Author Topic: Slot holes around LTZ1000(A)  (Read 9462 times)

0 Members and 1 Guest are viewing this topic.

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6924
  • Country: ca
Re: Slot holes around LTZ1000(A)
« Reply #25 on: May 16, 2018, 03:34:28 am »
That pic came from a search engine caching amosbbs, it has pin numbers so I repost it here. Thanks for the translation, it seems fine.
OT, but I see six rings of heater resistors, two are used normally and four go to special factory test pads. A seventh thick ring might be for thermal equalizing.
Looks like two parallel transistors for each. Such lovely art.

Earlier discussions on LTZ thread page 74 here saying... slots are useless, the HP3458a does not use them and is proven. LTZ airflow and vibration can be worse with slots The 34470a ref is not metrology-grade so take it lightly. etc. etc. This topic has probably been talked about before.

I think slots have merit but maybe not. I'm going to try FEMM and see what I can learn modeling slots/pads on a pcb.
« Last Edit: May 16, 2018, 03:36:45 am by floobydust »
 

Offline z01z

  • Regular Contributor
  • *
  • Posts: 151
Re: Slot holes around LTZ1000(A)
« Reply #26 on: May 16, 2018, 08:58:28 am »
however 1 thing i dont understand, where is the 120ohm in this Vref?
There's a 100 Ohms resistor (R95?) beside the 1k one (R5), that could be it.

Regarding the model, shouldn't LTZcap connected to temp C?
« Last Edit: May 16, 2018, 09:12:46 am by z01z »
 
The following users thanked this post: 3roomlab

Offline 3roomlab

  • Frequent Contributor
  • **
  • Posts: 824
  • Country: 00
Re: Slot holes around LTZ1000(A)
« Reply #27 on: May 16, 2018, 10:07:06 am »
however 1 thing i dont understand, where is the 120ohm in this Vref?
There's a 100 Ohms resistor (R95?) beside the 1k one (R5), that could be it.

Regarding the model, shouldn't LTZcap connected to temp C?

oh yes indeed
 

Offline Magnificent Bastard

  • Regular Contributor
  • *
  • Posts: 139
  • Country: aq
Re: Slot holes around LTZ1000(A)
« Reply #28 on: May 16, 2018, 02:35:34 pm »
The slots attempt to accomplish 2 things:
  • Reduce heater power required (important on battery, less important with the 'A' version).
  • Keep the reference's heat out of the rest of the circuitry.
It's far more important to:
  • Keep the LTZ1000's leads at the same temperature.
  • Exclude air drafts from the LTZ1000's leads.
G10/FR4 board substrate material has a low thermal conductance, which is why the Wavetek 7001 has a ring but no slots.  The signal leads traces are on the next layer down with very little epoxy-glass material between the leads signal traces and the ring.  This helps maintain the same temperature.  In either case, an air-draft excluder (which can be a plastic pipe cap or even a copper pipe cap) is always necessary.
« Last Edit: May 16, 2018, 05:19:06 pm by Magnificent Bastard »
 
The following users thanked this post: TiN

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6924
  • Country: ca
Re: Slot holes around LTZ1000(A)
« Reply #29 on: May 17, 2018, 02:39:07 am »
As an amateur I tried FEMM for some slot temperature modelling. It's limited as 2D software, and exact models would be tedious to build.

I encountered many assumptions and errors in the modelling, so it's wrong- although shows some tendencies.

1. Each pad+pcb trace act as a tiny heatsink to the LTZ pin- which does not have infinite heat available at each lead. You'd have to 3D model it with the die+attach+bonding wire+glass+Kovar. Here I crudely set each pad equal to 35C but expect pad temps to drop, esp. with thick pcb traces. People feel the IC's leads have different temperatures, not all equal.

2. If in FEMM you select a material i.e. copper, it is solid through the plane thickness (i.e. 1.6mm PCB) which is not realistic for PCB traces. I tried the "smearing" approach where you average the conductivity based on volume (of Cu and FR4). In the end it made no big difference as FR-4 is way more of an insulator.

3. The slot's air temperature is awkward if you have no forced airflow as there is convection and radiation mechanisms at work.

4.  For pads, solid copper verses solder and Kovar and copper not a big difference as metals are pretty good conductors of heat. It's the FR-4 and air that really drive things.


My observations, that may be wrong:

The LTZ pcb center area and pads are pretty much isothermal, NOT including the thermal resistance of the die/attach/bonding wires/glass/leads, which may make effects much worse. I'd have to model more and see if the imbalance is more of an issue.

