is a question I often answer,
and it turns out that the real answer especially with Sn/Cu or SAC soldering
is really a better alternative supplier, or even just better shop floor
practise.

Hope this helps

Regards

Mike Fenner
Indium Corporation
www.indium.com

T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]





-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kerry McMullen
Sent: Monday, December 11, 2006 3:01 PM
To: [log in to unmask]
Subject: [TN] 58Bi/42Sn

Hello esteemed Technet,
      I am looking comments on what you would consider the advantages and
disadvantages of this composition of this solder paste. (58Bi/42Sn)
Lower MP.  138 Degrees C.
Lower fatigue resistance
Wetting to ImAg??
Wetting to ImNiAu??
No Clean?  Residue?
Intermetallic Formation?
Mixing with 63/37Sn?
Our application is an RF board where we would use this paste post Wave /
Hand Solder.
Any comments appreciated.

Cheers,
Kerry


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