Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "Andrew" <andrewm1973@...> 
wrote:
> 
> Data sheets for most of the positive
> resist I have seen would indicate that
> black light (long wave UV) bulbs are
> faster at exposing than germicidal bulbs
> (short wavelength).
> 
> Is the negative resist different to the
> positive stuff ?
> 
> I know that _some_ gemicidal lamps have
> much higher wattage ratings per unit
> length that common black light ones (10's
> of watts 100mmm as opposed to 18W in
> 600mm).
>
Do the datasheets really show 600nm is the best wavelength?

What in the datasheet for the positive resist shows that longer 
wavelength UV will be more effective, i.e., faster, given the same 
intensity of light at each wavelength one compares?  I would assume 
that germicidal lamps (i.e., low-pressure mercury lamps) produce 
predominately the mercury line at 254nm.  I forget the exact number, 
but as I recall, it is on the order of 90 to 99% of the light. The 
tube itself, which acts as a filter, and various coatings can change 
that maximum to give most of the light at longer wavelengths, such as 
366nm or more.

With reference to a previous posting about the efficacy of germicidal 
lamps, one thing to consider is that regular glass blocks virtually 
all of the light at 254nm.  Thus, if one uses common glass over the 
PCB, a germicidal lamp may seem to be less effective.  That is not 
because the wavelength of light per se is less effective, but because 
the light is blocked by the common glass (or even the transparency).

I suspect that lamps designed to produce wavelengths in the near UV, 
say around 366 nm to 450nm(?), should be used, just because that 
simplifies the rest of the system.  

Can you give a link to the data sheet for the positive resists?

Thanks. John