Dave and Others:
Having done a lot of porosity testing I can tell you this:
Any porosity test which detects Ni will probably "Fail" any immersion Au
surface:
why:
well, the critical point in the porosity curve of Au platings (Knee) is
typically around 30 microinches (for electroplated Au). Any deposits of Au
thinner than that will have micro pores which will show up as indications in
porosity tests. The same level of performance should be expected with
Electroless Au, since surface cleanliness, roughness and the molecular nature
of Au all come in to play with the pores.
The pores expose the Ni, which shows up in the porosity testing . Thus, even a
highly solderable surface will have pores, and you will likely get caught
in the sensitivity of test method web.
Rather than a porosity test , if the immersion Au is used as a solderable
surface, I suggest you take XRF readings and do solderability testing.
When solderability testing fails, take coupons to SEM techniques - do dot
mapping, high mag, and backscatter. Also, Fluorescence techniques are
useful to find organics Etched cross sections, will probably need to be
evaluated on the SEM, since optical magnifications do not show well the 3-10
microinch thick Au, nor the intermetalic formations. A skilled technician can
tell you if you are seeing the right materials, grain structures etc.
If on the other hand, the immersion Au is used as a contact interface, as in a
pin and socket connector -- buy more Au thickness if you want more than one
mating and any kind of reliability at all (unless your in a sealed environment.
Hope some of this helps. It's good discussion one way or the other
Jim Herard
[log in to unmask]
IBM Microelectronics
---------------------- Forwarded by Jim Herard/Endicott/IBM on 06-02-97 10:34 AM
---------------------------
ddhillma @ cacd.rockwell.com
05-30-97 12:59 PM
Please respond to [log in to unmask] @ internet
To: TechNet @ ipc.org @ internet
cc: SUbl @ bldg4.plexus.plexus.com @ internet
Subject: Re: FW: ASSY:Au/Ni Coated Boards
Hi Scott -
I would be careful in applying the Fuming Nitric Acid porosity test on
Immersion Au/Ni surface finishes. This test was developed for use on
electrolytic Au/Ni finishes - my experience with the test method is
that is somewhat subjective and not enthusiastically accepted within
the user community (in other words, if a better test was available
many of us would use it instead of the FNA test). I would suggest
talking with your Immersion Au/Ni finish supplier about the
applicability of the test. You could also talk with some of the
connector suppliers (e.g. AMP) to get some feedback on their
experiences of the FNA test before investing a bunch of time and
effort with the test method. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: FW: ASSY:Au/Ni Coated Boards
Author: [log in to unmask] at ccmgw1
Date: 5/29/97 4:21 PM
Hi,
Is this poristy test, something that can be used to check immerision Au/Ni
or is it only good for Electroplated Au/Ni? Also, how porous can immersion
gold be( 1%,2% -10%) to allow for good solderabilty characteristics using
no-clean flux for SMT reflow and wave soldering?
Scott
----------
From: TechNet-request
To: [log in to unmask]
Cc: 'GM'; John Kelleher
Subject: FW: ASSY:Au/Ni Coated Boards
Date: Thursday, May 29, 1997 10:04AM
Zoran,
Two things about Au over Ni to note. First of all, over time, Ni will
diffuse thru gold (mostly along the grain boundaries) and get onto the
surface where it will oxidize. Enough Nickel oxide on the surface can
give you solderability problems. A strong flux may help. The time it
takes for this to happen depends on the thickness of the gold. I
don't have any numbers but 6 months does not seem like a very long
time.
Secondly, the quality of the gold plating is just as important. If the
gold has a lot of porosity, Cu and Ni can diffuse to the surface. Try
a Nitric acid vapor test (gold porosity test) to ensure that porosity
is not a problem. Hope this helps. Perhaps others can give you some
advice on what, if any, flux would work.
Regards,
Paul Terranova
Phone: 508-467-3109
FAX: 508-467-6796
DIGITAL's Analytical and Environmental Test Services Lab
Digital Equipment Corporation
M/S: MRO1-3/D2
200 Forest Street
Marlboro, MA 01752
-----Original Message-----
From: "MRMTS::MRMTS::MRGATE::\"DASMTS::PMDF::ipc.org::TechNet-request\"
"@sandz.mro.dec.com
[SMTP:"MRMTS::MRMTS::MRGATE::\"DASMTS::PMDF::ipc.org::TechNet-request\"
"@sandz.mro.dec.com]
Sent: Wednesday, May 28, 1997 11:45 PM
To: Paul Terranova
Subject: ASSY:Au/Ni Coated Boards
From: NAME: Raich, Zoran <[log in to unmask]@PMDF@INTERNET>
To: NAME: [log in to unmask] <[log in to unmask]@PMDF@INTERNET>
Dear Technetters
We have some printed circuit boards with gold over nickel coating
which are experiencing solderability problems, ie very poor wetting.
I have heard that this may be due to nickel migration through the
gold over time. The boards are approximately 6 Months old. Is there
a particular shelf life for this type of coating before we need to
dump the boards?.
Regards
Zoran Raich
Process Engineer
NetComm (Aus) Ltd
(+612) 9878 7394
[log in to unmask]
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