Believe it is Immersion Sn surface finish. I usually  use a pencil eraser to slightly rub against those pads and it works well. I hate immersion Sn finish. Take care of ESD.

________________________________

From: Guy Ramsey <[log in to unmask]>
Date: 22 December 2017 at 6:01:45 AM SGT
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] BGA pad non-wetting

Wow, thanks

On Thu, Dec 21, 2017 at 4:58 PM, Stadem, Richard D <[log in to unmask]> wrote:

> I agree with George that these pictures exhibit classic black pad or
> immersion tin intermetallics of some type, but even with those some wetting
> normally takes place on any given pad.
> I do not think it is a thin soldermask film on the pads, but it could be.
> The reason I don't think it is soldermasking is because if you look
> carefully at the picture, some pads appear to be fully wetted, and some do
> not, but no pad has any partial wetting.
> It is either "on" or "off".  However, the pads that appear wetted may not
> really be wetted. As Werner Englemaier once pointed out, you can achieve
> complete "wetting" of solder on a toothpick if you know how.
> But at $45,000 per assembly, it would be a career-enhancing move to find a
> way to clean up the oxidation, whatever the cause. I have a process that
> has worked for me, tell you in a minute.
> But before you attempt to clean up any oxidation, please make one more
> check. See if the pads that never wetted are connected to power, ground, or
> reasonably heavier traces than the pads that DID wet, or at least appeared
> to wet.
> The root cause may be a perfect storm of insufficient heat in the reflow
> profile, right on the edge of getting the board hot enough to reflow
> unattached pads or pads not severely heatsinked, along with some
> finish-related oxidation issues, and perhaps a flux not quite active enough
> to remove the oxides at the current reflow temperature.
>
> Otherwise, I have used very fine Dremel #463 blue-oxide abrasive tips
> impregnated with abrasive to remove oxides, rough areas, small burrs and
> scratch marks.  See one here:
>
> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.google.com_search-3Frls-3Dcom.microsoft-253Aen-2DUS-253AIE-2DSearchBox-26ei-3D&d=DwIFaQ&c=nvOEfClv88r3To4eOyYtQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwVeFoas7eghr5IF6QpUBreyGR1qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_QqE&s=80pAGk1ZIDMwG9Yl4yta_g7WK1A6ibC-_9lxLJZVSmM&e=
> LCI8WrLkFIiJ0gK3p6YCw&q=Dremel+blue+rubber+polishing+
> tips&oq=Dremel+blue+rubber+polishing+tips&gs_l=psy-ab.3..
> .3284.20303.0.21161.40.39.0.1.1.0.263.5212.0j26j4.30.0....0.
> ..1c.1.64.psyab..9.28.4637...0j0i10k1j0i131i67k1j0i131k1j0i
> 67k1j0i22i30k1j0i22i10i30k1j33i22i29i30k1j33i21k1j33i160k1.0.Sug1ul99SsE
>
> They have a tiny yet firm point to clean pads of oxides or other
> contamination. It takes a very fine touch to do it, is difficult on pad
> diameters smaller than .020", but it can be done and for a single component
> it takes less than a minute or two on the unwetted pads only. An overhead
> ionizer is used to prevent or minimize any ESD charge buildup, and the area
> to be reworked is masked off prior to the polishing such that only the BGA
> pads are exposed. A vacuum with a grounded tip is used to clean up the
> debris as you go along. Then you wipe the area with a drop of IPA on a foam
> swab, and you apply a thin veneer of a good tacky flux like Alpha WS820
> immediately after the last vacuuming. You then manually solder coat the
> pads, leave the flux on but add a little more, place the part and reflow
> with a hot nitrogen reflow machine. When finished, unmask under the
> ionizer, prewash by directing a low pressure spray under all four sides of
> the BGA, then follow that with a pass through the in-line or batch cleaner,
> blow off the water at 30 PSI max (but thoroughly), bake at 105 C for just
> 20 minutes, visually inspect using a good scope and prism, and re-test. If
> it passes, touchup the coating if present.
> If this seems to work, perform some type of thermal cycling that makes
> sense for the final use environment on two or three CCAs to ensure the
> oxidation or reaction does not re-occur over time. Get the customer buy-in
> if selling those units as downleveled CCAs.
> If that all seems to work, then continue with the rework on a larger
> sample of CCAs and closely monitor the BGA performance at test and final
