Practically the only time they re-engineered part of an old design was if parts availability became a problem or significant flaws were found.
Ups why did they not simply use the old existing boards where they have thousands in their ageing boxes?
jus a rough run down of costs
3458a qty 34470a qty
$60.00 $20.00 Z foil 3 $30.00 $10.00 Z1206 3
$1.20 $0.20 Ra 6 $3.50 $0.50 R0603 7
$6.00 $2.00 Rb 3 $0.30 $0.10 E1/2/3 3
$0.75 $0.25 C 3 $0.84 $0.14 C0603 6
$0.14 $0.07 D 2 $0.30 $0.30 CR1 1
$0.14 $0.14 Q 1 $0.03 $0.03 TO92 1
$1.00 $0.50 5pin 2 $0.50 $0.50 8pin 1
$40.00 $40.00 LTZ 1 $40.00 $40.00 LTZ 1
$2.30 $2.30 DIP8 1 $2.10 $2.10 SOIC 1
$0.50 $0.50 PCB 1 $0.50 $0.50 PCB 1
$112.03 sum $78.07 sum
come to think of it, i am not sure if the SMD zfoil cost is $10, but does anybody think 34470a ref board is more expensive than 3458a?
assumption 1 - i think 3458a is a huge 12kg thermal mass. 732a is also 12kg, 5xxx calibrators are all over 20kg. the speed at which these things heat up (mitigated by its own mass?) might be a factor in long term stability. from andreas TC measurements, i suppose that precision resistors do not like fast warm up? am i right?
in addition, mashing up some numbers in excel, i find that slots improve thermal isolation only if the traces are small enough (<10mil = more than 5% bonus). so i would assume the way all major heat sources are spread out and isolated with slots make for a slower temperature ramp. plausible?
im not the slot police trying to protect its use or lecture anybody, but as a real noob, i think we need more reasons to call something voodoo
1oz 6mil wide 12mm long Cu trace ~ 5900C/W
1.6mm thick 5mm wide FR4 12mm long ~ 6200C/W
as we can see, when R(Cu trace) is high enough, R(FR4) becomes a significant player. so its there, not magic voodoo.
so they are/could-be trying to reduce the speed of temperature change to "save" the lousier non-LTZ precision parts from "thermal shock". shock might be too large a word, but isnt SMD parts more likely to go out of spec faster than THT parts under similar thermal cycling conditions?