Just one comment to the very good explanation of Erik
The black pad is also correlated to the electrical potential in the path.
We mainly found black pad if small pads are directly connected to big cu
plated areas.
Siggi
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-
Kind regards / Mit freundlichen Grüßen
Dr. Siegmund Zweigart
New Technology Manager
Solectron GmbH
Solectronstrasse 2 Fon. ++49 7032 998 194
D - 71083 Herrenberg Fax ++49 7032 998 222
e-mail: [log in to unmask] <mailto:[log in to unmask]>
internet: www.solectron.com <http://www.solectron.com>
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From: Erik de Kluizenaar [SMTP:[log in to unmask]]
Sent: Donnerstag, 12. Dezember 2002 17:53
To: [log in to unmask]
Subject: Re: [LF] "Brittle Fracture"
Dear all,
Let me try to explain some metallurgy behind the nickel-gold board
finish (also called ENIG; terrible acronym for a chemist/metallurgist like
myself) and the failure modes that can occur.
1. The electroless nickel plating has a built-in phosphorus content
of about 8 mass-%. This is inherent to the replinishing type of deposition
process and can not be avoided.
2. The gold flash is produced by a chemical exchange process. Nickel
dissolves and the more noble gold ions are deposited. The thickness of these
gold layers is typicallly 0.05 - 0.2 micrometers. This implies that these
must be micro-porous. Otherwise, the reaction would stop at a layer
thickness in the order of 2 to 3 nanometers.
3. The copper lands on which the finish is going to be applied, is
activated with zinc, extremely finely deispersed, to provide proper adhesion
and a fine-grained nickel microstructure.
To arrive at a good quality finish, all the process steps mentioned
above, should be well optimized and controlled. (Following the suppliers'
process prescriptions is crucial for such processes.) If something goes
wrong, the following imperfections may occur:
- coarse nickel structure with irregular phosphorus distribution
- wrong phosphorus concentration (typically more than 8 %)
- passivation between nickel and gold (mainly determined by the
rinsing steps)
- poor gold deposit (too thin; too porous; uneven)
Soldering with tin-based alloys, both lead-containing and lead-free,
will produce a ralatively complex intermetallic interface, from nickel to
solder:
- nickel phosphide, Ni3P (thickness sub-micron and very regular)
- nickel-tin, Ni3Sn4 (thickness typically 1-3 micrometers and very
irregular).
This intermetallic double layer has an intrinsic weakness against
bending and mechanical shock loading. This is particularly pronounced with
large and stiff components such as large BGAs. The fracture surface lies in
the interface between these two layers and looks as an adhesive fracture. No
brittle or ductile fracture details on either side of the fracture to be
found using SEM. (More detail should be investigated using TEM and replica
technology.) Microprobe analysis and microsectioning have further proven
that this fracture mode is as described above.
The colour on the nickel side is dark grey. So, this can be called
"black pad".
All this does not mean that such joints are bad, but any chain has a
weakest link and with soldered joints on this type nickel-gold finish, this
intermetallic interface is the one, particularly with incidental overloading
such as mechanical shock.
Now if the nickel quality is poor, this interface turns out to be
really weaker in many cases. Then, the strength may be insufficient. In
those cases, often but not always, the phosphorus content exceeds 8%. But we
also found good quality finishes with 9% so, phosphorus is not the unique
reliability indicator. However, processes are running out of control with
too high P-content so, it always requires action to be taken.
The gold is micro-porous, see above. This implies that the nickel
can be attacked through the pores if improper rinsing procedures are
applied. This will result in the formation of corrosion products in the form
of a passivation layer, that cannot be removed by soldering fluxes. If a
passivation layer is present between nickel and gold, the gold will readily
dissolve and the solder will see an unsolderable surface. On large solder
lands this will be seen as severe dewetting and black pad, and on small
lands the solder will be covering the lands without really adhering to
those. So, fracture will occur at very low forces and the pads will also be
greyish-black. This type of "black pad" is the dangerous one.
Best regards,
Erik
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-----------------------------------------------------------------
Erik E. de Kluizenaar
PHILIPS CFT - Electronic Packaging & Assembly (EP&A)
Building SAQ-p, p/o box 218, 5600 MD Eindhoven - The Netherlands
Tel/Fax: (+31 40 27) 36679/36815; E:mail
[log in to unmask]
PHILIPS homepage: http://www.philips.com; PHILIPS CFT homepage:
http://www.cft.philips.com
Internal PHILIPS only:
http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
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------------------------------------------------------------------
Dave Hillman <[log in to unmask]>
Sent by:
Leadfree <[log in to unmask]>
2002-12-11 00:09
Please respond to "(Leadfree Electronics Assembly Forum)"; Please
respond to ddhillma
To: [log in to unmask]
cc: (bcc: E.E. de Kluizenaar/EHV/CFT/PHILIPS)
Subject: Re: [LF] "Brittle Fracture"
Classification:
Hi Bev! I have been using the following "definitions" of the two
phenomena;
a) Black pad: a defect of immersion gold/electroless nickel (ENIG)
pwb
surface finish associated with process control issues with the ENIG
plating
chemistry/deposition process; b) Brittle Pad: a defect of ENIG pwb
surface
finish associated with metallurgical phase formation and elemental
diffusion issues. This is how I have been keeping things straight
between
Diet Coke's.
Dave
PS - I know you and Vladimir work in the same group -that's why I
added the
"this is going to sound silly"... :)
Bev Christian <[log in to unmask]>@ipc.org> on 12/10/2002 07:23:13
AM
Please respond to "(Leadfree Electronics Assembly Forum)"
<[log in to unmask]>; Please respond to Bev Christian
<[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [LF] "Brittle Fracture"
Dave and Dave,
We (Vladimir and myself) are being told that Black Pad and Brittle
Pad are
DIFFERENT phenomena. Any enlightenment would be appreciated.
regards,
Bev Christian
Research in Motion
PS Vladimir works for me and we sit in the same double cubicle!
-----Original Message-----
From: Dave Hillman [mailto:[log in to unmask]]
Sent: December 9, 2002 5:27 PM
To: [log in to unmask]
Subject: Re: [LF] "Brittle Fracture"
Hi Vladimir! This is going to sound silly but I recommend you check
with
Bev Christian at RIM (his office should be close to yours!) - Bev
should
have the last couple of IPC Conference Proceedings which contain a
number
of good papers on the "black pad phenomena".
Dave Hillman
Rockwell Collins
[log in to unmask]
Vladimir Igoshev <[log in to unmask]>@ipc.org> on 12/09/2002 03:27:19
PM
Please respond to "(Leadfree Electronics Assembly Forum)"
<[log in to unmask]>; Please respond to Vladimir Igoshev
<[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: [LF] "Brittle Fracture"
Dear All,
I'm looking for any references related to so called "Brittle
Fracture"
phenomenon, which sometime can be obeserved in solder joints between
BGA
componets and ENIG finished boards.
Vladimir Igoshev, Ph.D.
Research in Motion
Canada
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