Hi Joe,
There are many trade shops that can provide good quality artwork
Finding one that can meet your needs for large format phototools may be a
bigger challenge. I don't know any that can provide glass phototools in
your size.
There can be many reasons for size problems with artwork. The
characteristics of the materials being used need to be understood, and
the conditions in which these materials are handled and used must be
controlled.
In general, two types of materials are used for PCB artwork:
photo-sensitive coatings on either polyester or glass. By far, most of
the artwork masters and working phototools are produced on 7-mil thick
polyester films. (MYLAR is actually a trademark of the DuPont Company
for polyester packaging films and is incorrectly used in this context.)
These materials include both silver halide films and diazo films. Glass
artwork is used primarily where extremely tight dimensional tolerances
are required, or where humidity variations in the areas where the artwork
is used cause films to change size unacceptably. 4-mil thick polyester
film is used commonly in the graphic arts industry, and occasionally for
"check prints" in this industry, but is generally considered too unstable
for production of PCB's. Other types of substrates have been evaluated
for this application, but none have been found to be suitable.
All these materials change size when exposed to changes in temperature
and humidity, as do most materials in this world. Most of the size
change characteristics of these materials are determined by their base
material, glass or polyester, though the coated layers also have a
noticeable effect. I don't have actual coefficients of expansion in
front of me (though they are available), but roughly, for a given change
in temperature polyester will change size about twice as much as glass.
For a given change in relative humidity, polyester will change about 10X
as much as glass. Again in round numbers, polyester based films will
change size by 1 mil over 24" when exposed to a temperature change of 4ºF
or a relative humidity change of 4%. These responses to temperature and
humidity are independent and additive. The response to a temperature
change occurs quickly while the response to a change in relative humidity
occurs over several hours.
Obviously, glass is a more stable material and may be the material of
choice for some applications. On the other hand, it has several
disadvantages versus film. It is expensive, heavy, and fragile. The
fact that it is not flexible means that it must be plotted on a flat bed
plotter (most plotters today are designed for film positioned on a curved
drum) and must be processed either manually (in trays) or through
specially designed & built equipment. A decision to use glass should
include a thorough evaluation of all the costs versus the benefits.
Film (both silver & diazo) can be used with satisfactory results in even
the most demanding applications, if the environment is appropriate and it
is handled properly.
Because the film adjusts to a change in humidity slowly, and therefore
continues to change size for several hours after the humidity changes, an
understanding of how the film and the environment interact is important
to achieving good dimensional stability. The basics include:
out * Ensure that all the rooms (and the
exposure equipment) where the film is used are at the same temperature
and relative humidity. Remember: ±4ºF and ±4% RH => ±4 mils over 48".
* Ensure that the film has the same moisture content as the air
before using it. In general, the film must be equilibrated to room
conditions before it is plotted.
* Ensure that processing (primarily drying) conditions are
consistent.
* Allow the film to re-equilibrate to room conditions after
processing.
* Store (and transport) films at conditions that are similar to their
use (avoid extreme variations).
If you have your artwork plotted by an outside agency (or
even another department in a different building), you will need to pay
particular concern to ensuring that the temperature and relative humidity
are really the same where the films are made and where they are used,
that they are not subjected to large temperature or humidity changes
during transportation, and that they are re-equilibrated to room
conditions before they are measured or used at your facility.
This is the short version of the discussion on "size holding". If you
would like more information, please feel free to contact me directly.
Bob Seyfert
DuPont Electronics
Technical Manager - Phototools
[log in to unmask]
(919) 248-5133
At 04:18 PM 4/2/98 -0500, Zhong-You Shi wrote:
>My name is Joe Shi. I am very new in this field. My project at Ford
Motor
>Company involves photo imaging. We are not happy with our current
>contractor who does our artwork ploting on Mylar. Could anyone of you
>recommend some best suppliers that can do a better job? Also we are
>concern about the expansion or shrinkage of the Mylar artwork
>with humidity and temperture flatuations. Could anybody suggest a
better
>Mylar or a better polymer for UV (380nm) printing?
>In order to overcome the shrinkage and expansion of the Mylar artwork
we
>are considering to use glass mask as a back up plan. Do you
>know anybody who can plot a 38 x 62 inches mask?
>If I try to control the printing-room's humidity and temperature to
some
>degree can the Mylar's shrinkage and expansion problem be solved?
>Any help is greatly appreciated!
>
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