Temperature Considerations for Circuit - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 15, 2020, 03:00:19 pm
content: Previously, I was building a transimpedance amplifier circuit to convert a current into a voltage. Thanks to the supportive community here, I've managed to create several prototypes of my circuit. Right now, it's being tested with the AD795, and so far, tests seem to be working favorably for the most part. Further testing is required, but I'd like to thank you guys so much for the help given. With that being said, we're thinking about environmental situations where the board will be used, and I'm a bit worried that my board will not handle it well. Since this is about a separate issue than functionality, I thought it would be best to ask about this in a separate post.In my test circuit, it's a simple TIA using a 100 Meg-ohm feedback resistor. You can see my test schematic below. The lack of a feedback capacitor is a concern, but the 100 Meg ohm resistor should have some parasitic capacitance, and it doesn't seem to be a problem for now. Circuit aside, the issue I'm concerned with is whether it will hold up in a vacuum in future tests. It may be premature to think of this now, but it's something that may be a concern.We are considering putting the TIA in a vacuum along with the other components it connects to (an STM). Looking at the worst case scenario, we were planning to run it in a vacuum at 150 degrees Celcius for 48 hours non-stop. According to common literature, FR4 is rated to have a glass transition temperature of 130 degrees Celcius. Going above that, it loses its solid form and becomes unstable. Similarly, the operating temperature of the AD795 is rated at 70 degrees Celcius. Going by these numbers, it seems that the circuit will not work at my stated scenario, and if that's the case, what would be good ways to ensure that the circuit will work? Should it be covered up or enclosed in something to withstand high temperatures, or should I switch out the FR4 PCB material for something that's more robust?On another note, when using a SMD oven to reflow solder components onto FR4 PCBs, temperatures go way above the glass transition temperature. In this scenario, is that okay because the circuit is not running and it's just 'baking'?

author: pwlps
author_rank: Frequent Contributor
timestamp: October 15, 2020, 11:50:58 pm
content: You may consider trying similar opamps specified for higher temperatures, e.g. OPA140 (JFET,125C) TLC4501M (CMOS, 150C).

author: Cerebus
author_rank: Super Contributor
timestamp: October 16, 2020, 12:47:07 am
content: Quote from: LoveLaika on October 15, 2020, 03:00:19 pmIt's not uncommon in high impedance TIAs to find that the parasitic capacitance from components in the feedback loop is already too much and you have to play little tricks to get around it.QuoteFirst off, switch to a specialist high temperature PCB substrate. FR-4 is just not going to cut it at temperatures above its glass transition temperature for sustained periods. Also I have some doubts about its outgassing properties in vacuum at those temperatures - you'll probably end up 'plating' everything in your vacuum setup with a thin layer of epoxy breakdown products. The obvious choice is a ceramic substrate, that may be overkill but at least you know it won't outgas. You're also going to have problems with operating points shifting, both from temperature itself but also from stresses set up by heating and cooling.As far as running parts at 150ºC, you're rather going to be on your own. Most commercially available parts are only qualified out to 125ºC. If you're lucky they will still operate at 150ºC, but you won't be able to rely on the datasheets at all, you'll have to characterise the part (running at that temperature) yourself. You'll also have to allow for the part self-heating in an environment where it can only rely on radiation and conduction to shed heat (no air, so no convection cooling). Thatmaypush the part temperature up significantly, it may not. (Your AD795 has ~30mW of dissipation assuming ±15V supplies, or 30 mJ/s. Push that out to 48 hours and it has over 5000J to get rid of somewhere, somehow.)It's not inconceivable that you'll be running at temperatures where creep, if not outright melting, in your solder is a problem. You might be better off with wire bonding on a proper hybrid ceramic circuit module. Analog Devices do make some of their parts regularly available as unencapsulated silicon chips, ideal for this type of hybrid use.Beware of JFET op amps, the gate leakage current of JFETs doubles approximately every 10ºC. So something specified at 25ºC will have a leakage current nearly 6000 times higher at 150ºC. For MOSFETs this still happens, but something like 100 times rather than 6000.Basically, all of physics is working against you here. If you can find a way to keep this down to a more conventional temperature range you'll have a much easier life. Then of course you'll have to deal with thermal EMF effects as you move from your 150ºC environment to a more balmy one - "swings and roundabouts" as they say. Oh, and did I mention thermal noise? OK, so it's "swings and roundabouts while juggling with club hammers". Good luck!Your only alternative to doing a lot of work yourself is to see if you can find a pre-packaged and qualified design from people who specialise in the aerospace world or scientific instrumentation in 'orrible environments like you're facing. Expect to pay handsomely for a suitably qualified pre-packaged solution.

