Slot holes around LTZ1000(A)

[Flinstone]

Hello Voltnuts,

                      it is more or less a rhetoric question (for me) to all of you using slot holes around the LTZ1000A ... ?
Can anyone provide the rationale why board designers are copying these VooDoo holes - do people have scientific facts why these should be in place ... ?
Why would one avoid these slot holes using in particular in conjunction with the LTZ1000A ... ? Slot-holes can be effective but are they useful in conjunction with LTZ1000A ... ?

Best regards
Fred Flinstone


Yaba Daba Doo !

Fred Flinstone
55 Cobblestone Road
Bedrock

[Synthtech]

Would it be to reduce mechanical stress due to any board flexing?

[branadic]

I once investigated LM399 with short and long leads with and without slots.

-branadic-

[TiN]

They look cooler and lure new baby volt-nuts better than usual boring looking board .

I don't do slots. 

[Andreas]

Hello,

My opinion
Slots or not is a question of the whole thermal concept.

You have always to pay attention wether a thermal measure increases or decreases thermal balance.
Keep in mind that 1K temperature difference between the KOVAR leads of 2 pins
of the LTZ and the copper on your PCB creates 40uV (6 ppm) EMF.

So the concept may be different wether you use a 1 Layer or a 4 Layer board.
In all cases you may want to have as much thermal isolation around the pins as possible and keep air drafts away.

My personal opinion (from branadics thermal measurements) is that slots which are very close to the pins
increase thermal gradient in the area between the pins,
so you will need more thermal isolation (at least with short leads) to compensate for it.

On the other side: slots at a certain distance (>1-2 cm from the center of the LTZ) help
to keep termal gradients from non constant heat sources away from the pins.

I have definitely reduced the PSRR error from around 150 ppb/V down to 50-60ppb/V
by introducing a slot between the LT1763 voltage regulator and the LTZ pins.
So slots introduced at the right point definitely increase stability and are no voodo.

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg875274/#msg875274

datron goofed up (no offence intended to Pickering)

no bashing please if you have no facts.
according to specs the Datron references are on par with Fluke 732B see summary here.
https://www.eevblog.com/forum/metrology/teardown-voltage-standards/msg902849/#msg902849

with best regards

Andreas

[SvanGool]

I searched again, but can't find it anymore:

I once saw a very old video (I think it was a Linear video on Youtube, but couldn't find on their channel anymore) where they scientifically tested/compared the "slot technology" thermal behavior with normal pcb-plates and concluded that it did make a difference. At least they convinced me with that video.

[Edwin G. Pettis]

According to the folks at Linear Tech, that video wasn't put up by them and that is probably why you can't find it.  Once again, from the source who brought you the LTZ1000/a, "they don't like slots because it creates TOO MUCH airflow on the board and close to the device."

There you have it, any slots on a LTZ board is not sanctioned by Linear Tech, they have found that slots cause more problems than they may solve.  The longest running continuous board design, the 3458A's reference module, does not and never has used slots.

[David Hess]

The slots decrease thermal conductivity and reduce strain on the package however as mentioned, increased air flow needs to be taken into account.

Strain on the package is more of a problem with surface mount parts and you will sometimes find precision operational amplifier with slots cut around them.  Analog ICs in plastic packages use a special encapsulation within the package to isolate the silicon IC from the package material.  For digital ICs this is irrelevant.

[splin]

The longest running continuous board design, the 3458A's reference module, does not and never has used slots.

Good point. The 3458A would never have sold for as long as it has if they'd designed the reference board with slots. 

[Andreas]

But why does Keysight then use slots on the (successor) 34470A reference???

[janekm]

But why does Keysight then use slots on the (successor) 34470A reference???

Your second picture shows the plastic baffle they are using to restrict airflow, so it seems they get the best of both worlds with this design.

[Andreas]

Hello,

You have not understood how to maximise profit:

They will not do a re-design of the existing board.
But they will now keep back the best reference boards
with the lowest noise + ageing rate for the new 34480A.

with best regards

Andreas

[SvanGool]

@Flinstone:

Thanks !

I saw these videos/pictures, while trying to dig up the video I meant, but those were not the ones. It was an old kind of "sixties style" digitalisation of a video=tape-recording, it showed a comparison of quite big pcb-panels that had segments having slots and others having no slots around multiple heated central points. I searched again but can't find it anymore.

@Edwin G. Pettis:
I appreciate you saying that the video was not from Linear, but do you (or your contacts at linear Tech) know then which video it was and where to find it?
You are probably right, that there is more to it then just thermal stress.

It might be that the video was not specific LTZ1000 related, but it was specifically for those slotted hole structures on a pcb and thermal gradients/stress. It also might be that it was only a part of a larger video, that is why I probably can't find it.

[David Hess]

You have not understood how to maximise profit:

They will not do a re-design of the existing board.
But they will now keep back the best reference boards
with the lowest noise + ageing rate for the new 34480A.

Tektronix did the same thing which was well documented.  Practically the only time they re-engineered part of an old design was if parts availability became a problem or significant flaws were found.

[Andreas]

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?

[David Hess]

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?

I am not sure what you mean but the puzzling thing to me is why Tektronix did not reused designs between different products.  For instance every 100MHz 76xx oscilloscope mainframe seems to have a different but similar horizontal and vertical amplifier assembly when they could have all shared.

[Andreas]

Hello,

what I mean is the re-use of the 3458A reference board which could have been used in the 34470A.
But of course it is a different form-factor. So probably there are space limits.

with best regards

Andreas

[3roomlab]

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

Code: [Select]

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?

