All:
Thanks for the responses; I've now got some good leads to follow up on to help alleviate our problem.
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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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
Sent: Friday, July 25, 2014 8:02 AM
To: [log in to unmask]
Subject: EXTERNAL: Re: [TN] Tombstoning
Hi gang - the majority of the industry considers the Sn63Pb37, Sn60Pb40 and
Sn62Pb36Ag2 equivalent alloys so the substitution wouldn't be a big deal reliability wise. However, the potential changes in your stencil process could be a concern. Investigating solder paste volume changes is a much less intrusive option to explore.
Dave Hillman
Rockwell Collins
[log in to unmask]
On Thu, Jul 24, 2014 at 4:56 PM, Steve Gregory <[log in to unmask]> wrote:
> 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]
> >> <http://listserv.ipc.org/scripts/wa.exe?LOGON=A3%
> >> 3Dind0209%26L%3DTECHNET%26E%3D7bit%26P%3D1153010%26B%3D--
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> >> 3B%2520charset%3DUS-ASCII>
> >>
> >>> @[log in to unmask]
> >>>
> >> <http://listserv.ipc.org/scripts/wa.exe?LOGON=A3%
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> >> part1_cb.280b0562.2ab010a2_boundary%26T%3Dtext%252Fhtml%
> >> 3B%2520charset%3DUS-ASCII>>
> >> on 09/10/2002 02:16:08 PM
> >> Please respond to "TechNet E-Mail Forum." <[log in to unmask]
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> >>
> >>> ;
> >>>
> >> Please respond to [log in to unmask]
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> >> Sent by: TechNet <[log in to unmask]
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> >>
> >>>
> >>> To: [log in to unmask]
> >> <http://listserv.ipc.org/scripts/wa.exe?LOGON=A3%
> >> 3Dind0209%26L%3DTECHNET%26E%3D7bit%26P%3D1153010%26B%3D--
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> >> 3B%2520charset%3DUS-ASCII>
> >> 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-
> >>
> >> ************************************************************
> >>
> >> 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]
> >> <http://listserv.ipc.org/scripts/wa.exe?LOGON=A3%
> >> 3Dind0209%26L%3DTECHNET%26E%3D7bit%26P%3D1153010%26B%3D--
> >> part1_cb.280b0562.2ab010a2_boundary%26T%3Dtext%252Fhtml%
> >> 3B%2520charset%3DUS-ASCII>
> >>
> >>
> >>
> >> 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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