Hi Peter! Let me redefine a misconception - if you are truly going to "age"
a pwb sample then the aging method would produce either the same oxide
conditions, the same intermetallic conditions or the combination of
oxide/intermetallic conditions that the pwb sample would have after sitting
in storage for X amount of time. Unfortunately surface finish oxidation
doesn't behave well in accelerated conditioning - typically you
force/produce oxide thicknesses/species which are not present under normal
storage conditions. Intermetallic growth is better behaved and there have
been some good data published showing those relationships. I recommend you
look at two conditioning methods: the use of humidity and the use of
temperature. Tin surface finishes tend to degrade quickly under humid
conditions in terms oxide growth. The use of temperature will highlight
intermetallic growth problems. Now what recipes to use? Many investigations
have used 85C/85% RH as their humidity parameters for oxide growth - you
get to pick the time. Many investigations use either 125C for 8 hours or
150C for 16 hours as their temperature parameters for intermetallic growth.
Instead of expecting to "age" the surface finish,  looking at your
conditioning as attempting to determine the "robustness" of the surface
finish for a set of conditions (you will be much happier in the long run).
You will have to decide just how "robust" you want the coating. For
example, are acceptable wetting balance values after 48 hours of 85/85
exposure robust enough for your manufacturing process to have high yields?
Only you can determine that. The IPC Alternative Final Finishes task group,
chaired by Denny Fritz of MacDermid,  is in the final stages of issuing a
report that attempts to answer the question " is there a
time/temperature/humidity conditioning parameter set which can be used for
a variety of finishes to predict robustness?" - the JSTD-003 committee
intends to use the Alt Finish committee efforts to revise the current
"steam aging" methodology.  Hope this helps.

Dave Hillman
Rockwell Collins
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Peter Hefti <[log in to unmask]>@ipc.org> on 04/25/2002 02:29:50 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
       to Peter Hefti <[log in to unmask]>

Sent by:    TechNet <[log in to unmask]>


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cc:

Subject:    [TN] Accelerated test for simulation of storage of PCBs with
       surface              finish white tin



Hello TechNet:

We are searching for a method of accelerated aging of PCBs with surface
finish in white tin (chemical  tin).
This method should be recognized , in order to simulate the storage of 6 or
12 month under normal conditions.
(85/85/1d or 2d  test ??)
After such an accelerated test, the solderability will be tested by the
wetting balance method.

Thank you for your answer

With best regards

Peter Hefti


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