Homebrew PCBs

Stefan Trethan wrote:
> On Thu, 29 Apr 2004 18:36:48 +1000, Russell Shaw <rjshaw@...> 
> wrote:
> 
> ...
> 
> Ok, well, i see our design goals a a bit far apart.

The reason for the large machine is that as well as for drilling
and milling, 100 reels of components uses up a bit of length.

> However, one thing it there that i want to ask.
> You say you don't use the plastic syringe, how do you fill any dispensing
> mechanism "air bubble free"?

I hadn't really considered it a problem, because the dispenser
software can predict the total volume dispensed, so that i can
refill it before air is sucked thru the mechanism. By squirting
new stuff in thru a tube where the tip is below the surface,
air is avoided. The other way is to connect a vacuum pump (or
simple reversed bicycle pump) and suck it down for 5-10s, which
pulls out small bubbles as very large bubbles under vacuum.
Letting the air back in causes any remaining bubbles to shrink
away. I have a simple tube fitting on it for that purpose if
needed. The valve arrangement avoids paste being sucked back
up the outlet.

> I did not consider buying paste in larger quantities or in jars mainly 
> because i think it is impossible to fill a syringe (or similar) without
 > adding air, which would in any case (maybe air pressure feed a bit less)
 > influence the paste amount delivered, and of course totally mess things
 > up when the air bubble actually reaches the tip.

My idea is to be able to push thru consistant volumes of paste, even
when the stuff is months old and would seize a normal syringe.