Thanks, Robert, more than two cents for me, rather two bucks.
Extreme case: we lift off a suspected 600I/O superBGA, and find that the board pads look extremly gray and funny, SEM=no SnPb left, nearly nothing, optical=looks like molding something with a wax form, you see the contours of BGA solidified balls footprint, yes like a mirror of them. TOFSIMS on pads tell about lots of Ni and P and also Cu. Foot on BGA balls all have 2 micron thick Ni3Sn4 (think it was that one)which is normal. So, something happened between the "wanted" intermetallic and the bulk board pad nickel. You didn't solve our problem, but added essential knowhow and inspiration to go on searching, thx.
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: Furrow, Robert Gordon (Bob) [mailto:[log in to unmask]]
Sent: den 6 september 2000 22:22
To: 'TechNet E-Mail Forum.'; 'Ingemar Hernefjord (EMW)'
Subject: RE: [TN] Increased Phosphorus in Electroless Ni
I have heard rumblings on several occasions regarding high P% causing
brittle intermetalic failures at the nickel / solder interface. I guess I
just wanted to ask this as a sanity check to see if I am comprehending the
immersion process correctly. If the nickel layer is codeposited in an
electroless process with say 6% to 9% of phosphorus, wouldn't the nickel
layer nearest the solder joint show a higher concentration of P relative to
nickel? I say this based on the immersion gold process, where one atom of
nickel goes into solution as one atom of gold replaces it. Wouldn't this
mean at the gold / nickel interface that there are just a many atoms of P
still present whereas a substantial amount of nickel has been replaced by
gold. When soldering is accomplished, the gold dissolves into the bulk
solder joint and the reduced amount of nickel is left around the original
number of P atoms. If analyzed, the thin interfacial layer of nickel (less
than 10 microinches) would have a high P content relative to the underlying
nickel. Isn't this a natural consequence of the immersion gold process and
to be expected? If so, isn't it more correct to say all soldered connections
near the nickel / solder interface are expected to have an elevated P
content relative to the bulk nickel rather than a thin high P% layer caused
a "black pad" brittle fracture,.
Thanks,
Robert Furrow
SMT Process Engineer
Lucent Technologies
978-960-3224 [log in to unmask]
> -----Original Message-----
> From: Ingemar Hernefjord (EMW) [SMTP:[log in to unmask]]
> Sent: Tuesday, September 05, 2000 11:28 AM
> To: [log in to unmask]
> Subject: Re: [TN] Increased Phosphorus in Electroless Ni
>
> Have little time to read TN nowadays, sorry for that, have to erase most
> before reading, but this one gripped me, because our process engineers
> have just got nervous attacks caused by BGA-soldering problems (double
> sided with 100-600 I/O PBGAs)on FR4 boards, electrical and mechanical
> disorder that is under investigation just now. Small project, fortunately,
> not the large numbers like in our telecom fabs, anyway, one issue that
> came up was the high P% of the electroless Ni, from 7 up to 12%. During
> the up/down ramp for normal soldering intermetallics will be created,
> which is normal, but you may also get a strange interface layer between
> this tin-nickel and the nickel barrier itself, superthin, strange,
> unpredictable layer, hardly visible even in high magnification, can cause
> severe disrupture from CTE-mismatch or other impact. Some of our
> metalurgist say this is connected with too high a P% in Nickel. When other
> baths were used, with much lower P, the problems disappeared. T!
> hat is why I'm interested in what other experiences may come up.
>
> Ingemar Hernefjord
> Ericsson Microwave Systems
>
> -----Original Message-----
> From: Meschter, Stephan J [mailto:[log in to unmask]]
> Sent: den 5 september 2000 15:02
> To: [log in to unmask]
> Subject: [TN] Increased Phosphorus in Electroless Ni
>
>
> Hi:
> We are presently using electroless Ni/Immersion gold metallization
> on our SMT PWBs. Our present requirements are:
> > A. FINAL FINISH TO BE IMMERSION GOLD PLATING, 10 MICRO
> INCHES
> > THICK
> > MAXIMUM, OVER 6-9% PHOSPHORUS CONTAINING ELECTROLESS
> > NICKEL PLATED
> > BASE 0.0003 TO 0.0007 INCHES THICK OVER ELECTRO
> > DEPOSITED COPPER.
> >
> One of our PWB suppliers intends to change their Electroless Nickel bath.
> The current
> phosphorous content of plated nickel is between 6% and 9%. The new bath
> will
> produce phosphorous content between 9% and 12%.
>
> Reason: The higher phosphorous content prevents occurrences of
> "black pad nickel" and also better resist excessive attack on the nickel
> by
> Immersion Gold. So we expect to reduce, or even eliminate, rejections of
> product due to excessive gold or for black nickel under gold.
>
> Does the increased phosphorous content eliminate black pad nickel?
> Is there going to be an effect on solderability or long term solder joint
> life?
> Will the PTH barrel become stronger and result in increased stress in the
> connections between the inner plane and the PTH?
>
> We are concerned that we may be trading one set of problems for another.
>
> Thanks in advance for any help you can offer.
>
>
>
>
> Steph
>
> Stephan Meschter [log in to unmask]
> Lockheed Martin Control Systems Phone :(607)770-2332
> 600 Main Street, MD R52F FAX :(607)770-2056
> Johnson City, NY 13790-1888 MARCALL: 8 * 255-2332
>
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