Surprisingly powerful UV light box - Page 1
Forum section: electronics / projects

author: JimmyM
author_rank: Regular Contributor
timestamp: March 10, 2014, 10:19:27 pm
content: So I've switched from the toner transfer to UV/photo resist method to build my PCBs. So I looked around at what a lot of folks were using and decided to use UV LEDs and found these with a dominant wavelength of 400nm.http://www.ebay.com/itm/NEW-50-PCS-0-5W-5-Chips-8mm-StrawHat-Hi-Power-UV-LED-30000-mcd-/290726120421?pt=LH_DefaultDomain_0&hash=item43b0a08be55 strings of 9 LEDs each. 80mA per string.With several UV LED projects I saw that they ran tests afterward to determine the correct exposure time. I ran a few tests for 1, 2, and 3 minutes as others have done. All were over exposed. After some more testing I saw that the best exposure time was 15-20 seconds. I just ran one now at 25 seconds and it's a little over done. I double up my transparencies since the black isn't completely opaque. I may run another test with a single layer transparency and expose at 10-12 seconds.Did I build a monster or were the others tests at 1-3 minutes the oddballs?Additionally, my power supply is a 32V printer power supply (~29V loaded with 400mA). I use opamps, FETs and current sense resistors in each string to keep current constant. Math/measurements show that the FET and CS resistor drop ~1.6V @ 80mA. They hardly warm up after 30 minutes of running in free air.

author: madires
author_rank: Super Contributor
timestamp: March 10, 2014, 10:41:35 pm
content: Wow, that's fast! Maybe too fast. You could add a PWM driver.

author: sleemanj
author_rank: Super Contributor
timestamp: March 10, 2014, 10:49:01 pm
content: Well the fact that you are using 8mm HALF WATT leds might have something to do with it.The rest of us poor disadvantaged people make do with 5mm more like 0.05W leds :-) 10 times less power per LED.

author: JimmyM
author_rank: Regular Contributor
timestamp: March 10, 2014, 11:09:37 pm
content: PWM seems like a hassle. I can just lower the current by adjusting the reference voltage at the opamps.Well it seemed like a tossup. More 5mm LEDs or fewer 8mm ones. The whole thing cost me less than the stupid $80 laser printer toner cartridge I need for the $110 printer I bought. Any way. I just etched the board. My God, it's beautiful. Sooo much better than toner transfer. Now I have to work on a heater for my etch tank/bubbler.

author: sleemanj
author_rank: Super Contributor
timestamp: March 10, 2014, 11:36:02 pm
content: Quote from: JimmyM on March 10, 2014, 11:09:37 pmHow is the resolution (what size trace did you achieve) and are you using a collimation grid? Do you get hot spots?I've been thinking about rebuilding mine using stronger and wide-angle leds as you are using but I'm concerned that the collimation may suffer too much.

author: apelly
author_rank: Unknown
timestamp: March 10, 2014, 11:42:16 pm
content: Subscribed!

author: JimmyM
author_rank: Regular Contributor
timestamp: March 11, 2014, 12:12:25 am
content: The traces were pretty thick in this design. Why use a 0.2mm trace when 1.0mm traces still fit perfectly. Although there was a tiny sliver of copper pour that came out nicely.No collimation grid. I was sort of worried about it, but I had the art work printed on the transparency so that it faced the PCB so it was in intimate contact. Only the thickness of the resist itself separated the copper from the closest layer of transparency toner.I did use 2 layers of transparency though due to the blow-through in the first test.The 15 second test came out very nice. I may reduce to 15 seconds from 20.I'm going to create a test pattern with traces from 30 mils to 5 mils and see what happens with a single layer of transparency and exposures of 5, 8, 10, 12, & 15 seconds.Of course I can always reduce the current to the LED matrix. The 45 LEDs fill a 7" x 5" area.

author: Unknown
author_rank: Unknown
timestamp: March 11, 2014, 03:07:08 am
content: Quote from: JimmyM on March 10, 2014, 10:19:27 pmSince you're new to photoresist, it's worth pointing out that exposure times can vary dramatically between different types/brands of board.For example, in my setup Kinsten takes ~90 seconds while Mega takes ~7 minutes. I also find Kinsten very critical to developer strength & very easy to overdevelop, while Mega is damn near impossible to overdevelop with any reasonable strength. Every other brand I've tried falls between these 2 extremes.

author: JimmyM
author_rank: Regular Contributor
timestamp: March 16, 2014, 05:15:56 am
content: Quote from: Tac Eht Xilef on March 11, 2014, 03:07:08 amYou have my attention...My only experience so far has been with MG Chemicals 600 series. That only goes up to 1oz. I did find another vendor with positive presensitized PCS with 2 oz copper. I'll make sue to run a test on them before a real board. Thanks for the heads up.

author: hlavac
author_rank: Frequent Contributor
timestamp: March 16, 2014, 03:44:22 pm
content: My160 UV LED exposition boxtakes under 20 seconds to expose the pcb stuffI use, so looks about right..,