AAGH! I doubt that this had anything to do with the mesh. I bet the
drier had hot air knives. The friction of rushing hot air on any polymer
(including the PCB laminate) *will* create a small (in microjoules)
surface charge, but relatively high in volts. This dissipates rapidly
(~1 min) through the damp polymer. However, tests have shown that these
are harmless because there was never sufficient energy at any moment to
cause damage.
I surmised that the charge was ± evenly distributed across the
substrate, but only a tiny fraction of it was in the vicinity of any one
conductor and that leakage through passive and active components (plus
the substrate itself) allowed the charge to dissipate locally before the
voltage built up to a dangerous level, in view of the high source
impedance. Anyway, as I said before, I doubt whether you have anything
to worry about.
Of course, if your customer insists... that is another matter. Even when
he is technically wrong, he is always right, because he pays for your lunch.
Brian
Rick Thompson wrote:
> Brian,
>
> That was our thought as well. However, our customer did an ESD survey and was able to detect fairly large charge build-up on the mesh that was coming out of the cleaner (after drying). They've demanded we stop using it because of this, even though we've used it for years.
>
>
> Rick Thompson
>
>
>
> -----Original Message-----
> From: Brian Ellis [mailto:[log in to unmask]]
> Sent: Tuesday, February 10, 2004 12:17 AM
> To: TechNet E-Mail Forum.; Rick Thompson
> Subject: Re: [TN] ESD Mesh
>
>
> Please explain to me how a wet plastic mesh of any colour can develop an
> electric charge. In the aqueous machines we developed and marketed for
> many years, we used polypropylene baskets, some with stainless steel
> inserts, some with polypropylene ones. Many customers were worried about
> this and tried to measure charges on the baskets and inserts and never
> found anything significant. The reason: the polypropylene was
> sponge-like and held sufficient water, after their first use, to allow
> the charges to be dissipated to all the earthed stainless steel they
> were in contact with. I have never heard of a case, in the 25 years that
> these were marketed, of a component damaged in a way imputable to these
> machines.
>
> Brian
>
> Rick Thompson wrote:
>
>
>>We've been using a black plastic mesh material to provide cushioning and protection in our stainless steel wash baskets for small boards that tend to get moved around and damaged during aqueous washing. As this material was bought before my time I thought I'd check it's ESD properties since I'd been told it was conductive (it's black, it must be ESD material ). Long story short, the material appears not to be conductive and builds up quite a charge from the cleaner.
>>
>>Question: Does anyone know of sources for a conductive mesh type material that could be used for this application? We've checked a couple of ESD materials providers so far without coming up with anything.
>>
>>Thanks,
>>
>>
>>Rick Thompson
>>
>>Sr. SMT Process Engineer
>>SMTEK International, Inc.
>>+1 (805) 532-2800
>>[log in to unmask]
>>
>>
>>
>>
>>---------------------------------------------------
>>Technet Mail List provided as a service by IPC using LISTSERV 1.8e
>>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 847-509-9700 ext.5315
>>-----------------------------------------------------
>>
>>
>
>
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 1.8e
> 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 847-509-9700 ext.5315
> -----------------------------------------------------
>
>
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
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 847-509-9700 ext.5315
-----------------------------------------------------
|
