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August 2007

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Wed, 15 Aug 2007 07:25:48 -0500
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Hi Technet! Pbfree component finishes - a very cool subject that gives all 
of us ulcers! I have no doubt there will be a Dilbert strip some day 
showings an engineer rambling about the "old days" when all we had were 
tin/lead surface finishes. It is easier to cover the topic of Pbfree 
component finishes if we break it up into a couple of segments.

Area Array Components (BGAs, CSPs, FCs) - there are a couple of industry 
camps: those folks who solder a Pbfree BGA in a tin/lead solder process 
with a reflow temperature that achieves good solderpaste/solderball mixing 
and those folks who rebump the BGA with tin/lead solderballs. There is 
plenty of industry data which shows that a nonhomogeneous area array 
solder joint has poor integrity so the industry folks has been focused on 
getting a uniform solder joint. The industry camp that raises the reflow 
temperature has to deal with the potential temperature impact on the other 
components on the assembly. The industry camp that rebumps the BGA has to 
deal with the potential impact on the BGA integrity.  Great discussion 
topic to exchange beers over!

Other Components - there is a persistent myth that Pbfree component 
surface finishes can not be used in a tin/lead solder process. I wish we 
could put that myth in the ground and forget about it sometime in the near 
future. In a tin/lead solder process we diffuse a non tin/lead surface 
finish into the solder joint, we don't melt them into the solder joint. 
The industry is very good at doing this - we have utilized gold, 
nickel/palladium/gold, tin, palladium silver, etc., for many years making 
solder joints with good integrity. Due diligence is needed on 
understanding that you don't have some weird metallurgical issue result 
from a component surface finish/solder joint combination but combining 
Pbfree component finishes with a tin/lead soldering process is very simple 
and not rocket science.

As for Ken's question: What about a SAC component surface finish? Well, 
its a Pbfree surface finish that has a high tin content. Hummm,  sounds 
like a matte tin finish with a little bit of silver added and the industry 
solders matte tin surface finishes all of the time. There will be a pretty 
decent industry data set on this topic by APEX. The gang at CALCE has SAC 
surface finishes under test, the NASA DoD consortia (formerly the 
JCAA/JGPP group) has SAC surface finishes going into test and a number of 
other organizations/investigators are chasing the topic. My opinion is 
that a SAC component surface finish is not a solder joint integrity issue 
if the reflow solder process uses adequate time/temps.  If the industry is 
competent with soldering matte tin surface finishes, then a SAC finish 
should not be any greater issue (and yes, I didn't mention tin whiskers on 
purpose!). 

Enough said - time to find a Diet Coke and harass Doug!

Dave Hillman
Rockwell Collins
[log in to unmask]




Gerald Bogert <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
08/14/2007 05:23 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Gerald Bogert <[log in to unmask]>


To
[log in to unmask]
cc

Subject
Re: [TN] SAC Soldered With SnPb






August 14, 2007

Good question. 

I also need info in this regard for SAC for all type of component
leads/terminations that may be finished with SAC.

Is it better from a reliability standpoint to just live with a SAC finish
for SnPb assembly level soldering, or otherwise re-work the components to
replace the SAC with SnPb? 

Are we at the point yet where we can reliably predict equipment life in 
the
field for SAC alloy as compared to SnPb alloy?  I have need seen any 
widely
accepted industry papers yet that equate accelerated life testing results 
to
expected product fieldlife using SAC alloy. 

On a related issue, for pure tin, I understand that NEMI is not endorsing 
it
unless there is a nickel under-plate.  Just annealing after plating for 
pure
tin plated parts is not acceptable per NEMI recommendations.

I believe Dave Hillman of Rockwell published some data regarding soldering
to SAC with SnPb solder.

If you get any info on SAC or pure tin plated leads/terminations, I would
appreciate it if you could forward to me.  Thanks.  Seems like when I post 
a
question to Tech-Net, I get very few answers.  Could be my questions are
difficult to answer.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Tuesday, August 14, 2007 4:51 PM
To: [log in to unmask]
Subject: [TN] SAC Soldered With SnPb

We are a Class 3 manufacturer and are looking at the different lead
finishes that can be soldered successfully with SnPb. One that we are
having a hard time finding information on is SAC lead finish soldered
with SnPb. I know that it's not good practice for BGA's but what about
SMT and through-hole parts?

Thanks in advance,

KennyB

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