Whoa there!  Do you have good references for this?
Not SMT or Circuits Assembly, I mean something tougher
peer review like Electrochimica Acta or J. Electrochem Soc.

1.  Electromigration requires DC voltages and currents.

        a. AC voltages and and current will not cause
           the electrodissolution of copper, silver,
           tin or lead.  The only materials that corrode
           with imposed AC (<~10 kHz). are amphoteric
           materials, like aluminum.

        b. AC voltage and currents will not allow the
           the electrodeposition of any metal that I know.
           The metal deposited during one half wave will
           dissolved during the 2nd half wave.

2.  Imposing a small AC voltage (5 to 50 mV) on top
    of a DC voltage and looking at the resulting
    small AC current is good way of studying
    electrochemical reactions like dissolution and
    deposition of metals.  The frequencies used range
    from 1 mHz to 100 kHz.  Frequencies over 100 kHz,
    such as RF, are not useful for looking at
    electrochemical reactions because the chemical
    kinetics of these reactions are too slow.

BTW 20 min. is ~ 1 mHz, and RF is >.5 GHz.

The problem with rosin/resin residues is that they have
interesting dielectic properties.  Leaving this residue
on a PCB board may change the RF properties that an RF
engineer spent a long time considering wrt the board,
especially if a lot of this residue is left on the board.
These effects have been examined by the Sandia and
the Georgia Tech studies to a small extent.


Graham Naisbitt wrote:
>
> Mark,
>
> Given that all fluxes leave residues, the issue is how will they inter-react
> with the other process chemistries you have on your assembly.
>
> In addition, we know that the higher the operating frequency, the more the
> electrons migrate to the periphery of the conductor so, where they interface
> with the board surface, you will have an area of high potential for
> electro-chemical reactions.
>
> Are you able to predict these and verify that they will not be harmful to
> long term reliability? SIR testing developments will permit you to do this
> by measuring the amount of leakage that is occurring. Use a voltage gradient
> of 100v/mm that will better mimic your assembly operating conditions, apply
> frequent monitoring so that you do not miss any dendrite formations
> (electro-chemical migration) and use current limiting resistors to preserve
> them for further analysis. Bear in mind that dendrites are known to form
> within 20 minutes under the right conditions.
>
> You should then be better placed to determine the reliability of your
> assembly. I hope this helps - let me know if you would like more
>
> Graham Naisbitt
>
> __________________________________________________________________________
>
> [log in to unmask]
>
> Concoat Ltd
> Alasan House, Albany Park
> Camberley GU15 2PL UK                        http://www.concoat.co.uk
>
> Application Engineers in Chemical Compounds to the Electronics Industry
>

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