It bugged me so I researched it and found that the evaporation of the solvent as it spreads across the board thickens the resist just enough to match the increasing centrifugal force. For silicon wafers they control the viscosity, speed and acceleration very closely. It's apparently much less critical with PC boards.
Boman33 <
boman33@...> wrote:
>Todd,
>I am not arguing, just thinking.....
>It helps when discussing concepts to emphasize for better clarity, so:
>If the resists is added an inch off center, I cannot see it moving toward
>the center to coat that area. Presumably the pour was sloppy enough to
>actually cover the center section.
>
>Similarly thinking about a very small drop sitting at the center, I would
>expect very little force to move it away and spread out.
>
>Finally, resist added in the center and spreading outwards has to cover a
>larger and larger area and therefor presumably be a thinner coating further
>out.
>
>Now, after writing all that there might be a completely different
>explanation:
>There might be a microscopic layer that attaches itself to the surface with
>a thin high boundary friction force and the excess just slides off, so as
>long as sufficient amount is added that thin surface layer stays constant.
>I do not know.
>Bertho
>
>-----Original Message-----
>From: Todd F. Carney / K7TFC Sent: Sunday, April 07, 2013 01:52
>
>∗I didn't ever know the speed of what I used to use, but in comparing it in
>my memory with a 45 rpm record (also from memory), I'd guess it was about
>twice that fast. One certainly kept one's fingers away. While it's true the
>perimeter would be moving much faster than the center as measured in
>inches/second (or some other linear unit), I think in this application it's
>angular velocity that matters and that's the same at all points. At 90 rpm
>that would be 32,400°/minute. ∗
>∗
>∗
>∗We used to mount the pcb material, start it spinning, and then carefully
>pour a small amount of emulsion at the center. Since "center" is really only
>a theoretical concept, it was always off center enough that the emulsion was
>flung across the board, hitting the sides of the open enclosure the thing
>was mounted it. It don't think we ever cleaned up that mess. It got pretty
>thick after a while.∗
>
>73,
>
>Todd
>
>