Hi Joyce,
Of course you are right, if customer spec'ed the paste then it is carved in
stone. But maybe they would allow a little leeway with a slight adjustment
in the alloy...
Steve
On Thu, Jul 24, 2014 at 3:33 PM, Yuan-chia Joyce Koo <[log in to unmask]>
wrote:
> 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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>>
>> 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
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