Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

A video http://www.youtube.com/watch?v=8WJn5YFANxU demonstrates exposing
the Riston-like Puretch negative photoresist film:

http://www.capefearpress.com/puretchpcb.html

using a vacuum bag for sealing clothes or food. This has a zip lock and
a valve for removing the air with a vacuum cleaner and retaining the
vacuum after the vacuum cleaner hose is removed.

Exposure is via a 500 watt halogen incandescent lamp from a hardware
store. I use a lamp such as this at a closer distance, with the lamp
pointing downwards (without its glass cover, to improve airflow cleaning
of the lamp). I use the glass cover to press the phototool onto the
PCB, which is supported by foam. However, it would be better to use
vacuum and a flexible top cover in place of the inflexible glass, since
the flexible cover and air pressure would press the phototool right up
to the PCB, even if the PCB was not flat, with less effects due to dust
between the phototool and the PCB, and with little or no problems due to
dust between the phototool and the flexible cover.

This vacuum-sealed bag approach looks inexpensive and effective.
However, I think it is important to blow air on the PCB so it doesn't
heat up too much. Temperature changes could cause the phototool to
expand at a different rate to the PCB, leading to loss of resolution.

As Harvey White wrote ("UV nail lamps - . . .":

http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/31264

it would also be possible to make a custom-size bag using PVC sheet from
a furniture store. Vacuum could come via a small tube inserted into one
corner of the bag, with the other end connected to a vacuum cleaner or a
pump from a desolderer. There's no need for a valve - just run the
vacuum cleaner while the exposure is taking place.

Since the ~0.5mm PVC is cheap and the bags can be made easily with
scissors and adhesive tape etc. in sizes which are most suitable for PCB
work, I think this may be a good alternative to a vacuum frame with a
flexible top and a solid or foam-and-solid base or to any vacuum frame
with a rigid (glass or acrylic) top.

The PVC would be less likely to wrinkle or become scratched during use
than the polyethylene of the vacuum-seal bags. The PVC is so cheap that
if it does become scratched, then it will be easy and inexpensive to
make another bag.

I will write to the list when I try this - which could be a few months
or more.

- Robin http://www.firstpr.com.au/pcb-diy/