Hello Ingemar,
The pictures are nice, indeed. Nevertheless, I'm still not sure whether the structure of the E-Ni layer has something to do with BF. I'm going to send a few pictures to Steve to publish, which show that the fracture path follows the intermetallic/P-enriched layer interface (seen as a darker contrast layer). By the way, the only reason we see the P-enriched layer is because Ni was "consumed" (diffused) to form the intermetallics. I wish I knew what is causing the BF to occur. Probably the guys you referred to know the answer.
Regards,
Vladimir
Vladimir Igoshev, Ph. D.
Senior Materials Researcher
Research in Motion
451 Phillip St.
Waterloo, ON, N2L 3X2
Voice: (+1) 519-888-7465, ext. 5283
Fax: (+1) 519-886-0863
E-mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: Tuesday, April 19, 2005 3:08 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG - no more problems?
Our experts believe BF was the case. Hope the pictures will come up on Steve's wall soon. The one that is named īnormalī nickel was without problems. 2,500 ball BGAs wouldn't fail even if you hit the PWB with a hammer blow, the one named 'abnormal' nickel was extremly fragile, BGA's lifted with a 'ping' when exposed to even little mechanical force or temp shock. The extremly high P content in the grain interface disturbed the normal intergranular strength, and caused the fragile solder line. When told about all this, the plater and the bath maker met and came to a decision what to do, and after that nema problema. WHAT they did, we don't know, but it had to do with the P content.
Mae Lin Syn at Xilinx made a simple demo for us at that time. She soldered BGAs on electroless resp. electrolytic nickel, made shear tests, cross sectioned and made microprobe analysis. The extremly fragile solder line (from soldering on electroless nickel) we talk about could have so high as 18 W% P. If you have Xilinx nearby, ask for her presentation. Can't find it in my puter at the moment, may be hiding in a server somewhere...
According to Safranek, you have to consider the band or lamellar structure of EN. A one micrometer thick EN layer consists of hundreds of nanometer thick layers, like the annual rings in a piece of wood. The phosphorous is sometimes varying a lot within such layers, and under some circumstance, the P is concentrated to last layer before the plating is finished. Safranek describes EN like this: "EN is Nickel in a metastable state that can be described as microcrystalline, polycrystalline or amorphous, and appears as a supersaturated solution of phosphorous in a liquid-like state. While electrolytic nickel is an absolute crystalline structure."
No wonder EN is harder to understand! I have a (bad) picture of a typical banded EN structure with 8-9% P. From Safranek. It's said, that the bands became coarser and more amorphous the more the P%. I'm not an expert on this myself, just referring to those who seem to know. If the experiments on ideal ground (Safranek) are comparable with the rough life in a plating industry, I don't know. (and it's a lot more I don't know!)
Steve, I'll send the pic to you for nailing on wall.
Ingemar
Ingemar
-----Original Message-----
From: Vladimir Igoshev [mailto:[log in to unmask]]
Sent: den 18 april 2005 16:51
To: TechNet E-Mail Forum; Ingemar Hernefjord (KC/EMW)
Subject: RE: [TN] ENIG - no more problems?
Inge,
That sounds very interesting and I'm wondering whether you are talking about "Black Pad" type of failure or "Brittle Fracture" one. That mysterious dark layer you are talking about is always at the Ni-P/Ni-Sn intermetallics interface. It looks darker than the underlaying Ni-P layer because it is depleted with Ni (so has a higher concentration of P). In both cases (BP and BF) traces of P can found with EDS on both of mating fracture surfaces. Can you clarify please what type of failure you meant?
Regards,
Vladimir
Vladimir Igoshev, Ph. D.
Senior Materials Researcher
Research in Motion
451 Phillip St.
Waterloo, ON, N2L 3X2
Voice: (+1) 519-888-7465, ext. 5283
Fax: (+1) 519-886-0863
E-mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: Monday, April 18, 2005 6:10 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG - no more problems?
Missed who started the question.
Anyway, we had our bad expensive experience and also made our expensive countermeasure job some years ago. And we found, that there was no simple answer, nor simple remedy, but fully controllable with some efforts. A lot of TNs helped me and we summed it all in a 180 page report. No ENIG problems sinse then. What was really interesting at that time was, that we found one central reason for brittle solder joints, in terms of too much Phosphorous between the nickel grains. This phenomenon was limited to just the uppermost fraction of a micrometer of the nickel plating. The solder wetted good enough, and a normal intermetallic layer was formed, but, somewhere within the joint, an abnormal layer was created, a brittle and unreliable one, we called it the dark mystic line, because of the less than one micrometer thickness. The typical nickel with cracked intergraneolous and P-richness can be seen on a couple of pics, which I send to Stephanos Gregorius.
We also learned that the normal Ni3Sn4 was not the bad guy, but a metastable NiSn3, that is normally growing only in pure Tin (edge connectors can be hit). That layer haunted us for a long time, escaped all observation and cheated us until Xilinx told us about it. That was the end of the 'mystic' thin line.
I also add, that you can make several faults in the analysis, go lost in the chemical labyrinth and walk in circles. Thanks to a lot at TN and whole Ericsson specialist ensemble, we cracked the nut(s). The remedy list containted nearly 30 paragraphs!
I think Jack Crawford got a shorted version of my report for the IPC archive. It's not a techy salvation report, but filled with arguments and opinions and facts and numerous references and names on knowing people in the ENIG game.
Nowadays, no ENIG problems (crossed fingers)
Good Luck, whoever the questioner was
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]
Sent: den 14 april 2005 17:54
To: [log in to unmask]
Subject: [TN] ENIG - no more problems?
Just listened to a Lead Free presentation by one of our assembly materials
suppliers. They tested different pastes on different board finishes using a
variety of different patterns, spacings, aperture sizes, etc,...
By far and away, ENIG was the best finish for performance, for wetting,
defects, voiding, ...
The guy making the presentation said that he felt a lot of the initial
concerns re ENIG - black pad being the biggest - have been resolved, and
that they are no longer a concern, that they have determined what the cause
of it is and eliminated it. Anyone want to comment on that? I'm not sure
if he is correct, or just out of the loop.
It was interesting as well: ImmAg got a very bad rating from them in
performance, even though many on this forum really praise that finish. It
exhibited dewetting (they printed solderpaste pads on an area with no
soldermask, and the solder did not stay and spread where it was printed, it
crept together with other pads into a big blob, like you might see a water
drop do) and the amount of voiding was impressive!!
About 4 years ago I joined this forum, and one of the reasons was because I
was looking for info on ENIG. We had done a prototype run of a couple of
boards and they failed miserably. They were never analyzed in depth, so it
wasn't figured out what exactly caused the failure. It was the excuse we
needed to not choose this finish and some of the extra costs associated with
it.
However, with the Pbfree push now, I don't know if we should re-examine
whether to use the finish. For products where we needed a flat finish, we
have gone to ImmSn, but we still use HASL wherever possible.
Genny
"Genius may have its limitations, but stupidity is not thus handicapped"
Elbert Hubbard
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