Homebrew PCBs

Hi James,

Assuming your film is the same in principle as the Riston MM540 I get
from the Czech Republic: http://www.ebay.com/sch/gaminn/m.html I think
it is vital not to bend or create uneven pressure on the film.

The photosensitive material is soft enough that it can be bonded to
copper with a temperature of 100C to 120C.  This means it will creep
(become thinner) if pressed unevenly.

The MM540 is supplied in a roll, without a core, with a diameter of
about 3cm.  This is flattened somewhat in the styrofoam box it arrives
in, and I leave it in that for storage.  The box itself is white and
this is enclosed in black plastic.  I cut open one end of the black
plastic so I can withdraw the roll and peel some off the outside.  I
store this in a cardboard box in a relatively dark place.

I think it is probably important to keep the roll closely wound so no
air can get to the film in general.  The oxygen in air definitely stops
the photosensitive process - this is my experience and I think it is
documented somewhere.  The protective film which covers the
photosensitive film once it has been bonded to the copper is good enough
to keep oxygen out for long enough to do the exposure, however I am not
sure how good it is for longer term storage.

The softer film which is removed before lamination might be even less
protective against oxygen.  So I am not convinced that loose sheets of
this material will store well, since this exposes oxygen to both sides
of the three layer material with the softer layer, I guess, being not
such a good protection.

As long as the material is in a roll, without any air gaps (as is the
case with the Riston from the Czech Republic) this resembles the large
rolls on which it is packed in from the manufacturer.  Normally, the
moment (or minutes or maybe an hour or so I guess) the 3 layer film
comes off the roll, the soft protective layer will be removed and the
photosensitive layer will be bonded to the copper with the harder
protective film on top.  Once bonded to copper, no oxygen can come from
that side.  Pulling the hard protective film off the copper before
exposure will make the film less sensitive or totally insensitive to light.

Perhaps the internal loop of Riston at the centre of this roll I get
would have oxygen permeating through the soft film, so I would expect
that last piece of material to be unusable.

  - Robin