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May 1997

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Sat, 10 May 1997 17:27:56 -0400 (EDT)
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In a message dated 97-05-09 02:37:20 EDT, you write:

<< Could anyone explain to me what is the difference between screen coating
 and curtain coating? and also, I  would like to know what is the
 difference between convection( batch and tunnel) oven and turbine ? 
 
 if I extend tack dry time, while keep the same temperature setting  at 
 170F, Would it be more chance to leave ink in the holes? I think the
 epoxy polymer only can be activated at much higher temperature 300F. What
 impact could have, if  panels are over backed at final cure stage.
 
 Thank you all in advance!
  >>
1) Screen coating can be applied on specific areas and/or one can flood coat
using 60 to 80 mesh screen and squeegee.  The curtain coat is is pumped from
a fountain and flows down an apron which spreads the coating over an area
that is usually wider than the board. The excess is collected an reused as
one does with screening.  In the case of curtain coating, this process is
easier to automate. 
2) Convection batch and tunnel dry is usually associated with poor air
distribution and dead spaces where coatings may not dry properly.  the
addition of turdry promotes extreme air flow which greatly assists removal of
unwanted solvent that normally would be retained in the film.
3) Extending tack dry time at the same temperature could create a developing
problem not only in the holes, but leave an invisible film on the copper
areas that you want to develop off. This could/will cause a solder problem
later.
4) Overbaking at the final cure stage should not hurt provided the board has
been totally and correctly processed.

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