Slots:
- good for preventing mechanical strain on the LTZ due to PCB flex. If you have the LTZ located on a cantilever at one end of the ref PCB, it's not important.

- not ideal as you have to have FR-4 & Cu "spokes" for the pcb trace runout which means the heat loss surrounding the IC pads is not symmetrical. A slot close to a pin means the pad runs hotter.

This all would be better if the lead's thermal model was accurate.
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6924
  • Country: ca
Re: Slot holes around LTZ1000(A)
« Reply #30 on: May 17, 2018, 07:16:38 pm »
The LTZ1000 thermal modelling I looked at on eevblog does not include the glass on the Kovar leads.
Example Schott 8250 borosilicate is 1.2W/m-K so a good insulator.

I expect the LTZ leads would be supplied heat through that high thermal resistance (TO-5 base-glass-lead), meaning PCB pad temperatures even more sensitive than models suggest, to slots, airflow and copper traces.

I've stopped working on this as others have done 3D modelling, compared to this limited 2D work which can be misleading.


edit: about slots on SMT packaged references from AN82 Understanding and Applying Voltage References:

Board Stress
Burn-in can help “relax” a stuffed board, but additional mechanical stress may be introduced when the board is mounted into the product. Stress has a directly measurable effect on reference output. If the stress changes over a period of time, it may manifest itself as unacceptable long-term drift. Circuit boards are not perfectly elastic, so bending forces may cause permanent deformation and a permanent step-change in reference output  voltage.
Devices in metal (TO-5 and TO-46) packages are largely immune to board stress, owing to the rigidity of the package and the flexibility of the leads. Plastic and surface mount packages are another matter.
Board stress effects are easily observed by monitoring the output of a reference while applying a bending force to the board. A controlled experiment was performed to measure  the effect of board stress on an LT1460CS8-2.5 surface  mount  reference; see pic
« Last Edit: May 17, 2018, 08:17:12 pm by floobydust »
 
The following users thanked this post: exe

Offline 3roomlab

  • Frequent Contributor
  • **
  • Posts: 824
  • Country: 00
shape of Slot holes around heat sources
« Reply #31 on: June 01, 2018, 10:53:38 pm »
there was a secondary question nagging in the back of my mind for some time regarding the effect of the shape of slots. i wasnt specifically sure how to simulate this problem/question, but then i came up with this idea (see pic). 1 of the questions which i sort of answered myself using simulations is that, i understood small widths and long thermal paths help to isolate the inside from the outside. but since PCB is not infinite size, will thermal isolation be different if we squeeze the size into a square? also for the sake of fitting square things on a square PCB.

*updated, a few more versions is done for this simulation, up to where the square and spiral leak are similar, but the square appear to require a slightly larger area, and have isolines spewing guts over a larger area. it would be pointless to expand the square further so that it performs "better" than the spiral but waste more PCB area to contain the heat. i guess that somewhat satisfies my curiosity about shapes :D if you use a rule to compare the sizes on screen, the spiral advantage is just about missing the square corner bits. now for reality, who would sit a VREF in the middle of a spiral? but in any case, this spiral/square thing is quite a surprise and fun to simulate.
« Last Edit: June 02, 2018, 08:39:24 am by 3roomlab »
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6924
  • Country: ca
Re: Slot holes around LTZ1000(A)
« Reply #32 on: June 05, 2018, 04:08:09 am »
I think you need symmetry around the LTZ pads to ensure equal heat loss at each pad. Equal area of FR-4, copper, air gaps etc. as best is possible.

Using slots, notice some pads have lots of FR-4 surrounding them, others have little FR-4 and more insulated because they are close to air (slots). So some pads would be cooler or hotter than others which we do not want.

I'd think the copper path is dominant for heat transfer but FEMM simulations aren't really showing a difference in pad temperature due to a PCB trace cooling the pad.
A PCB trace is thin copper foil on FR-4 laminate, I was not sure what numbers to use 'volume weighted, effective thermal conductivity, in the thermal modeling of a printed circuit board' numbers in various papers are all over the place.

I see mistakes in LTZ thermal simulations, i.e missing the glass/Kovar interface, so the heat output from a LTZ can lead is less than we are assuming (infinite), and plotting flux vs isotherms, no convection or radiation losses, etc.

So it's hard to tell how much upset slots can cause to the pad temperatures.
 
The following users thanked this post: TiN, Mickle T.


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf