Hi Steve,
You are correct, 2% silver does have a small pasty range. This alleviates
the problem of paste becoming liquid at different times on either side of
the 0402. While one side of the 0402 may become fully liquid, the other
side will at least be partially liquid, thereby giving it some surface
tension to resist the pull of surface tension on the fully liquid side.
Another source of tombstones is a difference in wetting speeds from one side
of the component to the other. A pasty range alloy will act like the pace
car at the Indy 500. It will allow some wetting during the pasty range
before going full throttle when fully liquid. This helps prevents solder on
one side of the component from grossly out racing the other side when
wetting the termination and pad.
Panasonic has also done thorough testing on tombstoning on 0402's. They
likewise found a enormous reduction in tombstones when using the 2% silver
solder.
Ryan Grant
-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Tuesday, September 10, 2002 1:16 PM
To: [log in to unmask]
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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