Your experience is not uncommon.  I used PAC glass in a previous life
that required
shims whenever fine line work was run.   We use Picard glass that I
designed to fit
our 18" x 24" panels with less than 1" of un-supported glass between the
edge of the
core and the vacuum seal.  Our limitation is about three mil spaces.
Any product
below four mils just needs shims around the perimeter.  The shims we use
are made
from a scrap core.  Cut one into strips on a shear.  Cut the core,
resist and mylar
together to insure you have the correct overall thickness.

> -----Original Message-----
> From: John Lennox [SMTP:[log in to unmask]]
> Sent: Friday, August 28, 1998 3:32 PM
> To:   [log in to unmask]
> Subject:      [TN] Multiline glass frames
> TechNet:
> We are currently evaluating the Multiline Multiframe glass frames on a
> Colight 1330 exposure unit.
> We are seeing loss of contact resulting in reduced spacing and/or
> bridging
> of circuits. Although the exact location of the off contact varies, it
> seems to be concentrated in an approximately 3-inch wide band at the
> front
> (closest to the operator) and back of the glass along the length of
> the panel.
> The frame installation has been thoroughly inspected to ensure that
> the top
> glass can close flat against the bottom with no binding or
> misalignment.
> The vacuum pressure has also been inspected and appears to be working
> fine.
> The majority of the image tests have been done using .0025" cores with
> 1
> oz. copper, although the off-contact condition has been seen with a
> variety
> of core thicknesses.
> Our dry-film vendor suggested placing .005" shims between the glass
> around
> the perimeter of the panel when imaging the .0025" material, which
> appeared
> to work pretty well (the image quality was much better with no
> noticeable
> off-contact).
> However, we were told that the use of shims would be unnecessary with
> this
> system and would prefer not to use them if possible.
> If anyone has any suggestions or has had any experience with the
> Multiline
> frames your input would be appreciated.
> John Lennox
> Process Engineer
> EMC
>

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################