DJ Deloraine wrote:
> Adam Seychell <a_seychell@...> writes:
>> Whats the grid pitch ?
>
> About 1.2 inches. Just enough to solder the leads together :-) That
> means 99 LEDs in a 9x12 inch area, or 11 strings of 9 for the power
> supply.
>
> The density is about 0.2 leds/cm2.
>
>> Whats the LED brand/model ?
>
> The 395nm UV from BestHongKong. I got 100, and 100 of them worked -
> perfect score! I built up a current regulator on a breadboard and
> checked each one before inserting it into the board.
>
>> I'm interested in your exposure times. I get 60 seconds with
>> negative dry film, grid density 0.28 LEDs/cm^2, BestHongKong 390nm
>> UV LEDs at 20mA, about 100mm PCB to LED gap, measured with inkjet
>> transparency + 5mm window glass in light path.
>
> I picked up a Stouffer step gauge (from stouffer.com - just call them,
> they can ship direct, not expensive) so I'll know for sure. The
> distance to board is about 5 inches, the current 20 mA. I calculated
> about 30 seconds initial exposure based on the "math" of the setup,
> but I'm sure reality will correct my assumptions there :-)
>
> The next step is to laminate some resist and cut it into test strips.
> I don't get a lot of free time for these, so stuff happens as I find
> time to do it. I've got some photolith paper but it's not JetStar
> (they sent the wrong brand but they're working on getting the right
> stuff) and some glass of dubious UV qualities (cheap hardware store
> glass). We'll see how it goes.
>
Thanks. I got 400 of the BestHongKong LEDs and got 0% failure. The ultra
cheap brand I got 100 LEDs and 1 showed < 1/2 brightness, and another
one didn't work. The BestHongKong were also 2x energy output so my
choice was easy.
For calibration, you just take a strip of pre-sensitised PCB, and expose
it in increments of 10 seconds. You do this by marking lines across
the photoresist so has about 10 divisions. You expose each marked
section incrementally by shifting some black opaque film across the
sections every 10 seconds. That's a terrible explanation.
Anyway , the right exposure time is found when your photoresist shows no
signs of surface attack after developing.