The ionizer will not help due to the fact that it takes one to two =
seconds
to dissipate the built up charge, depending on its magnitude. =
Preventing
the generation of the charged field is the only way to prevent ESD =
damage to
susceptible components. See ANSI/ESD S20.20 for an ESD program =
guideline.
A clean gas stream hitting a pcb will not generate a static charge. The =
gas
must be full of particles in order to generate a charged field on an
insulator. The static charge comes from the person handling the pwb if =
they
are not grounded while transporting the pwb.
Al Cash =20
-----Original Message-----
From: Szilassi Zolt=E1n [mailto:[log in to unmask]]
Sent: Friday, March 03, 2000 11:55 AM
To: [log in to unmask]
Subject: Re: [TN] ESD and Polyimide stencils...
Dear Techneters,
I'm very new in the forum, so here are some things about me. I'm =
working for
a small company in Hungary, which is selling all sorts of electronic
assembly machines, materials, services, basically everything, which can =
be
sold to electronic manufacturing companies. I'm mostly involved in the
technical support of our sales activities.
About the ESD problem with polimer stencils. I had the same kind of =
problem
at one of our customer's site, a CEM in Hungary. They were using =
Loctite
Varidot stencil for glue printing. (I do not know what sort of polymer =
is it
made of and also the company could not find it out.) After a while they =
have
realised, that some BGA packaged components would die during the =
printing
process. They have tried the ionic fan idea, but it did not give =
preffered
results. The final solution was to use metal stencil for this =
application.
The process is working fine ever since.
Questions:
They were using standard polyurethan sqeegee blades, not the one =
supplied by
Loctite. Does anyone has any experinece if the static charge buildup =
occures
if they use the Loctite blade for the Loctite stencil?
Why these stencil producing companies not use conductive polymers for =
such
an application?
Best regards
Zoltan Szilassi
-----Original Message-----
From: Murugan Vasudevan <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: kedd, febru=E1r 29, 2000 4:11
Subject: [TN] ESD and Polyimide stencils...
>Hi all!
>With yesterday's discussion on Polyimide stencils and
>the associated ESD risk factors (Boy! I gotta admit
>that I was a little scared..oops after weeks of
>stencil design and evaluation, and assumption that it
>is the best we've got, now we end up fighting against
>ESD..), we decided to run a quick ESD test on our
>polyimide stencils..and here is what was observed.
>- The poly stencil, without doubt, is definitely more
>ESD prone than a metal one. Some places on the stencil
>was measured holding more than 2000v of charge. But,
>an interesting observation was that when the stencil
>is put on the printer and after a run or two, when the
>charge was measured again, it was very small (around
>100v). Something was discharging it and it was not the
>PC board. So it was a kinda mixed reaction that was
>observed. We dont want to take the risk of going ahead
>because, well, sometimes it may happen that the
>humidity falls, then these stencils would accumulate
>charges, and oops do i have to tell what'll happen!.
>We would be printing adhesives using this stencil on
>the bottom side, with ICs and lotta ESD sensitive
>stuff on the board.
>- One of the suggestions were to use an ionic fan
>(replace the regular fan on the rear of the printer
>with an ionic one), so that we can be safe from ESD
>problems, but..i was still kinda worried..and here i
>am!
>
>Any bouncing back of ideas/suggestions...???
>
>Regards,
>Murugan
>__________________________________________________
>Do You Yahoo!?
>Talk to your friends online with Yahoo! Messenger.
>http://im.yahoo.com
>
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