[SI-LIST] Re: Fixing a black pad

Gil,

Unless you have lead-free BGAs that are being soldered with lead =
containing solder paste, it is pretty
unlikely you are getting cold joints - any assembly house with a reflow =
system (unless the system is
new or not functioning properly,for which there should be a monitoring =
system!) will have a limited
number of reflow profiles (maybe just two), and these will not be too =
cold. Plus, if there are no lead-
free factors in the game, you would probably notice cold joints in other =
places.

An exception to this would be if the pads you are talking about are in =
the middle of a large BGA on a
thick board, where it easily takes 30 seconds longer to reach liquidus =
than at other exposed places
(for a hot air reflow system). Then, a profile for a 'normal' board may =
result in not hot enough temps.

However, for the nickel to be exposed and oxidised, if the reflow =
profile was too cold, the only
possibility I can think of would still be a bad ENIG plating process - =
otherwise, the nickel would be
covered in gold, and would not oxidise (the gold will disolve into the =
solder joint, of course, only if
the joint has reached the necessary temperature, and then the nickel is =
covered with solder).=20

It is actually more likely that a unit is soldered too hot or too long =
(at liquidus) so that the already
compromised solder joint oxidises more, and quicker.

Would you care to let me know what kind of BGA it is (at least: housing =
type and dimensions, pitch, ball
size), plus board stackup? A reflow profile and solder paste type (from =
your assembly house) would also
be very helpful in ruling out assembly problems. Knowing the pad size on =
the board for the BGA also helps.

____________________________________
Sol Tatlow, M.Eng. (Oxon)
ProDesign Electronic & CAD Layout GmbH
Product Developer
Albert-Mayer-Str. 16
D-83052 Bruckmuehl
Phone: +49 (0) 8062-808-302
Fax:   +49 (0) 8062-808-333
Mailto:sol.tatlow@xxxxxxxxxxxxxxxxxxxx
www.prodesign-europe.com
____________________________________=20


-----Urspr=FCngliche Nachricht-----
Von: Zilber Gil [mailto:gzilber@xxxxxxxxxx]=20
Gesendet: Mittwoch, 29. Juni 2005 16:20
An: Sol Tatlow; si-list
Betreff: RE: Fixing a black pad


Hello Sol,
Thank you for your detailed reply. Is it possible that not enough =
heating during the reflow caused, in some extent, too cold soldering  =
and due to that bad wetting of the Ni followed by oxidation of the Ni =
pad. The phenomena may look the same after removing the components, =
black unwetted pad, but in this case the reason is the assembly house =
and not the PCB manufacturing.=20

Thanks,
Gil Zilber
Elta system Ltd.
Tel. +972-8-8577240
Mobile +972-54-4983222
http://www.iai.co.il/site/en/iai.asp?pi=3D17887
=20


-----Original Message-----
From: Sol Tatlow [mailto:Sol.Tatlow@xxxxxxxxxxxxxxx]=20
Sent: Wednesday, June 29, 2005 11:38 AM
To: Zilber Gil; billw@xxxxxxxxxxx; si-list
Subject: Re: Fixing a black pad

Hi Gil,

The 'Black Pad' phenomonen is well known in the industry - the root of =
the problem is generally the nickel plating applied during the ENIG =
(electroless nickel immersion gold) plating process, where a corroded =
nickel layer leads to weak solder joints after assembly.
This phenomonen therefore only affects boards with ENIG plating, and =
although reflow temperature/profiles may exacerbate the problem, they =
are not the real cause.

The worst part is, you cannot detect it optically before assembly - it =
only becomes apparent when the joints fail, as you found out.
And, if you have this problem with one board, you will most likely have =
more boards coming back to you with the same problem, since it is =
related to the plating bath (which was no doubt used to produce a whole =
batch of your boards).

AFTER failure, there is not too much you can do about it - with an =
aggressive flux you may succeed in soldering the component back onto the =
board, but since the problem is with the nickel plating (which, due to =
corrosion, allows the copper underneath to interact with the joint), =
reworks are likely to suffer a similar fate (even if the reworked =
component SEEMS to be properly soldered). PLUS, even if only one BGA on =
the board has failed, if there are others, they are also likely to fail =
at some point. The safest solution, if also the most expensive (short =
term!), is to replace the board.

For the future, since finding a PCB manufacturer who can produce a =
reliable ENIG finish is easier said than done (not because they don't =
exist, rather, because you only know if they weren't reliable when they =
prove it!!), for the future you may want to consider immersion tin or =
silver. These have other disadvantages (there is no perfect surface =
finish), but they tend to give overall better results.
The best thing to do is do some Googling/reading about the subject, and =
decide on the basis of your specific needs.
____________________________________
Sol Tatlow, M.Eng. (Oxon)
ProDesign Electronic & CAD Layout GmbH
Product Developer
Albert-Mayer-Str. 16
D-83052 Bruckmuehl
Phone: +49 (0) 8062-808-302
Fax:   +49 (0) 8062-808-333
Mailto:sol.tatlow@xxxxxxxxxxxxxxxxxxxx
www.prodesign-europe.com
____________________________________=20



-----Urspr=FCngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
Im Auftrag von Zilber Gil
Gesendet: Dienstag, 28. Juni 2005 16:33
An: billw@xxxxxxxxxxx; si-list
Betreff: [SI-LIST] Re: Fixing a black pad


Hello Bill,
Thanks for the response. The pad finishing was ENIG and the BGA have =
SnPb solder balls. I saw the Black pad only after removing the BGA =
components that was mounted more then a year ago (due to =
disconnections). Is it possible that it is not a real black pad but too =
low reflow temp in this components (partially could soldering) causing =
to oxidation of the Ni?

Thanks,
Gil=20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Bill Wurst
Sent: Tuesday, June 28, 2005 2:46 PM
To: si-list
Subject: [SI-LIST] Re: Fixing a black pad

Gil,

I'm wondering if the black pad is silver (Ag) oxide.  Have you recently =
switched to lead (Pb) free processing?  Or worse, has your BGA vendor =
begun to supply you with lead free parts without your knowledge?=20
Unfortunately, I'm not very knowledgeable when it comes to silver solder =
processes, but would like to hear from anyone else with more knowledge =
than I in this area.  We may all need to learn about this quickly as the =
electronics industry makes the lead-free transition.

     -Bill


       /************************************
      /         billw@xxxxxxxxxxx         /
     /                                   /
    / Advanced Electronic Concepts, LLC /
   /           www.aec-lab.com         /
   ************************************
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D
Zilber Gil wrote:
> Hello all,
> After removing a BGA components a black pads was seen on some of the=20
> pads. Is it possible to fix it? If trying to use activated flux it is=20
> possible to coat the pad with solder (receive back the wetting). Does=20
> the black pad phenomenon will appear again on this pad (will I=20
> encounter it later on other pads?)?
>=20
> =20
>=20
> Thanks,
>=20
> Gil Zilber
>=20
> Elta systems



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