Is it 250F or 250C ?
If it is celcius, it is way too high !
use a leveled hotplate and put your sample on it at RT. ramp it up to 95C for
about 4-5hrs and ramp it down ! wait for 1 hr, expose, postbake from 25 to 95
for 1 hr and ramp it down again. wait 3hrs and develop
200 um thick is not that thick. you should not have any problem with it !

florian

Quoting "J.J wang" :

> Hi, Dear all
> Right now I am using SU-8 100 as one of my structure layers. I want the SU-8
> layer to be as thick as possible and it need to stay after the process. My
> spinning speed is about 1000 rpm for 30s, the approximate thickness is around
> 200 um. I tried to cure it in nitrogen oven at 250 degree for 3 hours, but
> the SU-8 cracked.
>
> Does anyone by chance know the full cure heating profile ?