What is the lowest temperature that makes sense?
It depends on what the mechanism for the initial drift actually is.
Eric Modica argued at MM2021: "... Another important aspect that carries over to the ADR1000 is the benefit of bake and burn process. So when I have first started working with buried zeners and Vrefs in general the conventional wisdom was that bake and burn speed up the aging of the part and that one could accelerate the early life drift to finish up sooner. That’s this area here, that’s the majority of your drift on parts like this, your uncertainty. So I’ve learned enough in my years working in analog design not to make broad generalisations, but what I’ve observed is that bake and burn is helpful primarily for low frequency noise reduction in this context. We have had tier 1 manufacturers tell us the same thing when we send them ADR1000s. Prior to instituting a bake and burn most of the parts look okay from a low frequency noise perspective, but if you look at it enough you find one that is extremly noisy and actually remedies with the bake and burn. So only being a process hobbiest myself, the only explaination I can give there is that you have these dangling hydrongene bonds at the surface of the SI and sometimes that amounts to surface charge and if that ends up in the vacinity of the zener, if you are unlucky enough, then this is the kind of thing that you get. And it’s really the only thing that would move at such low temperatures where you are running a bake and burn, say like a 150 °C...."
I don't know if this is the real deal and wouldn't argue against it, cause I can't prove him neither right nor wrong. However, I believe from what I've learned over the years on packaging of integrated circuits that two mechanisms are involved. One being the heavily doping in the silcon latice and the second coming from the assembly of the die within the package using die attach. All sorts of plastics, thermoplastic and thermoset, do suffer from stress after being (injection) molded or applied and cured. A thermal treatment afterwards can to some extend remove this stress in those and there are many examples where such thermal treatment process is being applied.
I don't know if there were ever studies to rule out what is actually the reason for the initial drift of voltage references in hermetically sealed packages and if they can be seperated from another and what is just stupid hypthesis, rumor or wisdom.
Nevertheless, the 150 °C treatment for 168 h Eric mentioned as well as the cycling that Cern performs on the LTZ1000 gives evidence that at least the stress due to the die attach is a major contributor. I do have build at least one LTZ reference, that was heat-treated using the cycling methode and what I observed on it was, that it settled within about two weeks. I know other forum members, not to mention any names here, that observed similar effects. However, I couldn't get enough people together to perform a double-blind experiment on that to give it more evidence.
So the answer to that die attach contribution would be: storing the Vref above or cycling around the glass transition temperature, whatever that is for the die attach used in the ADR. Chances are, it's in the range of 110 to 120 °C as for many epoxy materials in that field, but we simply don't know and Eric couldn't answer that either.
-branadic-