Homebrew PCBs

Gentlemen, may I add my 2 cents worth to this discussion.

Over the last 30 years I have made many hundreds of PCB boards using the 
photographic method.

I have used Florescent tubes (3 x 20W UV tubes) at about 150mm from the 
board. This used to take about 8 minutes to expose.

99% of the time I use a mercury vapor  lamp scrounged from a street light. 
This is mounted in a box and is about 300mm from the PCB.

Exposure time is 2 minutes. The same lamp has been in service for thirty years.

This lamp has one serious drawback!!. It must be on for at least 4 minutes 
before it can be used and cannot be switched on/off like the florescent tubes.
This has never been a problem.

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.

The reason for such thick glass is so there is no bending and thus having 
the artwork not pressed firmly on the board.

When I only did single sided boars I did make up a vacuum frame which 
worked well but I haven't used single sided boards for many years.

The sensitized PCB I use is -ve acting so I have to produce a negative of 
the artwork.

The PCB material is cut 10mm bigger than required before exposing.

The artwork is printed on a laser printer on overhead transparency film. 
Two copies are printed and laid over each other to achieve the required 
density.

To make the -ve I use a Ilford daylight photographic reversal film. This 
film can be used in subdued light and is exposed for 7 seconds in the same 
light box I use for the PCB exposure.

The film is expensive and came in a box of 100 sheets letter size. I keep 
this in a refrigerator along with the PCB material. It is a messy process 
having to develop the film and fixing it as in any normal photographic 
process. However the results are worth the effort

While this process sounds complicated and is expensive to setup up in the 
first instant the results are worth the effort. I never have a failure 
unless I cut corners or forget to do some part of the process (like 
forgetting to turn the board over while exposing double side board)

As I said earlier I have used this process for over thirty years and would 
not change as I can guarantee results every time.

If you only want to make one or two boards then use another method, but 
once set up you can make as many boards as you want as often as you want.

Finally the boards are drilled and then tinned in a tinning machine.

I trust this presents another method of producing successful PCB boards

David


At 07:00 PM 2/20/2006, you wrote:

>Thanks. So, as I se,, the light goes inside the box, and the board
>outside, facing down on the light.
>I suppose UV light is not good to take.
>
>I just meet a person that uses a fluorescent light tube, and it works
>for him. Just 10 minutes exposure at 10cm. He puts the crystal on top
>of the board, with some weights outside.
>
>I would like to do some test without gettin much into mess. So I might
>try this solution, and if it goes well, then build a proper box. Does
>the box have any special purpose appart from what I said??
>
>Also, if you want to use only a piece of the board. Can you just cut it
>before exposing?? what is best?
>
>Thank you
>
>
>
>
>
>Be sure to visit the group home and check for new Links, Files, and Photos:
>http://groups.yahoo.com/group/Homebrew_PCBs
>
>If Files or Photos are running short of space, post them here:
>http://groups.yahoo.com/group/Homebrew_PCBs_Archives/
>Yahoo! Groups Links
>
>
>
>

David Hopkins (VK4ZF)
Queensland
AUSTRALIA
davhop@...
Skype :-   davhop
S 27, 22.294  E153, 11.008