[wraper]

Crappy thermal image done under angle. Does not tell much.

Have a look to the PCB below - it is not perfect, much headroom for circuit improvements - but at least the PCB designer understood his job ...

Yeah, mechanical stress and temperature drop over kovar leads to the maximum  .

[3roomlab]

excluding the older versions of simulation i have done before. for this post, i did FEMM in a curent/amperes method to see temperature gradients.

much more interesting than slots i think is conduction. or iso-thermal rings? a few months ago i think i simulated a scenario where a mini iso thermal pad can be used to try and equalize the pin temperature at LTZ base.
https://3.bp.blogspot.com/-svBNjYX9wvI/Wvq4VsaX5BI/AAAAAAAADAE/o8SBdL46xHUMQPBWDEN9MFENVUIaVyuoQCLcBGAs/s1600/031.gif
https://3.bp.blogspot.com/-JIeiXx3xyxk/Wvq4VISXhDI/AAAAAAAADAA/DpHRSbvTNOwW5aP5XguQwHjZSDGrmDyUQCLcBGAs/s1600/033.gif

in this round, i flipped 1 to the outer edge of the PCB.

no outer iso-ring, no slot, normal Cu inner iso-ring (per iso line estimate = 0.1C)
https://1.bp.blogspot.com/-iC7SY9ebk8U/WvqoXSJQOsI/AAAAAAAAC_g/OFLNs5uJGqE_1yycYYf4ym43W6bMq8ldACLcBGAs/s1600/038.gif

+slots, +beefy equiv outer iso rings, +1 large trace inside PCB
https://3.bp.blogspot.com/-zllZwYd9MZQ/WvqnHhIuM2I/AAAAAAAAC_M/zNeM0vPf3s8ZC2_Qii5qr9xJ9UZQC76EwCLcBGAs/s1600/036.gif

it simply also mean that it is possible to use conduction to eliminate temperature gradients in passive areas w/o a heat source (see the top left of this PCB)
https://4.bp.blogspot.com/-R1vCAhz7vIo/Wvqu3WGIeoI/AAAAAAAAC_w/LMA6UtYl73wF94QrLwfVScZ4M_Rsw1oigCLcBGAs/s1600/039.gif

while a small ring outside a LTZ has some effect in trying to normalize temperature, the real temperature gradient killer appears to be a beefy outer ring.
and since the simulation is done in a VA manner, it is like putting extra capacitor to reduce AC ripples

slotting for reduction in thermal RC or conduction increase in thermal RC, i guess we can call it thermal guarding? just like op amp input guarding?

everything said above is now "old", but still might be informative
because i tried to do a "calibration" on FEMM. ie : apply 1W on a 10C/W material, but it doesnt result in 10C rise unless parameters are changed. this problem appears in both current mode and heat mode. weird isnt it?

ie : either you re-scale your materials or you rescale the input watts/heat. while reverse theta works in current mode, the air doesnt seem to behave normally, but heat simulation mode, the air appears to be normal.

but still after numerous simulation, the "trick" remains the same, air isolation slot enhances thermal resistance only if the path/Cu/bottleneck is small enough.

[floobydust]

The FEMM work is very interesting.
Can I back up a step before going to the use of copper strips as heat dams and an equalizer.

We know slots lessen conduction heat loss from the ref IC pads through the PCB. This would lower heater demand, and another slot can prevent the heater transistor from warming up other parts on the board. It also can make a layout more compact instead of all that real estate of FR-4 around the LTZ to deal with the gradient.

Second, slots isolate any PCB flex/bending moment from applying strain to the ref. IC through its leads. Ideally the slots cover both X, Y planes, leading us to voodoo patterns and crop circles. Voodoo slots may carry over from a designer's experience with parts shifting after any flex, during assembly/manufacturing.


The 34470a ref pcb I see slots around the LTZ pads to lower heat loss (except at the spokes); the LTZ is on a thermal island. But there's no bottom-side cover, so fan airflow cooling the bottom, leading to extra heat loss there.

The 3458a ref pcb has a cover on both sides of the LTZ, so minimum convection heat loss but no slots so extra (conduction) heat loss there.


Note the 34470a ref pcb extra pads on pins 3, 7. I think if these were brought out to a big split circle under the IC, it would be better, with no slit in the copper, to keep these pins isothermal.

You seem to have two LTZ pins with greater heat compared to the other six?

[3roomlab]

The 3458a ref pcb has a cover on both sides of the LTZ, so minimum convection heat loss but no slots so extra (conduction) heat loss there.
You seem to have two LTZ pins with greater heat compared to the other six?

i think LTZ pins are never equal in heat output, so i purposely try to show that.

[Andreas]

Hello,

there is one pin with 3 bond-wires, is that also in your model?

with best regards

Andreas

[floobydust]

The FEMM simulation seems to be showing almost the same temperature (gradient) a fixed distance away- whether FR-4 or air. I don't think that's correct if air has 1/10x the heat transfer compared to FR-4. What numbers are you using.

Thermal Conductivity (Tc) of Cu  ~ 355 W/m·K
Thermal Conductivity (Tc) of FR4 ~ 0.25 W/m·K
Thermal Conductivity (Tc) of Solder Mask ~ 0.21 W/m·K
Thermal Conductivity (Tc) of Air ~ 0.0275 W/m·K

https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html


I see three bonding wires on heater pin 1 (?) It may be a tiny bit more heat transfer but the glass and Kovar I'd think would swamp that.

[gamalot]

I'm learning English, so I tried to translate it.