UV-sensitive PCB material

[Boncuk]

I am planning to build an UV-exposure box using a modified scanner. A scanner is small compared to the huge (and heavy) wooden boxes normally used for exposure boxes and requires one row of 42 LEDs only.

There are different wave length UV-LEDS on the market ranging from 340 to 400nm.

I know from experience that 400nm LEDs require an awefully long exposure time for good results.

340nm-LEDs cost US$ 51 per piece ordering 100 pieces.
350nm-LEDs are still very expensive (about US$22 per piece odering 20 pieces)

For a good compromise between cost and effectiveness of the exposure box it is essential to know the wavelength of the photoresist's sensivtity.

Does anybody have info about it?

Boncuk

 

[jpanhalt]

Check the recommendations for the particular photosensitized boards you are using. Mine recommend the BL-type of fluorescent light, which emits a max at about 365 nm (it can be confusing to actually pin down the emission max of those bulbs). Many resists have a max absorption in that same area, but some actually have a max absorption at about 400 nm. Matching your light source to the max absorption could save you money.

If your experience shows that the cheap 395 nm UV LED's won't work, then the 350 nm (or 365, if available) would be a better choice. Remember, the glass hold-down plate and transparency will absorb shorter wavelength UV more, so using the much more expensive, shorter wavelength UV LED's may just be wasted money.

John

 

[Boncuk]

Hi John,

thank you for your information. There are many hobbyists using the cheap 400nm LEDs with extreme long exposure time (about 10 to 20 minutes).

In the meanwhile I've read UV-sensitive PCB material is sensitive in the range from 340 to 420nm decreasing considerably with increasing wavelength.

340nm LEDs are US$51 per piece, 350nm are still about US$22 per piece and 360nm are considerably cheaper at US$3.15 per piece. Since those LEDs emit at an angle of 120 degrees I still need 42 of those to cover normal DIN-A4 width. 340 and 350nm LEDs don't emit any visible light, and that makes them extremely dangerous. 360nm has a small portion of visible light which might warn the user not to look at the LEDs directly.

I have on mind just to put in the exposure time and the MCU calcuates for stepper motor speed (clock frequency) and if necessary for full and half step as well to get a board exposed in one run (forth and back).

Supposedly the math will cause me more headaches than getting appropriate LEDs. One run must always be at one speed for equal exposure. The second run might be different in clock speed and step.

I'm not worried about the glass since scanners normally are fitted with glass not filtering UV-light.

Regards

Hans

 

[jpanhalt]

I have a large pdf document (too large to attach) that has absorption spectra for photoresists. Since Novolak is common, here its spectrum:

Note that a sensitizer is often added to move the active absorption to longer wavelengths. The effective max is at about 360 nm. (The I line refers to the mercury emission spectrum and is 366 nm, as I recall. Edit: Wikipedia lists it at 365.4...close enough.) A less intense absorption is at 400 nm, which explains why the longer wavelength UV also works.

Long story, short, I would use the 360 nm LED's at $3.15 each and pocket the change. Are the outputs of the 360 nm and 395-400 nm LED's about the same?

The glass hold-down is probably not as big a consideration as the transparency. To my dismay, my favorite transparency (Pictorico Premium OHP film for Inkjet) was re-formulated to include a UV blocking agent. It's main use is in graphic arts and absorption of UV makes the image more vivid or something.

John

 

[Boncuk]

Thanks again, John,

so with 360nm I'll just be right on the money then.

There are no data available for the 400nm LEDs at my electronic supplier. I only know the LEDs look pretty dull and even looking directly at them won't harm an eye.

340 and 350nm UV-LEDs don't emit visible light, but 360nm do. The power output of the 360nm LED is 500 to 1000µW.

I'll try it and keep you informed about the outcome.

Regards

Hans