Homebrew PCBs

Does any one know if LCDs are transparent to UV? Would it be possible 
to replace the backlight on a LCD display (like a LCD computer monitor) 
and do a contact exposure to a sensitized PCB with the PCB image 
displayed on the LCD? A number of monitors have sufficient resolution 
to do fine traces if when the LCD is clear UV is transmitted and the UV 
is blocked when the LCD is dark (or visa versa if you have positive or 
negative resist).
Having disassembled a LCD monitor it does not look like it would be too 
difficult to replace the cold cathode lamp with UV LEDs. (Unfortunately 
the LCD was cracked or I would have tried the experiment). This would 
eliminate the need for photo-plotting, printing on Velum, or overhead 
material. With a white light behind the LCD, you would get what you saw.

That one has occurred to me before. I even got as far as disassembling 
an old black and white laptop to investigate. The biggest problem I can 
see is the thickness of the glass. Unless you have extremely good 
collimation of the light you will get severe undercutting.

Les

Dave Miller wrote:

> Does any one know if LCDs are transparent to UV? Would it be possible 
> to replace the backlight on a LCD display (like a LCD computer monitor) 
> and do a contact exposure to a sensitized PCB with the PCB image 
> displayed on the LCD? A number of monitors have sufficient resolution 
> to do fine traces if when the LCD is clear UV is transmitted and the UV 
> is blocked when the LCD is dark (or visa versa if you have positive or 
> negative resist).
> Having disassembled a LCD monitor it does not look like it would be too 
> difficult to replace the cold cathode lamp with UV LEDs. (Unfortunately 
> the LCD was cracked or I would have tried the experiment). This would 
> eliminate the need for photo-plotting, printing on Velum, or overhead 
> material. With a white light behind the LCD, you would get what you saw.
>

On Mon, 18 Feb 2008, Leslie Newell wrote:

> That one has occurred to me before. I even got as far as disassembling
> an old black and white laptop to investigate. The biggest problem I can
> see is the thickness of the glass. Unless you have extremely good
> collimation of the light you will get severe undercutting.

I doubt it would work - the LCD would expose a grid onto your light
sensitive material (look at your LCD monitor closely - it's a grid of
pixels - positive exposure, you'd get tracks composed of very small
squares).

It would only work with diffuse, very un-collimated light (to avoid just
getting a grid pattern), which would mean you'd only be able to use it for
very large features.

--- In Homebrew_PCBs@yahoogroups.com, Dylan Smith <dyls@...> wrote:
>
> On Mon, 18 Feb 2008, Leslie Newell wrote:
> 
> > That one has occurred to me before. I even got as far as 
disassembling
> > an old black and white laptop to investigate. The biggest problem 
I can
> > see is the thickness of the glass. Unless you have extremely good
> > collimation of the light you will get severe undercutting.
> 
> I doubt it would work - the LCD would expose a grid onto your light
> sensitive material (look at your LCD monitor closely - it's a grid 
of
> pixels - positive exposure, you'd get tracks composed of very small
> squares).
> 
> It would only work with diffuse, very un-collimated light (to avoid 
just
> getting a grid pattern), which would mean you'd only be able to use 
it for
> very large features.
>

I wondered about that as well, however in their efforts to make every 
thing thin and light, the glass screens are getting very thin. If you 
had a UV source that gave you reasonably good collimation it might 
work. 

You might be right about the pixilation problem, however, even the 
ink jet and laser printers have that problem. Many times that problem 
goes away because when you iron or laminate the paper to the boards 
the toner spreads out a little. Perhaps the light spreading out a 
little from the thickness of the glass will act the same.

I have a small TV screen I think I will dissasemble and try it on, is 
there any one willing to sacrifice a 4" x 4" sensitised board? I have 
the UV LED source, but no sensitized boards.

> DaveM wrote:
>
> Does any one know if LCDs are transparent
> to UV? Would it be possible to replace
> the backlight on a LCD display (like a
> LCD computer monitor) and do a contact
> exposure to a sensitized PCB with the PCB 
> image displayed on the LCD? A number of
> monitors have sufficient resolution
> <SNIP>

What monitor do you have there?

The finest dot pitch LCD I have around here
(exluding <30mm monochrome projector LCDs
units) is a 10" SXGA off a fujitsu lifebook
P7000 series.

At 1280 dots across its 8" width only makes
160 DPI.

Your desktop monitor might have >1600 dots
across it but its going to be 20+ inches
diagonal so 16+ inches across which makes
for worse than 100 DPI