Steve, change paste is a major material change,  it might not flight  
if he does not have design authority even to change the pad layout.   
he might need go up to the food chain (i mean - design... that  
selected material/process and parts).
imo.
   jk
On Jul 24, 2014, at 5:16 PM, Steve Gregory wrote:

> 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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> on 09/10/2002 02:16:08 PM
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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]>
>>
>> P Please consider the environment before printing this e-mail
>>
>>
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