Well, I don't think that is exactly what is going on in both phenomena.
Here are some of the "disconnects" we have. :-)
1. The P-rich layer on top of a layer of electroless Ni forms (at least
to my knowledge) as a result of Ni depletion to form Sn-Ni intermetallic
during re-flow. I don't remember seeing it on a virgin board.
2. If there is no cavitation in the layer of electroless Ni, then there
is no reason to suspect BP.
3. A proper layer of SN-Ni intermetallics can be present at the
solder/electroless Ni-with a P-enriched layer interteface even if BP
(cavitaion) is present.
4. If BP is present to begin with, then it's there and it can not get
any worse. That is why I said it's not time dependant. In turn, BF is
time dependant, but I'm not aware of the actual mechanism.
5. Thanks a lot for the reference in the previous e-mail :-)
Regards,
Vladimir
-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Wednesday, June 18, 2008 4:18 PM
To: Igoshev, Vladimir; TechNet E-Mail Forum
Subject: RE: [TN] [LF] [TN] SN100 for Reflow Application
EN deposits usually contain between 5%-18% P to counteract corrosion
later during the gold deposition. The amount of P is determined by the
Ph of the Ni bath. If the electroless nickel has too little P (usually
less than 7%) after plating there may be more corrosion during the IAu
plating. This in turn leads to formation of a P-rich layer on top of the
nickel plating, under the gold. During soldering the gold is dissolved
into the solder, but there is then a gray or black P barrier (or nickel
oxide barrier) to overcome, and thus poor wetting and this will show up
readily as a poor IMF formation (Classic BP). The P prevents a good
reaction between the Ni and the solder. But if the P formation is not
excessively bad, some wetting will take place and the problem goes
undetected until later.
If, on the other hand there is much more than 7% P concentration after
plating there is little chance of corrosion during the IAu plating and
thus a much better chance that the solder joint will appear to "wet"
normally, but later after more thermal excursions are seen there will be
a P buildup as more NiSn IMC continues to form.
In Amol's case, the appearance of a good NiSn layer is normally a good
indicator of good reaction between the solder and the finish. But the P
in the EN finish is separate and is not included in the formation or
subsequent growth in the NiSn IMC, and thus it will continue to
accumulate along the nickel surface underneath the NiSn IMC, independent
of but in reaction to the continued IMC growth. As more thermal cycles
pass, even while the CCA is just sitting there in a room, but more
rapidly while in service, the NiSn layer will tend to increase and
simulataneously, the P layer also, leading ultimately to weakened SJs
and failure.
This is why BP is considered to be a time-variable phenomena.
So the SJs may appear "healthy" initially, but that does not mean they
will remain so.
-----Original Message-----
From: Igoshev, Vladimir [mailto:[log in to unmask]]
Sent: Wednesday, June 18, 2008 1:22 PM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: [TN] [LF] [TN] SN100 for Reflow Application
I think you are talking about two different effects Black Pad and
Brittle Fracture, as one phenomenon, even though they are two completely
different things. BP isn't a (life)time variable effect, while BF is.
Of cause cratering has nothing to do with BP. It just serves as an
indicator in that particular case, that the solder joints are healthy
from the UTS point of view.
There is one observation, though; Amol mentioned I'm a bit concerned
with. He saw "...a rough Ni surface". I'm wondering whether the layer of
Ni cavitated or not.
Regards,
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, June 18, 2008 2:10 PM
To: [log in to unmask]
Subject: Re: [TN] [LF] [TN] SN100 for Reflow Application
No it doesn't. BP is a time-variable defect.
But I agree that it probably isn't BP, more likely a CTE mismatch issue
due to improper cooling. However, it can be both. The pad cratering has
nothing to do with BP or vice versa.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir
Sent: Wednesday, June 18, 2008 12:37 PM
To: [log in to unmask]
Subject: Re: [TN] [LF] [TN] SN100 for Reflow Application
Not at all. The fact that you also got cracks in laminate rules out BP
:-)
Regards,
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Wednesday, June 18, 2008 12:51 PM
To: [log in to unmask]
Subject: Re: [TN] [LF] [TN] SN100 for Reflow Application
Dear Valdimir,
Can you please clarify your E-mail below? Do you think I may be dealing
with black pad?
Regards,
From: Igoshev, Vladimir [mailto:[log in to unmask]]
Sent: Wednesday, June 18, 2008 10:55 AM
To: [log in to unmask]; Kane, Amol (349)
Subject: Re: [LF] [TN] SN100 for Reflow Application
Hi Amol,
The thickness of intermetallic layer does not indicate whether you might
deal with BP or not, but the fact that you got laminate cracks does.
Regards,
Vladimiir
----- Original Message -----
From: Leadfree <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Wed Jun 18 10:33:12 2008
Subject: Re: [LF] [TN] SN100 for Reflow Application
Hi Pete,
We will not be using the DICY cured laminate in a LF process. It wont
withstand LF process temperatures. yes, we did observe a P rich area and
a rough Ni surface. However, a presence of a thick intermetallic layer
rules out black pad. it does reflect a plating bath that Is potentially
unbalanced wrt the chemical composition. However, the SnPb version of
the board is also ENIG and from the same board supplier. Therefore all
things being equal (plating wise), we see cracks only with the LF
assembly.
All the obvious SMT process steps (Solder deposition, solder ht, reflow
temps etc.,) were checked to ensure accuracy.
Regards,
Amol
-----Original Message-----
From: Pete Houwen [mailto:[log in to unmask]]
Sent: Wednesday, June 18, 2008 9:53 AM
To: [log in to unmask]; Kane, Amol (349)
Subject: Re: [TN] SN100 for Reflow Application
Amol,
If you are joints that much more prone to failure, don't rule out your
process just because they aren't failing during the process.
Are these ENiG plated boards? When you did your cross sections, did you
do any analysis of the interfacial? Are you getting P-enriched zones?
Are there voids - large or micro, in the joints or balls? Are the
soldermask openings and registration the same on both versions? Do you
inspect the paste deposition to make sure the stencil is releasing the
lead free solder as it did with tin/lead?
While the fractures aren't showing up until you apply mechanical stress,
the process itself could be weakening the joints, and that doesn't
always show up on all of the balls, or even the same ones consistently.
Though pad cratering indicates that at least at those pads, the joint
was stronger than the laminate.
Be careful using dicy cured epoxies with lead free process temperatures.
A
whole 'nother can o' worms.
Pete
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