Dave

I thought you were going to say that that PCB house burnt down :-)

Brian


Dave Hillman wrote:
> 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
> [log in to unmask]
>
>
>
>
> 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]>
>
>
> To:    [log in to unmask]
> 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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