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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