Unless you use a package that has true hermetic sealing (borosilicate glass and Kovar steel, with the cover silver-soldered on), I'm not certain that you would be able to keep the humidity out. Water vapor has a strange way of finding a path through most materials-- including most epoxies and most silicone compounds. Like this (for example):
http://www.sinclairmfg.com/catalog/hybrid.html
The problem with the Kovar/Pyrex hermetic packages is that the Kovar leads make *great* thermocouples when in contact with copper-- this is *not* what we want-- (is there another way that is effective)?
I'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this? How do we make a high-performance hermetic package using materials a hobbyist can obtain? We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes. (The temperature we will control actively).
I'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this? How do we make a high-performance hermetic package using materials a hobbyist can obtain? We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes. (The temperature we will control actively).
QuoteI'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this? How do we make a high-performance hermetic package using materials a hobbyist can obtain? We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes. (The temperature we will control actively).
Well, there are a few possible ways. You could use second-hand double oven oxco's in the bay and reuse the package, buy hermetic packages from companies like Schott:
http://www.schott.com/epackaging/german/overview/products/gtms/index.html
or construct your own hermetic package using rf-metal cases, copper or brass sheet metal and feedthrough capacitors for the electrical contacts from inside to outside.
Avoid sealing materials such as silicon, this is not hermetic at all, no epoxy glue nor plastic. To reach hermetical sealing prefer soldering, welding and ceramic or glas as they are diffusion resistant, exactly what you want. I again would also recommand to use LTCC, the substrate you will find if you open for example an oscillator. This could be cheaper compared to your mentioned Rogers 6035HTC.
Dry the package and fill it with nitrogen before closing it.
The most sensitive point on this circuit is the base connection to the temperature sensor transistor on pin-6 of the LTZ1000. You could create a guard-ring (which is just like all other traces in your PC design package) that surrounds all of the connections to this node (on all layers of the PC board). The guard would be connected to the collector of that same transistor (pin-8 of the LTZ1000). The rest of the circuitry will work just fine without any guarding.
If you use higher quality PC material (the material I already suggested in a previous post, or the ceramic material already mentioned by others), then this guard ring could be left out-- *however* do *NOT* use water-soluble flux for soldering-- this leaves a residue behind that is very difficult to remove-- and it is conductive enough to cause problems with sensitive circuits such as these.
...LT1881, it seems that this part doesn't have sufficient common-mode [Vcm] down to ground on the Inputs...bias the LTZ1000 *must* work down to 0V on the inputs.
The LT1051A has excellent specs for this purpose-- and you can probably find original units in the ceramic package...
I'm guessing Minco-Tech will charge you ~$10-$20 for each one in small quantities. ...
...I would use an LT1051A, because it has the best long-term (time) drift and temperature drift of all of the possibilities...
If your not going for the highest accuracy possible, then, what's the point?
So far I have pre choosen ROSE housing like
http://www.reichelt.de/Rose-Aluform-Gehaeuse/ROSE-04-08-12-08/3/index.html?;ACTION=3;LA=5;ARTICLE=102470;GROUPID=5200;artnr=ROSE+04.08+12+08
It is not exactly hermetic housing but at least IP66 sealed.
But I also would like to see your solution, when we meet.
Dear countyman, you better use TEKO 371 / 372 / 373 / 374 / 392 / 393 / 394 / 1680 / 16120 or 16160 instead of your IP66 case.
I clean my pcbs with IPA in a bag in a heated ultrasonic cleaner, that works fine for me.
And that rinse should be a spray rinse not a dip rinse.
There are bigger rf cases available...
You got me right, that is exactly the way I clean my circuits...
After cleaning I dry the board with compressed air, but you can also put it in an oven for a while.
I don't know a manufactor for LTCC or the mentioned Rogers 6035HTC in germany, but maybe the use of RO4003C is an alternative choice? There is a german guy who manufactures them for a fair budget www.pcb-devboards.de also in 1.6mm. I expect this material has lower moisture absorption compared to FR4 and the Tg is much better Tg>280°C DSC (IPC-TM-650 2.4.24).
Even if it is said to be not nessecary I would nevertheless provide the slots in the substrate. Do what ever you can to get minimal stress to the leads. You will find statements that quote that this package is not sensitive to stress, but I wouldn't count on that if you want the last ppm.
And for my soldered fully populated one, I still feel uncomfortabe.
About the slots in the board, I still do not know, how to do it in Diptrace
You can do the cleaning procedure twice, but up to know I had absolute no problems. As said I heat the ultrasonic bath too.
Just draw the slots in the dimension layer and make them as suggested by the pcb manufactor (normally >2mm for the milling tool), that's it.
https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg219527/#msg219527
But why not use an "off the shelf" program such as Eagle Light or KiCad?
I can tell for sure as my wife was working there, spending a lot of time explaining customers (like me) how to do stuff with EAGLE, as that was the CAD system she was responsible for
@Quarks: There is a PCB distribution house right in your town, chances are good that you can ask them for Diptrace hints.
It's not totally "easy" to make a voltage transfer standard that holds 2ppm per year-- but is can be done by the average engineer using average (and well known) approaches to the problem. But-- (short of going out and buying a million-dollar Josephson Junction Array)-- to get below 2ppm per year takes a lot of study, hard work, patience, and help from your friends.
I already own a Fluke 732B, which (according to it's published specifications) will hold 2ppm (or less) per year. So, I want something better. I want *less* than 1ppm per year, and my goal is (somehow) to achieve 0.1ppm/year-- a lofty, but worthwhile goal (for me at least). For most people, the Fluke 732B is "Good Enough"-- but not for me.