The trick is not to pull a vacuum in a large cavity beneath the glass, but to pull it between two sheets of glass, or between a sheet of glass and a clear flexible sheet over it.
The plastic would tend to seal down and prevent a good vacuum from being pulled all across the surface, maybe some paper towels placed right up to the edge of the board would allow air to be pulled out.
Steve Greenfield
--- In Homebrew_PCBs@yahoogroups.com, DJ Delorie <dj@...> wrote:
>
>
> William Alford <walford@...> writes:
> > I've built several vac hold downs over the years and the trick is to
> > have the substrate supported everywhere.
>
> Maybe I missed a bit of info... the first time I tested it, I had
> nothing in it - just the glass panel with the gasket around the edges.
> I guessed that the panel would be pulled down evenly, and I could
> measure how far. What happened, obvious in retrospect, is that the
> glass bowed down further in the middle. My solution, obviously, is to
> use the pcb to support the middle of the glass. The trick will be to
> support it enough but not too much - I want the glass flat on the
> board at full pressure, not bowed down due to insufficient support or
> arched over due to too much.
>
> The gasket is 125 mil, the boards range from 8 to 65 mil, each sheet
> of card stock is 8 mil, the film is a few mil, etc. So I can stack it
> all up so that the glass is pulled down evenly over everything.
>