Fluro tubes have disadvantage producing of wide angled light. In
my experience this light source produces problems with light
under cutting the photomask and giving inconsistent track widths.
A bulb lamp at distance far enough so the uniformity is
relatively good across the panel, produces much sharper
photoresist patterns. That's why all PCB manufactures use a bulb
type. The lamp to panel distance doesn't need to be that great,
and Hans's setup looks perfectly adequate. With less light
undercutt the exposure time becomes more relaxed. So with such
wide time window, you can afford to overexpose in order to
compensate for non-uniformities.
In general bulbs will be slower than fluro tubes, unless you
start getting into high power, like the 2kW+ short arc HID lamps
used by professionals and manage a 15 second exposure time.
I use a 400W phosphorized mercury HID street lamp, at about 40cm
distance and get 3 minute exposures, but if I were to build
another light box I'd use 250W or 125W at go for closer distance.
Mike Putnam wrote:
> Hans,
> I looked at the picture of your UV exposure setup. I just wanted to comment
> that I would never use the round bulb. Use the florescent tubes and your
> results will be better. I have found that the round bulb tends to give more
> exposure in the center than at the outer edges. Even more apparent with
> larger boards. This is the reason all exposure units sold on the market use
> the florescent tube. You can compensate somewhat by having the bulb not so
> close, but this just increases your exposure time. The other bad thing about
> the round bulb is the heat. This will actually get in the way of your
> process, depending upon what type of photo etch you are using. I am trying
> to recall if it was a problem caused to the developer or if it was a
> consistency problem in the exposure. Nevertheless, I found it to be a
> problem several years back. If you modify your design to use the florescent
> bulbs, you will notice a marked difference in detail as well as less
> exposure time needed.
> -Mike
>