Hi Andreas (reply 423)
All resistors have many interacting mechanical forces on them, they are complex in nature and like most things, decreasing the effects of one force may amplify another. While the mechanical forces in PWW and film/foil resistors share many common effects, there are differences because of the different types of construction as well.
PWW have hoop stress from winding the wire onto a bobbin, film/foil do not but film/foil have a similar type of stress from being attached to a ceramic substrate. Encapsulation stress is present in both types for different reasons, both have partial remedies. Film/foil are sensitive to moisture, partly due to absorption through the encapsulation and around the leads, mine have no significant affects. Film/foil are particularly sensitive because the resistive element is bare and the circuit paths are very small and close together allowing water molecules to bridge them easily, baking is a temporary fix. Foil/film resistors have an applied voltage coefficient, PWWs do not, again this is a artifact of film/foil construction. Both types of resistors use very similar alloys, the basic aging rates are normally nearly identical. Drift rates are complicated, mainly the result of residual mechanical stresses which are very difficult to completely remove. This involves additional 'enhanced' (as Vishay likes to call it) processing to further reduce those stresses, i.e. costs more. Oil filled hermetic sealing tends to be more beneficial to film/foil as it removes more of the external sensitivities that film/foil has to their environment such as moisture and barometric pressure which PWW resistors have insignificant sensitivity to. One other important characteristic, film/foil are noisier than PWW resistors, particularly mine.
"I only asked because on other resistors up to now I have seen protrusions from baking the epoxy.
On your resistors I see shrinking epoxy like on some capacitors." - Andreas
I am not sure what is causing the protrusions you are seeing on the other resistors, it may be caused by a filler they are using in the epoxy. The epoxy end 'seal' on my resistors is purely mechanical in nature, to hold the resistor inside the shell and support the lead assembly, the epoxy may not form a complete seal around the metal lead, this is of no concern as it does not affect the resistor's electrical characteristics.
Aging tends to be of a nonlinear nature, as exhibited in the better resistors, aging (which is a form of drift) tends to decrease with increasing time reaching a fairly level, stable rate after some period of time. Just how much the initial rate is and the eventual stable rate becomes depends on the resistor technology and quality of construction. As I've pointed out before, the more 'perfect' a resistor you want, the more expensive it is going to be and that cost tends to rise exponentially. You are also going to rapidly run into the always present law of diminishing returns, that last little bit of improvement becomes so expensive, it isn't worth the trouble.
Speaking as an design engineer, respectfully:
Fluke, HP (Agilent, Keysight), ect. all recognise this, that is why their voltage references perform to a certain level for that cost because it is a happy balance. It is much cheaper to have those references checked against a primary standard periodically than to try and put a primary standard inside their instruments. Would anyone be willing to pay say five or ten times more for a 3458A with a primary standard inside? I really doubt it, the performance of the LTZ1000/A with good resistors around it provides the necessary performance at a reasonable price. The Fluke voltage standard ups the anti by using multiple LTZs to improve the performance but those aren't cheap and no one is going to make a single LTZ perform as well as that Fluke standard no matter how good the resistors are.
Those LTZ resistor sensitivity numbers given in the data sheets/apps are only approximate as stated clearly there, every LTZ is a bit different. If you want to know about this stuff, just ask Linear Tech, the people who designed the chip. If there were any relatively 'easy' tweaks that could be done to improve the performance, don't you think Linear Tech or Fluke would have already discovered them after all this time?