Hi again Steve,
I went back and looked for the thread about 2% silver solder paste and it's
effect on tomb-stoning. I wanted to find it because I remembered that Dave
Hillman gave a very good explanation of why it works, I was able to find
it. Below is the thread:
Steve Gregory <[log in to unmask]
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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-
************************************************************
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
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On Thu, Jul 24, 2014 at 1:57 PM, Vargas, Stephen M <
[log in to unmask]> wrote:
> Hello All:
>
> We are experiencing a high rate of tombstoning on two particular package
> styles (0508 and 0612 capacitors) on an assembly here. The rest of the
> board solders at a normal defect rate. Here are some of the things we have
> looked at and some aspects of our process:
>
> I've tried using two different profiles (straight ramp to peak and a ramp,
> soak, spike).
> I've moved the parts from our high speed chip shooter to our flexible
> placement machine to optimize placement accuracy.
> The pad layout (which is not an option for change due to the product
> having already been qualified by our customer) is very close to the
> manufacturer's recommended layout and the board finish is immersion silver.
> We are printing 1:1, no aperture micro-modifications.
> Stencil thickness is 6 mils. I'm concerned about moving to a thicker
> stencil due to having 20 mil pitch parts on the board.
> Our paste is a low residue / pin probe-able no-clean 63/37 (again not an
> option for change).
> We also looked at which side of the device is connected to ground,
> assuming that this side of the device would heat up more slowly causing a
> tombstoning condition pivoting at the non-ground side. But there was no
> trend indicating that this was the case.
> Solder mask height measured in between the two pads and it was determined
> to be at the same height as the pads
>
> I've attached a link to the datasheet for one of the devices for
> reference. I'm wondering if the forces of physics make this part more
> susceptible to tombstoning due to the terminations being on the long side
> of the device. Any ideas on how we can improve our yields? Thanks.
>
> http://www.avx.com/docs/Catalogs/licc.pdf
>
> Regards,
> Steve Vargas
>
> If you don't have time to do it right, when will you have time to do it
> over?
> John Wooden
>
> Polaris Contract Mfg Inc
> 15 Barnabas Rd
> Marion, MA 02738
> 774-553-6192
> [log in to unmask]<mailto:[log in to unmask]>
>
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