Most varieties of acrylic (Plexiglas) have very poor transmission of UV wavelengths below 370nm (i.e. they tend to block the Hg I-line). If the photoresist that you are using is Hg G-line or H-line sensitive, then most varieties of acrylic will be fine at least initially (they will yellow over time), but if your resist is only I-line sensitive, then the only variety of Plexiglass that would work for you is Plexiglas G UVT. Here are a couple of data sheets that back this up:
See figure 5 in this sheet:
http://www.plexiglas.com/literature/pdf/81.pdf And here is the data sheet for Plexiglas G UVT:
http://www.plexiglas.com/literature/pdf/227.pdf-Marc
________________________________
> To: Homebrew_PCBs@yahoogroups.com
> From: nuno-t@...
> Date: Tue, 17 Aug 2010 11:44:13 +0100
> Subject: RE: [Homebrew_PCBs] UV: alternatives to glass?
>
>
>
> Hi,
>
> Have you considered clear acrylic sheets?
> I've been using them for a while on the same UV box that came with normal
> glass on it.
> The difference is considerable in the UV behaviour. Much better for acrylic.
> Flexibility vs. rigidity was solved using a slighter ticker acrylic.
>
> Nuno T.
>
> > -----Original Message-----
> > From: Homebrew_PCBs@yahoogroups.com
> >
> [mailto:Homebrew_PCBs@yahoogroups.com]
> On Behalf Of DJ Delorie
> > Sent: segunda-feira, 16 de Agosto de 2010 21:55
> > To: Homebrew_PCBs@yahoogroups.com
> > Subject: [Homebrew_PCBs] UV: alternatives to glass?
> >
> >
> > Ok, shame on me, I broke the glass in my UV box. Too much vacuum.
> > We've checked a few local places, the glass they have has UV
> > blockers on it. I've got more places to try, but I thought I'd ask...
> >
> > What else can be used besides glass, which will still pass UV
> > without bending or blocking it? Something less brittle, more
> > flexible, but still stiff enough to mostly hold it's shape
> > (my UV box has a foam gasket along the outside edge of a
> > piece of plywood, the glass rests on it and is sucked down
> > onto the board).
> >
> > Even rubber-like flexibility is OK, I suppose - I can make a
> > wooden frame to hold the edges and use an even thinner
> > gasket, but that's more work, and it means I have to cut up
> > my films into individual sections so I don't bend or crease
> > them from the shrink-wrap effect.
> >
> > DJ
> >
> >
> > ------------------------------------
> >
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> >
>
>
>