author: bdunham7
author_rank: Super Contributor
timestamp: October 16, 2020, 12:50:59 am
content: Quote from: LoveLaika on October 15, 2020, 03:00:19 pmIs that operating at 150C or just baking the system prior to pumping out to high vacuum?

author: pwlps
author_rank: Frequent Contributor
timestamp: October 16, 2020, 01:46:41 am
content: Quote from: Cerebus on October 16, 2020, 12:47:07 amOr use a high temperature solder like one of these:https://www.indium.com/applications/high-temp-soldering/In a similar high temperature environment (200-250C) I have been using the Au-Sn eutectic alloy, it is expensive (still should be negligible compared to the cost of building an STM) but very strong even at 200C and can be used easily with a soldering iron (and appropriate flux).

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 16, 2020, 02:50:22 am
content: Cerberus, thanks for your reply. Thank you for your concerns about the parasitic capacitance. One trick I was introduced to was to put (in my case) two 50-meg ohm resistors in series, so I still get the resistance needed but I reduce the parasitic capacitance. If tests shows that I need to reduce the parasitic capacitance, then I'll look at these little tricks. The input load is a tungsten tip that goes straight into my TIA through a connector (no other circuit components, no datasheet since it's homemade). I'm envisioning it as an ideal current signal for now, though there's probably some capacitance that I don't know about.Thanks for your comments on the substrate. They're really enlightening. Looking at from another angle, is it possible to enclose my PCB circuit in some sort of ceramic enclosure (or any type of enclosure) and seal it up so that it will still function? Also, for pin headers, what insulation substrate would you recommend? The ones that I see on digikey that operate past 150 degrees Celcius have Polyamide (PA9T), Nylon 9T as their insulation, but will that hold up?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 16, 2020, 02:52:18 am
content: bdunham7, that's a good question. Sorry, I don't know the answer to that right now (this is someting that I'm just thinking about far off into the future), but if it's the latter where we bake the system prior to running it in a vacuum, would there be any problems?

author: bdunham7
author_rank: Super Contributor
timestamp: October 16, 2020, 04:03:20 am
content: I was just thinking that making the device able to operate at 150C is a lot more difficult that making so that it will survive 150C and then operate at normal temps. Your AD795 doesn't quite make it--it lists a maximum storage temperature of 125C and as for operating temperature, read page 11 of the datasheet.As Cerebus mentions above, you have to be pretty careful about what you put in a high vacuum system and baking may cause a huge headache. Plastic device cases, PCB resins, solder masks and so forth may give off enough crap to foul up the whole system. I've never tried to put anything like that in a vacuum system, only bare metal, teflon, ceramics and stuff like that. My first thought would be to only use metal or ceramic cased devices and make the circuit Manhattan-style without glue.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 16, 2020, 04:13:38 am
content: Quote from: LoveLaika on October 16, 2020, 02:52:18 amIt would make your life tremendously easier if it only needs to survive the bakeout process not achieve any particular performance at temperature. It just needs to work when things cool back down to a more sensible temperature. All electronics parts can survive brief excursions to 150 C as that happens during soldering. Reflow doesn't last 48 hours which isn't brief but it still opens up choices. Even parts not rated for storage at 150C may have tolerable problems such as a permanent shift in offset voltage that can be calibrated out. If circuit isn't powered during the bake-out it also means you won't have to worry about self heating to even higher temperatures from the operating power. That said, in a UHV system you probably need to worry about more than whether the parts survive the bakeout temperature, you need them to have low outgassing. So you probably want to avoid polymers as much as possible and use known good UHV compatible polymers like kapton and peek whenever possible.Honestly if you want to do STM with the sample at 150C you can probably get by with a high temperature circuit but I would seriously consider having a water cooled plate/box for the amplifier. Remember than in vacuum you don't have conductive or convective heat transport through air -- you can have hot and cold zones right next to each other if they aren't connected. You still have radiative heat transport but that can be dramatically reduced by having a heat shield and at 150C isn't that bad to begin with. You will have a signal cable going between them but that can be engineered for low thermal transport for instance by using a restive alloy. That would let you potentially increase the temperature range of the probe in the future. You can't use the cooling during bake-out but if the circuit doesn't actually need to operate during that time it's fairly easy.

