Russell Shaw wrote:
>
> Have you tried using the same higher current and just increasing the
> distance? Maybe the wavelength is increased at lower current, which
> would reduce the photon energy.
>
Yes, I tried 20mA at 120mm distance and still no exposure. I also tested
15mA at just 20mm distance and got complete polymerisation in under 2
minutes. So, I don't think its anything to do with wavelength change,
even if an LED's wavelength could change significantly with current.
LED arrays solve the problem because intensity is increased greatly, and
is also less dependent on spacing between the exposed surface and
LEDs. Increasing the spacing also improves uniformity because the
increased overlap of adjacent LEDs.
For anyone interested, here's some more results.
Here are approximate times for complete polymerisation of negative dry
film photoresist using a 3x3 LED UV array of 15mm pitch. The light path
was travelling through 3mm window glass plus one sheet of inkjet
transparency film to represent actual PCB exposure conditions.
LEDCurrentSpacingIncrementPolymerisation Time
Type (mA)(mm)(seconds)(seconds)
-----------------------------------------------------
A101501040
A20150520
B201501060
B20601050
LED types are:
A "UV 390-395nm 5mm NR F/R" ebay: besthongkong, price: AUD$0.21/ea
B "UV 3000mcd 5mm" ebay: winsome_house, price: AUD$0.11/ea
Its clear LED "A" is a winner here. But then again it might be better to
use the cheap "B" LED and then double the density in the array so
uniformity is improved. Both LED "A" and "B" when assembled in a 15mm
pitch square array at 150mm distance seems to give pretty uniform
looking light when shined on some white paper.