Interesting, thanks, I refuse to get into the Sn63 vs. Sn62 debate, but I
would just caution that when comparing the differences between alloys keep
in mind that the data for solder properties is [usually a consensus number]
derived from "lab perfect" specimens, commercial alloys may not have so
exacting alloy ratios, there will also be variation between suppliers for
the same notional alloy in other areas such as oxide content and so on. So
it could well be that the alloys you are looking at are "off eutectic" and
therefore effectively exploiting to a limited extent the antitombstoning
effect deliberately built in to formulated alloys such as our #100 and those
of other suppliers mentioned in other posts.
Regards
Mike Fenner
Applications Engineer, European Operations
Indium Corporation
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
W: www.indium.com
Leadfree: www.Pb-Free.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dave Hillman
Sent: Wednesday, September 11, 2002 2:06 AM
To: [log in to unmask]
Subject: Re: [TN] 2% Silver solder and tombstoning...
Hi Steve! The differences between Sn63 and Sn62 on the tombstone type
defects is not voodoo but has a science basis. The Sn63 alloy is a
near-eutectic composition with a melting range of slightly above 183C (183C
to 188C) - most everyone lists the melting point at 183C but only the true
eutectic composition (61.9 Sn) really melts at that temperature. Most of
the time this small temperature discrepancy doesn't influence the reflow
process. The surface tension of the Sn63 alloy is 490 dyne/cm. The melting
range of the Sn62 alloy is 177C-189C (the alloy is not an eutectic
composition and therefore doesn't melt a one temperature!). The surface
tension of the Sn62 alloy is 376 dyne/cm. And before I get flamed - yes, I
know I should be listing the surface tension values at the temperature they
were measured by I couldn't get my references quite lined up so please just
take them with a grain of salt! Now all you have to do apply the solder
alloy properties to the physical phenomena of tombstoning - tombstoning is
hugely influenced by the surface tension of the solder alloys and when
melting begins to occur. The Sn62 solder alloy has a slight advantage in
that the initial melting temperature is slightly lower and its surface
tension is lower meaning it will wet surfaces sooner. It's pretty easy to
see why some folks favor the Sn62 alloy over the Sn63 alloy. With all that
being said, you should consider one alternative avenue - a change of the
component pad geometry also impacts the surface tension forces and could be
used as a tombstone fix instead of switching solder alloys! And as you
mentioned - other changes in the solder alloy composition can be used too.
I have found that there are two very distinct camps - those who swear by
using Sn62 and those who don't - kinda like Doug and I arguing which is
better Coke or Mt. Dew! I know lots of folks who use Sn63 and have no
tombstone problems..... and lots of folks who use Sn62 and have no
tombstone problems! Just one of those issues in which you have to look at
your processes/pwb pad geometries and decide if a either direction fits the
way you want to run the solder process. And if you really want to
complicate the process throw in the impact of having a 1206 capacitor
versus a 0402 capacitor (or a mix of both)! Hope this helps (solder process
are just soooo simple!).
Dave Hillman
Rockwell Collins
[log in to unmask]
Steve Gregory <[log in to unmask]>@ipc.org> on 09/10/2002 02:16:08 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to [log in to unmask]
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: [TN] 2% Silver solder and tombstoning...
Hi All,
A while back there was some discussion about 2% silver solder paste and the
effect it had reducing tombstones with 0402's.
I was skeptical at first, but I decided to try it on a board we build here
that quite often has a problem with 0402's tombstoning.
To make a long story short, the results were dramatic. Since we've been
using the 2% silver paste on this board, tombstones are a very rare
exception.
I was asked by a new engineer here why I use 2% silver paste to reduce
tombstoning, I told him because it really works, I seen it work with my own
two eyeballs. But he asked why does that make a difference? That's where
I'm kind of stuck...I think I remember that it has something to do with the
2% paste not really being a true eutectic solder, that it's close, but
there are slightly different temperatures for solidus and liquidous for
that alloy...can anybody educate me again?
I did a search and found that Senju had done some studies and found that
slight additions of silver and antimony reduced tombstoning to below 10% of
that which occured with a 63/37 solder. I think it's either AIM or
Multicore also sells what's called a low tombstoning alloy as well.
As always, thanks in advance!
-Steve Gregory-
P.S.
This is non-technical, but I'd like to pass on something that was sent to
me...
September 11th is tomorrow. On that day, wear red, white, and blue to work
or school to show your support for those who lost their lives on 9/11/01,
and to honor the heroes who worked to save them, and the families left
behind. At noon your time on September 11th, 2002, no matter where you are,
or what you are doing, stop, put your hand over your heart, and say the
Pledge of Allegiance, and say a prayer for our nation. In addition, for
those of us that drive to work, please drive with your headlights on to
also give honor and rememberance to those that perished on September 11th.
If all of us work together in every time zone around the world, we will
have a powerful chain of thoughts surrounding us. Please share this with
your friends and family. On September 11th, hopefully enough people will
have read this and will join together in unity.
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