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 
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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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