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Subject:
From:
David Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Hillman <[log in to unmask]>
Date:
Thu, 6 Sep 2018 07:26:36 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (127 lines)
Hi Guy - my recommendation is to find out the specifics of the soldering
process that formed the IMC layers. If the soldering process is normal with
no anomalies then the IMC formed normally based on the phase diagram and
kinetics at the time of soldering. Did the lab just "observe" that the IMC
layers had variation or did they link it to a failure root cause?

Dave H.

On Thu, Sep 6, 2018 at 7:10 AM, Guy Ramsey <[log in to unmask]> wrote:

> We infer that an IMC has formed by visual evidence, wetting and spreading.
> In the case of bottom only terminations we seek evidence by other means. We
> can't see the contact angle or determine what forces created the spreading.
> In this case a lab cross sectioned a DDR3 memory device. The lab observed
> continuous intermetalic on the individual pads but characterized the IMC
> layer as too thin and inconsistent from pad to pad.  Where one pad
> exhibited 70uin of IMC the neighboring pad measured less than 10uin.  I
> found very little discussion of this in white papers. But, I did find a
> nicely done DOE, Effects of reflow profile and thermal conditioning on
> intermetallic compound thickness for SnAgCu soldered joints. This paper
> contained data about the thickness of IMC formed at different temperatures
> and dwell times. To some extent irrelevant because the base was OSP copper.
> However, the standard deviations in measured data on this experiment were
> much lower than the deviations the lab found at the DDR memory pads. Is a
> large variation a cause for concern. How thin is too thin? My first
> impressions of the report fell in line with Dave's Rant, that trying to
> establish a reflow profile to achieve some standard IMC thickness was a
> fool's errand. But, on reflection, variation from assignable causes is
> always the enemy.
>
> On Wed, Sep 5, 2018 at 6:01 PM Bob Landman <[log in to unmask]>
> wrote:
>
> > Hi Rich,
> >
> > You mean K100LD, right?
> >
> >
> > https://www.kester.com/products/product/k100ld-lead-
> free-silver-free-alloy-bar-solder
> >
> > Bob
> >
> > -----Original Message-----
> > From: TechNet <[log in to unmask]> On Behalf Of Stadem, Richard D
> > Sent: Wednesday, September 05, 2018 3:26 PM
> > To: [log in to unmask]
> > Subject: Re: [TN] R: [TN] Ni intermetallic thickness target
> >
> > That is also true, but there is really good information out there
> > regarding the fact that too thick of an IMC is worse than too thin.
> > That is because of the brittle nature of the alloy you end up with;
> > nickel, gold, palladium, tin, and with SAC305 you add silver and copper.
> > Although the gold, palladium, and silver are very low percentages, the
> > combination of these and a significant percentage of nickel with no lead
> > can make for a very brittle IMF, especially if it is at the 100 uinch or
> > thicker levels. So in that case, perhaps 20 to 70 uinches may be quite
> > ideal. I am trying to find the reports I saved in my bottomless stack of
> > "important stuff".
> > Also, the IMF formation is self-limiting, but it depends on the factors
> of
> > time above solidus, temperature, component plating, solder alloy type,
> etc.
> > These were all listed as having significant impact on the thickness along
> > with even very small amounts of germanium and other dopants. So, for
> > example, Kester KL100D has very different properties from SAC305, yet it
> is
> > almost 100% tin. KL100D is very similar to Sn63.
> > If there was a concern, then I would rather rely on actual reliability
> > tests taken over time, rather than the average thickness variation of a
> > bunch of microsections. It's very difficult to guess at exactly what
> > thickness might be ideal, but reliability results prove that, assuming
> you
> > know for sure what thickness you have with your samples. And remember,
> the
> > IMF grows over time; it is never the same 3 months or two years later.
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of SALA GABRIELE
> > Sent: Wednesday, September 05, 2018 1:39 PM
> > To: [log in to unmask]
> > Subject: [TN] R: [TN] Ni intermetallic thickness target
> >
> > Keep in mind also the terminal finishing.....
> > One reflow or two reflow ? etc
> >
> > Too  early to fix a reliable IMC thickness ..... too many variables
> > playing !!!
> >
> > GS
> >
> >
> > -----Messaggio originale-----
> > Da: TechNet [mailto:[log in to unmask]] Per conto di Guy Ramsey
> > Inviato: mercoledì 5 settembre 2018 19:34
> > A: [log in to unmask]
> > Oggetto: Re: [TN] Ni intermetallic thickness target
> >
> > ENEPIG
> >
> > On Wed, Sep 5, 2018 at 1:17 PM Stadem, Richard D <
> [log in to unmask]
> > >
> > wrote:
> >
> > > What is the finish plating?
> > >
> > > -----Original Message-----
> > > From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
> > > Sent: Wednesday, September 05, 2018 11:59 AM
> > > To: [log in to unmask]
> > > Subject: [TN] Ni intermetallic thickness target
> > >
> > > Recently, I was reviewing a lab report. It concluded that the
> > > manufacturer should increase the IMC thickness as a part of process
> > changes . . .
> > > It stated that, while there are no industry specifications for IMC
> > > thickness it s generally accepted that for Pb-free assemblies the IMC
> > > thickness should be in the 20 to 120 uin range. It seems to be
> > > critical of a process that produces IMC between 10 and 70 uin on pads
> > > across a single device.
> > > Does anybody have reference papers or texts that would support this
> > > target and process critique?
> > >
> >
>

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