I am using the 395-400nm LEDs. Since there is a strong UV component to
the LEDs I am not sure if a visual test for uniformity is going to be
accurate.
Since I am working in english units obvious I am working around 12.7
mm or .5 inch. Not a big difference.
To really measure exposure I need stouffer gauge. I may go ahead and
order one from Think Tink at some point.
As far as distances, the further away the longer it takes. So
initially I will be going for 1.25 to 1.5 inches. Based on my
calculations from yesterday in another message it looks like 1.42
inches is the distance I want so that the half power points sum up
properly. Since these point sources and not infinitely sized, the
illumination will never be perfectly uniform.
My design at this point is leaning towards a 4x6 board, which with .5
inch spacing will use 96 LEDs. I will also get the board sized so that
I can add more if I need a larger size, but my largest board to date
is 2.5 by 3.5 inches.
I am looking into also using LT3591 boost regulator for driving the
LEDs. Obviously it will take several of these to drive all 96 LEDs,
but I do not like the idea of current limiting resistors or just
putting them all in parallel.
--- In
Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@...>
wrote:
>
> I also got the 390-395nm BestHongKong LEDs. To test the exposure time I
> made a 3x3 square array as you did, but with 15mm pitch, not 13mm. To
> look for uniformity, just shine on some white paper and see how much
> variation there is on the reflection. I found an LED pitch of 15mm gave
> fairly uniform projection at 100mm or more. A 30mm distance as you are
> talking about looked way too non-uniform to my liking.
> There is no big penalty going further distance, except the edges of the
> projected light will tapper off more slowly the further distance away.
> All this means is your array will need 1 to 4 LEDs in size larger than
> PCB artwork area to compensate for tapering intensity. Remember an
> infinite LED array has constant intensity at all distances. So, it
beats
> me why your trying to get the LEDs close as possible.
>
> The best way to test exposure times is to expose without any image
> printed on the photomask. Then expose small sections at incrementing
> times. I did this by cutting strips of photoresist film about 100 x
20mm
> , and drawing 4 lines equally spaced across the strip to give 5
squares.
> I used some black plastic to mask out all but one square and exposed
> this area for a recorded time. Exposing in small steps, e.g
> 10,20,30,40,50 seconds, you can get very good estimation of time it
> takes to fully expose photoresist. After development you will see the
> squares which are slightly underexposed will look different (more
> damaged or loss of smoothness) than those 100% exposed.
>
> Only after doing that you should test artwork, because now you know
what
> the minimum exposure time is.
>
> javaguy11111 wrote:
> > I did a little more testing of UV LEDs this weekend. The LEDs are
> > plugged into a solderless breadboard and driven with a variable
> > external power supply I tried a 3x3 grid with 1 inch and .5 inch
> > spacing. The LEDs were run at 20mA and pulled about 10V.
> >
> > The 1 inch spacing at 1.25 inches above the board showed too much
> > variation in exposure. I would get areas of overexposure while other
> > areas would not get enough exposure. I tried times of 1,5 and 10
> > minutes. 10 minutes was way over exposed, 5 not so bad and 1 left some
> > areas okay, but other areas underexposed.
> >
> > I then did tests with .5 inch spacing with the LEDs at 1.25 inches
> > above the board. Uniformity of exposure was much better. I tried an
> > exposure time of 1 minute. I think I can drop that down to 45 seconds
> > as I still saw a little extra exposure in areas directly below the
LEDs.
> >
> > So I think for me .5 inch LED spacing is what I am going to use to get
> > uniformity. I will do a few more tests of increasing the distance to 2
> > inches above the board to see how that works.
> >
> > --- In Homebrew_PCBs@yahoogroups.com, "javaguy11111"
> > <javaguy11111@> wrote:
> >> I just did a quick test with my UV LEDs using a step pattern going
> >> from 10/10 spacing down to 1/1 spacing in thousands of an inch. This
> >> test pattern was printed by an Epson 2400 printer using Pictorico
> >> transparency.
> >>
> >> The leds were 1 1/4 inches above the board with a .5 inch spacing. I
> >> tested with 4 LED's running 2 in series and then 2 of those in
> >> parallel running 20ma at 6.6V.
> >> Exposure time was 60 seconds.
> >>
> >> After developing in sodium carbonate, I got good lines down to 3/3
> >> spacing. The 2/2 looked pretty good as well, but was a bit jagged.
> >> Possibly due to the 1440 dpi resolution of the printer. The 1/1 lines
> >> were all merged together.
> >>
> >> I will try 3/4 inch and 1 inch spacing over the weekend to see how
> >> those work, before I commit to a spacing. I will post the results of
> >> those tests as well.
> >>
> >
> >
> >
>