Water itself is ionic and therefore conductive. It is impossible to
obtain a conductivity lower than about 0.056 µS-cm at 20°C, no matter
how hard you try, because it always breaks up into H+ and OH- ions. OK,
this is not very conductive but it is sufficient to dissipate static
charges. The moment you add anything ionic, such as absorbed CO2 from
the atmosphere, the conductivity shoots up and DI water which is exposed
to the atmosphere usually stabilises at around 1 µS-cm, which is quite
conductive.
On the other hand, if you mix in a non-ionic substance, such as an
alcohol, this will dilute the free ions and the conductivity will drop.
An ultra-pure 75% IPA/water mixture can reduce its conductivity down to
~0,004 µS-cm, without exposure to CO2, which is very low, indeed, and
impossible to measure without ultra-sensitive instrumentation.
Terpenes are mixed with anionic surfactants and water mixtures are
therefore conductive. Saponifiers contain amines which themselves are
ionic and conductive.
Aqueous cleaning in suitable equipment is 100% safe ESD-wise. OTOH,
drying may not be, under some conditions. The presence of residual water
and high humidities will ensure safety in the early stages, but
excessive hot air-knifing may allow charges to develop by friction. I
recommend that air-knifing be stopped before complete removal of
moisture. If you leave 2 or 3% by weight of moisture in the epoxy in the
air knife section and rely on static drying for the rest, you will risk
nothing, making sure that the conveyor/basket is grounded.
Brian
- bogert wrote:
> October 1, 2007
>
> I recently reviewed an OEM'S ESD control procedure that required
> cleaning solvents to be conductive for use on any ESD sensitive
> assembly. i also seem to recall reading this same requirement in some
> ESD specification.
>
> Using conductive cleaning solvents does not seem like the right
> technical thing to do. Although it may help regarding ESD protection,
> seems to me we are at greater risk in event some of the solvent remains
> on the assembly after the cleaning process.
>
> Normally, our OEM'S use DI water and/or alcohol for cleaning non-rosin
> based flux and for RMA flux, a solvent like Terpene with DI water or a
> saponifier and DI water.
>
> Are the aforementioned solvents conductive? I don't believe DI water
> should be since it should not contain ions.
>
> Any info on this topic would be of use. Thanks.
>
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 15.0
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt or (re-start) delivery of Technet send e-mail to
> [log in to unmask]: SET Technet NOMAIL or (MAIL)
> To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: http://listserv.ipc.org/archives
> Please visit IPC web site
> http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
> information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
> ext.2815
> -----------------------------------------------------
>
>
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
