Homebrew PCBs

----- Original Message ----- 
From: "Vlad Krupin" <vlad.cnc@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Tuesday, February 21, 2006 2:18 AM
Subject: Re: [Homebrew_PCBs] Re: Photolithography


> David, thanks for the great description of your process. I have one 
> question
> however:
>
> On 2/20/06, David Hopkins <davhop@...> wrote:
> [snip]
>
>>
>>
>> To hold the board and artwork together I use two sheets of 10mm glass 300
>> x
>> 300mm. The sensitized board and the artwork are held between the two
>> sheets
>> of glass and they are held together with large spring paper clips. The
>> reason for two sheets of glass is to do double sided boards.
>>
>> Before the glass I used to use 10mm Perspex (Acrylic) however this
>> scratches easily and one of the trainees spilt developer on them and
>> ruined
>> the Perspex.
>
>
> Doesn't glass (or at least most kinds of glass) tend to absorb UV rays? I
> was under the impression that this was the case. As a result I first tried
> using acrylic, and after finding that it scratches too easily, ended up
> using the thinnest glass I could find -
> http://www.krupin.net/serendipity/uploads/pcb_making_exposure_frame.jpg
>
> It sounds to me that using thick glass would seriously slow down the
> exposure time, especially for people who are using a less powerful light
> than the one you have.

Glass doesn't absorb the long-wave UV used for PCBs., unlike, say the 
shorter wavelength used for EPROM erasure.

Leon