I decided to perform a bit of experimentation here, and soldered up some resistors to measure their drift.
One group was normally laid out, another had extra long leads, and another hung far enough off the board to sit in an alcohol bath while being soldered. There was also a "control" group which I had soldered ~6 hours earlier, so the "before" and "after" measurements for those don't involve effects from soldering. I also logged the time and ambient temperature at a few points along the way.
(See the attached screenshots of the setup and results)
The results were somewhat surprising -- soldering doesn't seem to make much of a difference at all for these cheap 1% metal film resistors (they either came from Tayda or eBay, I can't remember). The soldered resistors drifted about as much as the "control" resistors.
(Note: all measurements taken with a UNI-T UT61-E)
those cheap Chinese resistors drove me crazy when I used them to build an instrumentation amplifier (for a milliohm meter)
according to your tests it looks like their quality now is much better, but I still prefer only brand 1% 50ppm resistors when I need a fair precise and stable value (of course depending on your requirements and budget there are many other solutions: PGA, INA, precision resistor network, 0.1% and better resistors, etc.)
so far I've tested vishay, yageo and royalohm who provide a datasheet where you can know about maximum voltage etc.
I paid just 1-2 euros for 100 pieces of the same value and since you do not need so many different values for differential/instrumentation amplifiers that's a good bargain
IMHO it just doesn't make sense to buy unknown quality components and then spend a lot of time wondering why your circuit doesn't behave as expected (that's not your case, but a generic advice
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