printed circuit board exposure

[The Old Crow]

On Sat, 1 Jan 2000, Rob wrote:

> Well, up until recently, I have had the luxury of a uv exposer for my pcb
> boards. I was wondering what the usual exposure times were for the following
> sources:
> 
> sunlight
> 
> halogen lamps
> 
> black lites
> 
> Thanx... Im having a hell of a time getting my exposures correct.. This is
> about my third try using these "alternate" methods of pcb exposure.. arrgh.

  Buy some UV "plant growth" fluorescent tubes (18" or so, I can get the
part# for you if needed). and use those in your lightbox.  longwave UV
lamps are a poor substitute.  The typical exposure times for the Kepro PCB
photoresist/copperclad is 2 minutes under shortwave UV, and up to 5
minutes under longwave UV.  I lament the passing of the no-longer-made
orange UV film Kepro made.  That stuff was wonderful--one could expose a
negative directly from a laserprinted page.  Now I have to laserprint to
positive on cels to use their flimsy black film.  These days, I find it
less of a headache to just generate checkplots for artwork and pay a board
house to make even my prototype boards.

  Don't even bother with exposing to sunlight-the insolation of UV is too
variable based on air density/water vapor.  As for halogen lamps, in
order to get UV out of them you have to remove the quartz glass (as on a
halogen work light), and then the device becomes quite dangerous in terms
of UV exposure.

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