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.
>>
>
>
>