Hi Robin,
I 100% agree with you about the limited performance of toner transfer.
My question is: why exclude positive resist options?
Years ago I used to make custom prototype PCBs but now I am too busy so I
usually order the quick turnaround PCBs.
Independently of the photo resist is a question of how to generate high
contrast fine line artwork.
You can find resolution charts and printer testing info on my website:
http://www.vinland.com/USAF-1951.html Have a great weekend!
Bertho
-----Original Message-----
From: Robin Whittle [mailto:
rw@...]
Sent: Friday, May 18, 2012 10:32
To:
Homebrew_PCBs@yahoogroups.comCc: Bill Maxwell; Boman33; VK3YV
Subject: Why use Riston or some other negative photoresist dry film?
Hi Boman33, Don and Bill,
I am responding to your replies to my message "Riston precoated FR4 in
Australia, Riston sheets and rolls".
I am pursuing Riston, or potentially other negative photoresist films,
because its my impression it can reliably produce finer lines than the other
alternatives.
For instance, from this list (2012-02-14):
∗ <
http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/29673>
http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/29673 Bob Balderstone wrote to the effect that Toner Transfer was not as easy or
repeatable as Riston type laminated film, that spray-on or sponge-on
photoresists don't work, and that "pre-coated boards" (negative, positive? -
at least as distinct from laminating your own Riston negative resist) were:
>> Good sometimes but expensive and sometimes poorly cut (raised edges
>> have to be filed down before exposure) and occasionally unevenly
>> coated meaning it's hard to get consistent results. My experience is
>> they have a shelf-life (older = longer exposure time) which again
>> causes inconsistencies if you don't make many boards at one go.
I have read about toner transfer, and my impression is that it would be
tricky to get the transfer lamination process working reliably, with
expensive transfer film required for each attempt. Also, it is clear from
the Pulsar documentation:
∗ <
http://www.pcbfx.com/main_site/pages/direct_etch/the_8min_pcb.html>
http://www.pcbfx.com/main_site/pages/direct_etch/the_8min_pcb.html that the toner itself is not good enough, so there has to be a second step
of applying a "Green TRF foil" so the combined bonded "foil" and toner will
keep the etchant reliably off the copper.
These steps involve melting the toner on a flat surface and pressing it, so
I would expect it to spread out and so make lines wider, with fuzzier edges.
If I can laminate my own Riston or similar to FR4 material, rather than buy
pre-laminated FR4 material, then I can choose whatever thickness of
FR4 I want, with one or two copper layers, with the copper thickness
whatever I want, without having bunches of laminated boards degrading over
time.
I have some single-sided Riston FR4 from the 1980s. It is all polymerized
dark blue - so it can't be used. I understand the thin mylar film is to
protect the Riston from oxidation, which I guess causes the photoresist to
polymerize. I guess this can be avoided if the dry film is stored
correctly. Maybe it goes off after a few years, but the dry film is cheap.
Pre-laminated Riston FR4 boards are difficult or impossible to obtain and
would be much more expensive.
The low cost and simplicity of Riston developer and stripper would be hard
to beat.
There are plenty of etching options, so the remaining problems are:
1 - Choose and purchase some Riston film. I should have some 38 micron
MM540 in the next few weeks.
2 - Figure out how to laminate it onto single and double sided boards
- probably A5 size will be fine for me. I will write a separate
message about the laminator I bought today.
3 - Expose the Riston without too much fuss. I have a promising
technique which I will write to the list about once I have tested
it further.
- Robin
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