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May 2006

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 16 May 2006 12:11:56 -0500
Content-Type:
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text/plain (251 lines)
It is a big item, indeed, John.
Sometimes Kirkendall voiding is confused with planar microvoids that
occur from using improper copper types. I saw some of that with an
assembly using something called tough-pitch copper. When they switched,
the problem disappeared. 
I also know that improper reflow profiles can cause the planar voiding
seen with the use of immersion silver. It really is seen with all
finishes, but the root cause is too fast of a cooldown.
But the biggest contributor to microvoiding is the type of solder paste
and the condition of the pwb. I know of one solder paste that is
extremely prone to microvoids (as well as regular voids). 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Tuesday, May 16, 2006 11:57 AM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG and other surface finishing

None that I don't understand..............8-)

Surface finish topography is a big item. McDermid did a paper at APEX
last year which I believe covers most of the issues.

John

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Tuesday, May 16, 2006 9:46 AM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG and other surface finishing

Any concerns yet amongst this group regarding planar microvoids and
immersion silver?

http://www.smta.org/files/oregon_chapter_presentation0905.pdf

John Parsons

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, May 16, 2006 9:17 AM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG and other surface finishing

Yes, it depends...
To use ENIG instead of immersion silver finish as a means of mitigating
storage issues is a bad reason to switch, unless there are other
compelling reasons, such as having a storage environment with a high
chlorine content (sea salts) or other contaminants in the air that can
potentially react with the immersion silver finish on the pwb's.
Potentially, the ENIG is going to cause you a lot more problems than any
issues with storage and tarnishing of immersion silver boards ever will.
If you choose to solder to ENIG, you are soldering to nickel, not gold
as many think. The gold is absorbed into the solder and you form your
solder joints to the nickel underneath.
However, soldering to nickel is much more difficult than soldering to
the copper traces and pads on an immersion silver board, where the
silver is absorbed into the solder joint and you form your solder joint
directly with copper, which can be dissolved into the intermetallic
formation with the tin/lead solder alloy (or just tin in the case of a
Pb-free process) much easier than with nickel.
The solder joint formed with the nickel is inherently much more brittle
than one formed with copper, and is not as strong.
That is if the ENIG is plated properly. If it is not done properly,
which it often (still) isn't, then you will really regret ever
switching.
Packaging of immersion silver boards should be done with silver saver
paper between each board, and the entire brick is shrink-wrapped in a
special low-outgassing wrap that seals the air out of the package, along
with a dessi-pak. In a previous life, I had immersion silver solder
coupon boards that sat under my desk for 5 years exposed to the air. We
needed some working scrap assemblies for test set-up. Even though the
boards were somewhat discolored (slightly tarnished) they soldered right
up, and have been in use for three years as calibration units without
any failures. But there is very little sea salt in Minnesota air, and
most factories control their air quality very well.
If you do choose to change your finish, qualify the process before you
jump into changing them over. I would suggest performing an evaluation
using a small quantity of immersion silver boards and a small quantity
of ENIG, and then running durability studies that include thermal
cycling and perhaps some type of shock or stress testing.
Werner can provide more information on the appropriate types of testing
than I can.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roger Stoops
Sent: Tuesday, May 16, 2006 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG and other surface finishing

I just received a question from someone inside my organization asking
about why not use ENIG instead of Iag because of storage issues...
Doing a quick Internet search took me to a paper at
http://www.mse.uiuc.edu/Faculty/Shang/Preprints/1997-2006/JMR00-pilin.pd
f#search='tin%20nickel%20intermetallic%20strength'
discussing "a comparative fatigue study of solder/electroless-nickel and
solder/copper interfaces."  It's dated from 2000 and does not include
any study using Pb-free solders.  Final thought is that Sn-Ni SJs do not
behave well under high stress environments after aging.
Found a few others, Daan's website was also very helpful, but time does
not permit further discussion.
We have used ENIG, Iag, HASL, Pb-free HAL, and it seems our choices (as
far as I have anything to say about it) will likely be Iag and Pb-free
HAL, due to our operating environment and quality requirements.
So, should one use ENIG instead of other SFs?  In the now immortal words
of (can't remember who, sorry), "It Depends"...

Roger

-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Monday, May 15, 2006 4:50 PM
Subject: Re: Question about ENIG and other surface finishing

Good points, George.
Trouble is, in order for the temperature under a large BGA to reach 220C
for any duration, the rest of the component leads on the assembly will
be close to what? 235C? Not a good process for some components.
The problem with the logic that the ENIG will work better at higher
reflow temperatures seen with Pb-free processes is this:
The plating issues that cause the nickel oxide formation during the
immersion gold step (related to the phosphorus content, amongst other
variables) are exacerbated by subsequent temperature excursions. If the
plating is properly done, then yes, perhaps the IMF formed to nickel
will be more easily accomplished. However, if NOT done properly, the
higher temperatures will make the Black Pad and other nickel plating
process-related problems much, much worse.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of George Milad
Sent: Monday, May 15, 2006 2:15 PM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG and other surface finishing

I believe one of the key issues in assembling ENIG parts is that the
inter-metallic formed is a Ni/Sn. Ni/Sn inter-metallic requires  higher
peak assembly temperature and a longer dwell at peak temperature. The
right  re-flow temperature profile is a must.

The recommendation of ENIG assembly is 220 C for 20 seconds.

Some of the failures with ENIG may be attributed to colder assembly
temperatures.

The transition to Pb-Free should favor ENIG as the assembly temperature
for Pb-Free is going to be higher and the Ni/Sn inter-metallic will
always be formed  successfully.


Best  regards

George Milad
[log in to unmask]
National Accounts Manager  for Technology Uyemura International
Corporation Technical Center 240  Town LIne Rd Southington CT 06489
(516) 901 3874 (mobile)
(860)  793-4011 (office)
(860) 793-4020 (fax)

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