Brett:

Mark is right and there are some more specific considerations for you to
look at in the process.

After lamination prior to expose  (tenting effects):
Depending on the film type and work or storage area temperatures thed
hold times on tented boards can be problematic.  Once laminated to
boards using the film for tenting the process has created a potential
where the film can "cold flow" into the larger holes and get thinner
around the rim of the holes. This can then become a problem for good
tenting on the larger holes.  It can be seen as a light color of film at
the rim of the holes and after developing can show as tent failures.
This isn't normally a problem with any film until several days have gone
by but if the panels are stacked hot and not allowed to cool it can be
worsened.

After lamination prior to expose (lock in)
There is a designed reaction occuring between the copper and the film
chemistry.  The design of the dry film chemistry is to react with the
copper to enhance the bonding for adhesion during plating or etching.
Most films have enough control dieigned in to limit this reaction and
allow the film to be coated for days with NO PROBLEM but others can as
Mark pointed out "LOCK IN".  If they do it is hard to reactivate for
plating (resuidue after good developing) or you get a selctive slowed
start on the etching.  Again this is a normal hold point and most of us
will suggest for process control you process within 24 hours of
coating.  Truth is most films can still be processed for days or even
weeks with no negative effects but this depends on the chemistry of the
surface, type of surface (electroless or electroplated foil) and more
specificly the dry film used.

After exposure and prior to developing (side wall salvation)
Once exposure has been accomplished it is important to develop the
panels in a reasonable time because you have created exposed film in
direct contact with unexposed film.  The unexposed area of  film can
negativly react with the exposed resist area and vise versa.  This all
contributes to bad side walls.

After developing prior to plating (white light holding):
This is normally in a storage area prior to plating or etching BUT after
developing.  Some shops have the new UV rich Mercury ceiling lights or
just good bright general lighting which can damage the exposed and
developed film over time.  It will further polymerize the film causing
embrittlement.  This can result in breakdown in plating or etching.  It
is not uncommon in some of these situations to cover the panels when
extended storage is planned or lighting is severe.  normally a few days
is never a problem in any condition but is is a consideration.

After plating prior to stripping (stripping difficulties)
This storage is similar to the white light holding issues but shows up
then as a stripping problem. This white light exposure simply makes
removal more difficult as it is aged beond a reasonable time.
Reasonable is a few days before there are problems noted and again it
depends on the lighting in the area.

All in all any hold time can be worked with but because of the effects
on the process variation it creates on all films the best policy is to
identfy what you shop can or should be able to live up to and consider
that the norm.  Then, any time there is a major departure from the norm
it should send up a warning flare that the job "MAY" process with
variations.

Good Luck,

Kerry Grimes
Grimes Distribution, Inc.

Dryfilm wrote:

> Brett,
>
> I'm unclear at which point in the process you would like to extend
> hold times,
> so I'll do my best to answer your question.
>
> In general, reducing hold times during the image process is
> recommended.
> Most dry film resists like an acidic surface, and this will usually
> promote
> adhesion, and occasionally, cause lock-in.
>
> Heating panels prior to lamination can help achieve better resist
> conformation, but should not affect hold times under normal
> conditions.  If
> you are heating developed panels, again, lock-in is a possible
> result.  Be
> careful baking panels after developing; it is very easy to make the
> dry film
> too brittle.
>
> Hold times are generally determined by the resist formulation, and
> process
> environment also has a big impact.  Check with your supplier for
> recommendations.
>
> I hope this information is helpful.
>
> Mark Dowding
> Regional Technical Service Manager
> INSULECTRO
>
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