I tried a fresh bottle today fully expecting a more normal thickness but
it actually turned out a bit thicker!
My coverage wasn't 100% as I rushed the substrate preparation a bit, so
I'll try again after the weekend..
In a way it's nice as I do want to do very thick layers in the future
and this will make it easier but I would like to find out what is
causing it.
My cleanroom is not especially cold during the day (I measured 23C) but
probably gets cold overnight (there is a big single-glazed metal framed
window in there). Hence the thermal mass of the bottle could mean it is
significantly colder than ambient. If true, and temperature really does
have such a big effect then it occurs to me this may be a nice way of
increasing the thickness range of any particular SU-8 grade - just keep
it in a fridge (or oven) for a while prior to use.

Gareth

Brubaker Chad wrote:
> Whoops,
>
> I missed the mention that the SU-8 is old.  As Michi mentioned, once past the
expiration date, there is a loss of solvent and perhaps a slight amound of
premature cross-linking.
>
> Just solvent evaporation can be a major contributor.  If I remember correctly
(no guarantees), the SU-8 2100 is ~77% solids, and the SU-8 2050 is ~ 72-73%
solids.  So a swing of 4-5% solids is enough to triple the viscosity (from ~150
P - 15,000 cP - to ~500 P).