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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Guy Ramsey <[log in to unmask]>
Date:
Thu, 6 Sep 2018 10:32:26 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (157 lines)
The surface finish is ENEPIG

On Thu, Sep 6, 2018 at 10:21 AM George Wenger <[log in to unmask]>
wrote:

> I agree Richard.  The difference sounds like a solder wetting variation do
> to the pad and not the reflow process.
>
> RIGHT ON RICHARD OSP over bare copper is just below Immersion Tin which is
> at the top of my list of least favorite finishes.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D
> Sent: Thursday, September 06, 2018 9:56 AM
> To: [log in to unmask]
> Subject: Re: [TN] R: [TN] Ni intermetallic thickness target
>
> Dave's Rant and what you have posted below are all too true. But one thing
> I know for sure; OSP over bare copper is next to immersion tin on the
> bottom of my list of favorite finishes. I was wondering if the variation in
> IMF could be caused by some pads having thicker OSP coverage, and thus
> blocking or interfering with the wetting of the solder onto the pad during
> reflow? Is there a way you could mechanically or chemically remove the OSP
> on a scrap PWB on only some of the pads for the DDR3 part, then print the
> paste as you normally would and reflow the board using your existing reflow
> profile, then have them microsection that and see if you get
> different/better results? This would eliminate the part plating as a causal
> factor, and possibly prove the OSP is at least a major contributor to the
> issue, and possibly exonerate your reflow profile, thus eliminating several
> factors as the issue. My suspicion stems from the fact that if OSP is not
> properly applied immediately some of the copper pads may have oxidized,
> leading to the variation in IMF amongst pads. The simple fact that pads
> right next to each other have major variation pretty much rules out the
> reflow parameters unless there is a correlation between pads with heavy
> copper connections or some other cause.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
> Sent: Thursday, September 06, 2018 7:10 AM
> To: [log in to unmask]
> Subject: Re: [TN] R: [TN] Ni intermetallic thickness target
>
> 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-a
> > lloy-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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