> visual inspection using a good microscope and prism.
> No problems? Increase the sample size of the rework and continue.
> Everything still good? Then walk upstairs and barge into the Upper
> Management Meeting and tell them you deserve at least a 50% increase in
> salary, a corner office, and a company car with expense account if they
> want you to continue to keep the entire company from washing down the
> toilet just when Donald was about to give them a huge tax break.
> Let us know how it goes!
>
> Good Luck, and Merry Christmas!
>
> Odin
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of George Wenger
> Sent: Thursday, December 21, 2017 2:05 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA pad non-wetting
>
> Steve,
>
> I just went back and re-read your initial post and looked at the pictures
> again.  I originally thought we were looking at the pads on the BGA but now
> I think we're looking at the pads on your PCBA.  If they are the pads on
> the PCBA then my suggestion is to look at a new non-soldered PCBA and let
> us know if the pads look like gold or tin.  If they look like gold you have
> an ENIG Black Pad problem if they look like tin then you have an oxidized
> Cu-Sn IMC.  The real problem in either case there really isn't a reasonable
> fix.  The only think I would try is to use a very aggressive flux and try
> to pre-tin the pads with solder and then clean off the flux residue and go
> back through your normal repair process.
>
> George
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of George Wenger
> Sent: Thursday, December 21, 2017 2:58 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA pad non-wetting
>
> Steve,
>
> I totally agree with John.   My first thought was that the  pad looked
> like thin immersion tin where the IMC came to the surface an oxidized in
> which case solder will not stick to the pad.  But I don't know of any BGA
> where they use Immersion tin so I still suspect you have an ENIG Bad Pad
> issue.
>
> George
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
> Sent: Thursday, December 21, 2017 2:51 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA pad non-wetting
>
> Assuming it’s not ENIG???
>
> If not ENIG I would suspect immersion tin where the intermetallic has come
> to the surface.
>
> If an ENIG problem or an immersion Tin problem it is not realistically
> recoverable without impacting the assembly with some serious reliability
> issues.
>
> Sent from my iPhone
>
> > On Dec 21, 2017, at 11:39 AM, Steve Gregory <[log in to unmask]> wrote:
> >
> > Hi all,
> >
> > I've been asked to post this to the Technet by a dear friend:
> >
> > *We installed this little processor. At test we identified opens on
> > the device. When we removed it we saw a number of pads that were matte
> > gray with no evidence of solder adhered to the pad. * *We attempted to
> > bump the pads with solder, they did not wet easily. In some cases we
> > had to scrape through the matte finish to a shiny metal.
> > But, we did get all the pads to appear wetted.*
> >
> > *We cleaned the site well and then fluxed the bumps with a tacky flux
> > ROLO designed for POP, placed the BGA and reflowed it on a rework
> > station.*
> >
> > *The assembly failed test with similar result, opens under the
> > processor.*
> >
> > *The attached photo shows that the bump process only appeared to wet
> > the pads. The condition returned when we removed the BGA  second
> > time.*
> >
> > * https://urldefense.proofpoint.com/v2/url?u=http-3A__stevezeva.homestead.com_2017-2D12-2D21-5F11.21.17.jpg&d=DwIFaQ&c=nvOEfClv88r3To4eOyYtQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwVeFoas7eghr5IF6QpUBreyGR1qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_QqE&s=Du_vGyBf05tjpSvFet5EPytyLuSbCfhNQ4wlPVTfIK8&e=
> > <https://urldefense.proofpoint.com/v2/url?u=http-3A__stevezeva.homestead.com_2017-2D12-2D21-5F11.21.17.jpg&d=DwIFaQ&c=nvOEfClv88r3To4eOyYtQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwVeFoas7eghr5IF6QpUBreyGR1qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_QqE&s=Du_vGyBf05tjpSvFet5EPytyLuSbCfhNQ4wlPVTfIK8&e= >*
> >
> > *Have you seen this before? Is there any recovery? This assembly is
> > worth about $45K. *
> >
> > Thanks,
> >
> > Steve
> >
> > --
> > Steve Gregory
> > Kimco Design and Manufacturing
> > Process Engineer
> > (208) 322-0500 Ext. -3133
> >
> > --
> >
> >
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