On Mon, 7 Jan 2013 01:46:29 -0500, you wrote:
>I experimented with a ziplock bag and a vacuum pump. The transparency is
>held to the board very very well. Much better than glass can ever do.
>However, I found a drawback and that is the plastic forms imperfections and
>scratches from fingernails, stretching and pulling to get wrinkles out
>during vacuum, and around the transparency and copper clad edges.
>
>
>
>I question, as I have not tested, if these imperfections can cause diffusion
>of the light as it passes through and decrease results especially if the
>imperfections are over a thin trace or spacing between traces. I tried
>blanket storage bags, same problem. My conclusion is that very thin glass
>like microscope cover slide glass is the ideal material because it is thin
>enough to deform over the copper clad. I searched for glass as thin as
>cover slide glass but I could not find a place who sells it. Maybe
>something better than ziplock bag plastic can be used too that I am not
>aware of.
I used furniture store vinyl, thin gauge, seemed to work just fine,
calibrated exposure with the covering in place.
Did not use saran wrap or the equivalent, since it was too thin and
developed a static charge which tended to lift the positive from the
board and was messy to deal with. You could also use book cover
material, which may be equally nice.
Take something fluorescent, a black light, and see if it glows less
with the material between it and the light.
Harvey
>
>
>
>Jeff
>
>
>
> _____
>
>From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
>On Behalf Of Robin Whittle
>Sent: Sunday, January 06, 2013 9:17 PM
>To: Homebrew_PCBs@yahoogroups.com
>Cc: Vicent Colomar Prats
>Subject: Re: [Homebrew_PCBs] UV nail lamps - sharpness of shadow,
>phototool-PCB contact closeness, 500W floodlamp
>
>
>
>
>
>Hi Vicent,
>
>Some of the eBay ads which can be found by searching for:
>
>36W Nail UV Lamp Acrylic Gel Curing Light
>
>show that they contain four long U-shaped fluorescent lamps. This ad:
>
>http://www.ebay.com/itm/310556445104
>
>shows one of these types of lamp - and they have "365nm" written on
>them, which is most helpful! The Riston General Processing Guide at:
>
>http://www2.dupont.com/Imaging_Materials/en_US/tech_info/datasheets/index.ht
>ml
>
>indicates peak sensitivity is in the 350-380 nm near-UV range, with some
>sensitivity from 300nm (UV) to 450nm (blue).
>
>I am not sure what wavelengths other photoresists are most sensitive to,
>such as positive photoresists.
>
>I think it is much better to have a relatively compact source of light
>than something broad like these.
>
>In some previous messages to this list, such as message 30440, I
>described using a 500 watt linear halogen incandescent lamp with
>reflector - a cheap hardware-store "floodlamp". Such lamps put out
>enough near-UV to give me a sun-tan when I extensively used an old
>process camera which had four of these lamps. Glass would reduce the
>tanning/burning shorter wavelength UV rays to some extent but I think
>would hardly affect the transmission of the 350-380nm light of interest.
>
>The 500 watt lamp radiates most of its energy in the visible and
>infra-red (with some being lost as hot air), and only a fraction of it
>is the light we are interested in. I couldn't find a suitable source of
>dark-violet-looking "Woods glass" to make a filter which would reject
>most of the visible and IR I don't want. This visible and IR energy is
>substantial and will warm the board, which might cause differential
>expansion of the phototool and board. However, this can be largely
>solved by blowing a fan on the board, with its phototool and glass or
>other surface which clamps the phototool to the board.
>
>This way, most of the light comes from an area about 10cm wide and 45cm
>away from the board. These rays of light are relatively parallel, so
>the shadow from the phototool remains relatively sharp even if the
>phototool is not completely flat against the photoresist.
>
>With a broader source of light, as would inevitably be the case with
>these less intense, larger, fluorescent lamps, the shadow would be much
>less sharp.
>
>It is difficult to get the phototool sitting flat against the PCB. The
>PCB is reasonably rigid and is not necessarily flat. If the top cover
>is glass or acrylic, then any dust or mismatch between the flatness of
>the glass and the PCB will cause a gap and loss of sharpness.
>
>It would also be possible to use a vacuum frame with a soft, flexible,
>top surface, such as clear PVC sheet (retailed at fabric and other shops
>as a table covering), which would press the phototool close to the PCB
>even if the PCB was not completely flat. This would also largely avoid
>problems caused by dust, since it would tend to stretch the phototool
>over the dust particle and so press it close even one or two mm away
>from the particle, whereas with the glass or acrylic approach, the dust
>particle would lift the phototool and glass off the board for a larger
>surrounding area.
>
>Another likely problem with fluorescent lamps of all types is that their
>output would vary considerably with how long they have been on for,
>which would be affected by their starting temperature and therefore how
>long they had been on for in the last 30 minutes or so. This would make
>it difficult to run them from a timer. This might be OK if you ran them
>all the time and had a way of simply placing them over the PCB with
>phototool etc. However, I prefer to place the phototool and my existing
>glass sheet arrangement together by a safelight (a bunch of red LEDs)
>and place it under the lamp, before turning the lamp on for a fixed
>time. The 500W incandescent lamp would have its near-UV output affected
>somewhat by mains voltage fluctuations, but I doubt if this is a
>significant problem.
>
>- Robin http://www.firstpr.com.au/pcb-diy/
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