Gene
Sorry, I'm not sure about what you say in this context. We are not
talking about heat guns or the like, but air knives with air velocities
of 200++ km/h. At this velocity, any particulate matter will be blown
off the substrates and, if they were in any way bonded, will eave a
charge. More certain, with thermosetting resins, minute quantities of
boundary pre-polymer, which is insufficiently or not cross-linked, will
be removed with the water being blown off and this will surely leave a
charge behind. Air knives are characterised by violence. Even worse, if
the substrate has packed micellar contamination (van der Waal's forces),
the very act of blowing water off could create charges because of the
polar bond between the water and the hydrophilic part of the contaminant
molecule. Life ain't so simple, is it?
Brian
Gene Felder wrote:
> Make sure the air is clean. "Moving gases that are free of particulates,
> e.g. from air guns, fans, or heat guns do not tribocharge"
>
>>From ESD Handbook TR20.20 section 2.5 "Virtually all materials, including
> water and dirt particles in the air, can be triboelectrically charged."
> Per section 5.5.5 Tribocharging "Tribocharging from equipment can be
> caused by conveyer belts, hardware sliding on surfaces, and moving fluids
> (sprays) or solids (sandblasters). Moving gases that are free of
> particulates, e.g. from air guns, fans, or heat guns do not tribocharge an
> insulator or conductor. If rapid air movement causes parts or dust
> particles to rub together, tribocharging might occur. Grounded conductive
> surfaces can tribocharge an insulative surface. An insulative surface can
> charge an ungrounded conductor, which is the greatest ESD hazard."
>
> Gene Felder
> [log in to unmask]
>
>
> -----Original Message-----
> From: Mike Fenner [SMTP:[log in to unmask]]
> Sent: Tuesday, February 10, 2004 8:52 AM
> To: [log in to unmask]
> Subject: Re: [TN] ESD Mesh
>
> Some years ago in one of the "wars" between competing soldering systems
> some of the people advocating VPR and Convection IR made a claim that
> forced convection systems would cause static damage to PWA, as the blown
> air would cause a charge to build.
>
> At the time I was with a multi-division company so we (the soldering
> division) and they (the ESD division) spent a half a day with all sorts
> of static measuring instruments and found.... absolutely nothing.
>
> Our conclusion was that if the blown air was sufficiently loaded with
> charges to carry to the board, then the same blown air leaving the board
> was sufficiently ionised to form a conductive path to carry them away
> again - just like a regular ioniser.
>
> Regards
>
> Mike Fenner
>
> Indium Corporation
> T: + 44 1908 580 400
> M: + 44 7810 526 317
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>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: Tuesday, February 10, 2004 4:31 PM
> To: [log in to unmask]
> Subject: Re: [TN] ESD Mesh
>
>
> 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]
>>>
>>>
>>>
>>>
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