Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Short version:

10W ~380nm LED exposes Riston in about 60 seconds at 30cm.
A 3W one would be less expensive and require about 3 minutes.

Previously I used a 500W quartz halogen linear incandescent lamp (with
aluminium reflector) as my source of UV for exposing Riston. However,
it radiates a lot of visible and infra-red and so heats up the PCB and
the phototool at the distances I was using it at: about 40cm, with a 4
minute exposure. This could cause problems with the phototool expanding
at a different rate to the PCB and so messing up the image. I described
this on 2012-09-03:

http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/30595?var=0&l=1

including the ~7:1 contrast ratio phototools which I made with a Brother
laser printer onto a particular laser transfer film.

Riston, such as the MM540 I get from the Czech Republic:

http://www.ebay.com/usr/gaminn
http://www.tech-place.com/en/photosensitive-materials/23-photosensitive-film.html

is exposed with near visible UV light. According to the datasheet:


http://www2.dupont.com/Imaging_Materials/en_US/assets/downloads/datasheets/mm500series.pdf

the "peak response" of the material is 350nm (nanometre) to 380nm. This
is the same as in the other Riston datasheets I looked at, but I did not
look at them all. The shorter the wavelength of light, the more energy
per electron is coupled, so 350nm light is more energetic than 380nm.

UV LEDs are now obtainable in high power versions. A 1 watt LED is a
single LED chip and can be bought for a few dollars. 3W LEDs may have
three chips in parallel. 10W LEDs have three in series, in parallel
with two other sets of three in series.

I found a wholesaler in China which evidently is source of many high
power LEDs sold individually by eBay merchants. This wholesaler has
minimum order quantities such as 10.

In their 10W High Power LED section they list the following UV wavelengths:

365nm
380nm
390nm
400nm
410nm

The shorter the wavelength, the less light they put out, the more they
cost and the harder they are to find.

This is page for a 10W 380nm LED is evidently for the same LED I bought
from an eBay retailer:


http://www.leds-global.com/ultra-violet-380nm-high-power-led-modules-p-7.html


This is item number G-P10UC140A1-XT.

It is specified to run at 1050mA at which it will have 10 to 12 volts
across it. It will then produce 400 to 500mW of light in the 375 to
385nm range.

The nine chips are arranged in a 3x3 array with outside edges of 5mm, so
this is a nice small light source which should give sharp edge shadows
on the photoresist even if the phototool is not pressed into direct
contact with it.

I bought the LED for USD$50 including shipping to anywhere:

http://www.ebay.com/itm/120896174810

It arrived here in Melbourne Australia 9 days later with signed for
delivery and a hand-written "Made in China" note. This is a Hong Kong
retailer:

http://www.ebay.com/usr/lucky_guy2010
http://stores.ebay.com/lucky4u2bid/

It is titled "10W UV Ultra Violet 380~385nm High Power LED Light for
Recognize Banknote". There's no mention there of a part number.

Running a few mA through it I see a rather broad spectrum with violet -
like a pastel violet. I assume I can hardly see the real peak of
emission, so I won't be looking at it when it is running properly.

I attached it a heatsink with small fan from a PC video display card. I
found an 18 to 19 volt regulated switch mode power supply and made up 9
ohms of power resistors to wire in series with the LED. This is one way
of making a reasonably good constant current source. I got about 1 amp
going to the LED, including a few tens of milliamps for the small 12V
fan motor. This takes the LED voltage nearly to 10 volts. So it is
running pretty close to its recommended current and voltage.

I found the ideal exposure time with 30cm distance, as before through
6mm of glass and the laser-printed phototool, was around 60 seconds.

20 seconds resulted in inadequate exposure - some photoresist remained
but not enough. 40 seconds produced a good image. So did 60 seconds
and 100 seconds.

This is at least a 2:1 exposure latitude. Many people think that laser
printed phototools are inadequate for exposing photoresist. Perhaps
with low-contrast photoresist (maybe some positive or non-Riston
negative photoresists) this may be the case. However, I find this
Riston MM540 is high contrast. This contrast enables the approximately
7:1 contrast ratio laser-printed phototool to work well, without being
excessively fussy about exposure time. "7:1" means that when I use a
photodiode with visible light to see how much light gets through the
dark part of the phototool, I find 1/7th the light than comes through
the clear parts. The dark parts are not entirely even, so the contrast
between the clear sections and the lighter parts of the dark parts is
probably 4:1, 5:1 or 6:1. Anyway, it is still high enough to get good
results with Riston.

- Robin http://www.firstpr.com.au/pcb-diy/