author: Cerebus
author_rank: Super Contributor
timestamp: October 16, 2020, 02:28:32 pm
content: Quote from: LoveLaika on October 16, 2020, 02:50:22 amThere are hermetically sealed packaging technologies available, I've seen hybrids with a ceramic substrate and a glass or ceramic cover cemented in place. How those hold up in a vacuum I don't know as I've only ever seen ones that were designed for use at normal atmospheric temperatures where the hermetic sealing was for long term stability reasons (e.g. precision trimmed film/foil resistor networks). I don't know if they're strong enough to survive the forces from their own internal pressure if there's a vacuum outside.I'm a poor man to ask for advice on specific materials that are suited to a combination of high temperature and vacuum. I'm largely extrapolating from what I know (or strongly suspect) won't like those conditions. So no comment on the nylon, it'd likely be a great material for 'ordinary' extreme conditions like motors, your kind of extremes I just don't know. PTFE is always the go-to plastic for insulators and pass-throughs for difficult conditions, gets a lot of aerospace usage. Take a look atthe Wikipedia entry for Materials for use in vacuumfor some starting points.

author: Cerebus
author_rank: Super Contributor
timestamp: October 16, 2020, 02:38:22 pm
content: Quote from: ejeffrey on October 16, 2020, 04:13:38 amJust for a reference point, a bake-out at 125ºC for 12 hours at normal atmospheric pressure is an expected part of the life of packaged semiconductors that are in packaging at MSL2 levels or above (packages that are likely to be damaged by absorbed moisture when they are reflow soldered). This is, obviously, non-operating.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2020, 09:42:20 pm
content: You probably won't do this, but the old-fashioned RCA industrial Nuvistor tubes could be operated with a case temperature of 250 C, with some derating, but the standard maximum operating temperature was 150 C.http://electronbin.com/other/RCA/RCA_Nuvistors_NIT140.pdfJust like your vacuum apparatus, they are metal-ceramic construction. The Russian versions used flying wire leads instead of pins, and therefore you wouldn't have to use a plastic socket.

author: mzzj
author_rank: Super Contributor
timestamp: October 17, 2020, 08:00:24 am
content: 100M feedback resistor on a transimpedance amplifier and the op-amp input bias current is something like 10nA. 1 volt offset on the output?

author: Cerebus
author_rank: Super Contributor
timestamp: October 17, 2020, 03:08:16 pm
content: I think we're all assuming that the current circuit is just a "getting started" approximation and that trim and null settings will appear in the future once the OP has figured out whether he can deal with his environmental issues in principle before getting down to brass tacks.I'm not familiar with the precise charge levels that one needs to measure on an STM but instinct tells me that 100M is probably going to become 500M or 1G or more. STMs are dealing with rathersmall'capacitors' after all.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2020, 04:41:39 pm
content: It has to be at least the fifth thread on this topic.OP is replacing a commercial module which is no longer available. 100MΩ is the spec of the original and the spec also calls for bandwidth which would be proportionally harder to achieve with higher gain.Not sure if the original also worked at 150°C or if they are trying to "upgrade".

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 18, 2020, 03:48:05 am
content: This has nothing to do with the original run methods. We were looking into different runtime environmental conditions for the STM, so I'm specifically looking at what I have to consider as I move to a higher temperature range to operate. In this case, consider that the circuit, exposed to the environment, works as intended in room-temperature conditions. Putting it in a radically different environment to operate mixes things up, so that's what I'm trying to ask about.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 18, 2020, 03:52:00 am
content: For whatever it's worth, my test circuit as it is now is going through the stages of testing, and so far, testing is very promising. It's working well with an STM, all things considering. It seems fine with the current we're running it at.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 18, 2020, 04:52:49 am
content: While the 795 seems to work well, I also have an AD549 with lower bias current available to test.

author: KT88
author_rank: Frequent Contributor
timestamp: October 18, 2020, 01:22:14 pm
content: I won't be too much concerned about the high temperature in this application.When parts are specified, they have to meet the DS values reliably, as the DS is also a legal document - it relates to liability...Mainly two effects happen when a part is operated at temperatures beyond spec:Specs are not met anymore and the lifetime will be reduced. For lifetime one can do a crude estimation by halfing the lifetime every 10°C above spec - but be aware that this estimation is crude. It is also limited to a temperature where the package falls apart.The specs can/should be tested. If the part works - it works...Maybe testing a small lot of circuits at slightly higher temperature over a slightly longer period of time should give a good confidence.That would be for qualification/verification of the circuit.For the circuit you want to operate, you do only a short test up to the operating temperature in order to avoid any pre-wear...Another effect of high temperature is the softening of the epoxy of the package if a plastic package is used. Also this won't be much of an issue if there is no vibration and not too many thermal cycles are applied.I used Zinc based solder back in the days which was specified to operate up to 205°C. Was terrible to work with though...CheersAndreas