Hi Ted! The answer to you question is "it depends" on just how good of job
the hot oil reflow operator can do his/her job! The copper/tin
intermetallics will only become a long term solderability issue if you can
not keep a minimum of 100 microinches of Sn/Pb covering all surfaces
intended to be soldered. On SMT features this shouldn't be a problem but on
plated thru hole features the hole "knees" can be an issue. The growth of
copper/tin intermetallic is a pretty well understood phenomena and fits an
Arrhenius equation. The room temperature growth of the copper/tin
intermetallics (Cu3Sn and Cu6Sn5) are very slow so the key to long term
stability is to have good initial coverage. There have been some attempts
to use "as plated" Sn/Pb but as Werner mentioned there can be a porosity
issue which renders the solderbility useless for long term stability (I
have seen as plated Sn/Pb finishes withstand steam conditioning but ya have
to be porosity free which seems to be a rare accomplishment). Once the
copper/tin intermetallics reach the surface and become oxidized it is
nearly impossible to restore solderability without the use of some very
aggresive chemistries.  There are a number of reference sources which
contain the parameter data for the Arrhenius equation - Klein Wassink (ISBN
0-901150), AWS Soldering Handbook (ISBN 0-87171-618-6), etc. I have seen
one specific hot oil reflow operation produce pwbs which had 10 years of
shelf life on a consistent, repeatable basis. And no, that pwb house is no
longer in business (hot oil reflow is more expensive than HASL). What you
are looking for is possible provided you can maintain Sn/Pb coverage
thicknesses. Good Luck.

Dave Hillman
Rockwell Collins
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Ted Edwards <[log in to unmask]>@ipc.org> on 09/07/2002 12:39:58 PM

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

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


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cc:

Subject:    [TN] Copper Tin Intermetallic Growth Rate



I have received a question that I need to answer  from a customer who is
making a long term buy of solder reflowed tin lead finish  boards. Plated
tin lead seems to read in the .0006 inch range before hot  oil reflow on
the boards in question.
One of the customers concerns is the possibility of  growth of the tin lead
intermetallics that might in their mind result in an  unsolderable
condition later.  Projected storage is at least 5 years at  room
temperature.  Final assembly when done  will probably be RMA flux and wave
solder in most cases. Any help  would be appreciated.  Boards are
individually bagged with a desiccant  package and humidity indicator strips
and heat sealed.
I am looking for technical papers, other  literature, etc.


Werner:  I printed out and kept all your  TechNet e mails, and in your
September 3, 1997 Assembly: Dewetting; you mention  a temperature/time
dependency, are there charts or other reference  materials?

Thanks to all:

Ted Edwards
Process Engineering manager
Dynaco Corp.
Tempe, AZ.



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