Re: Platinum heater

-
is your serpentine pattern comprised of sharp 90 degree turns or are you using an "S"-type of pattern with smooth transitions. Sharp "corners" in film heater patterns can cause a failure site.
You indicated that you depositing onto a ceramic base. I presume your are using standard 96% Al2O3 substrates. If so, you will want to check the orientation of the areal surfaces. One surface is "smoother" then the other.
You might also want to consider a higher grade substrate. Possibly 99% Al2O3 which are used in Thick Film RF circuitry. These are more expensive, but have a much smoother surface finish.
Additionally, where the trace reduces in width from several millimeters to a fraction of a millimeter the "squares" of the conductor trace in this region increase with a resultant increase in "local resistance". Take your probes (use 4 point to compensate for the low resistivity of Pt) and measure the ohms at the beginning and end of the wide regions and at the beginning and end of the narrow regions. I expect there are major differences.
One means of preventing overheating in this region would be to increase the thickness of the trace in the narrow zone thus decreasing resisitivity.
-al