And I'll third that vote! All of the immersion type finishes have roles to
play for our electronic designs but ImSn is the least robust to assembly
processes. Also, I don't recommend ever using an eraser for "cleaning" a
pad. If you examine a pad that has been erased, you will find eraser
contamination embedded in the pad that can cause solder joint integrity
issues later. Using an eraser to troubleshoot is ok but no further use
should be conducted.
Dave
On Thu, Dec 21, 2017 at 7:42 PM, George Wenger <[log in to unmask]>
wrote:
> Tan Geok,
>
> The one think I absolutely agree with is that I also hate immersion tin
> surface finish
>
> George
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Tan Geok Ang
> Sent: Thursday, December 21, 2017 8:15 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA pad non-wetting
>
> 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_se
> > arch-3Frls-3Dcom.microsoft-253Aen-2DUS-253AIE-2DSearchBox-26ei-3D&d=Dw
> > IFaQ&c=nvOEfClv88r3To4eOyYtQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwV
> > eFoas7eghr5IF6QpUBreyGR1qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_Q
> > qE&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.Sug1ul99S
> > sE
> >
> > 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.homest
> > > ead.com_2017-2D12-2D21-5F11.21.17.jpg&d=DwIFaQ&c=nvOEfClv88r3To4eOyY
> > > tQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwVeFoas7eghr5IF6QpUBreyGR1
> > > qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_QqE&s=Du_vGyBf05tjpSvFe
> > > t5EPytyLuSbCfhNQ4wlPVTfIK8&e=
> > > <https://urldefense.proofpoint.com/v2/url?u=http-3A__stevezeva.homes
> > > tead.com_2017-2D12-2D21-5F11.21.17.jpg&d=DwIFaQ&c=nvOEfClv88r3To4eOy
> > > YtQN0213v_-JGWbujap2hCee4&r=-MAMt3MrjiRvYJiwVeFoas7eghr5IF6QpUBreyGR
> > > 1qs&m=ZtT8m__jsmKXx-NxB6DAVryCAm8iznmmCFPq9PL_QqE&s=Du_vGyBf05tjpSvF
> > > et5EPytyLuSbCfhNQ4wlPVTfIK8&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
> > >
> > > --
> > >
> > >
> > > This email and any attachments are only for use by the intended
> > > recipient(s) and may contain legally privileged, confidential,
> > > proprietary or otherwise private information. Any unauthorized use,
> > > reproduction, dissemination, distribution or other disclosure of the
> > > contents of this e-mail or its attachments is strictly prohibited.
> > > If you have received this email in error, please notify the sender
> > > immediately and delete the original.
> >
>
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