Hi recently I tore down this expensive four thousand dollar. Keithley DMM 75 10 7 1/2 digit precision Benchtop multimeter one of the best ones on the market. And normally inside you find precision thin film resistant. it works that look like this and they're ordinarily not that very exciting because as you can see, they're covered with a blue or some other protective film on the surround, over the ceramic, a substrate, and the resistive material themselves so you can't actually see the magic that's going on in there.

But it just so happens that this meter has two other thin film precision resistor networks in there that don't have the protective coating on them and we can see pretty much precisely what's going on in here. So I find this really fascinating and I thought we'd just take a quick look at how these precision resistor networks are manufactured and how they work. Now it wasn't true when I said this doesn't have a protective film. In fact, this has an awesome protective film.

As you can see, it's a glass sheet actually on the front. This is a letter what allows us to get in here and see all of the resistive element detail in here, which is fantastic. Now why they've used a glass protective sheet like this instead of your more traditional blue, epoxy, or whatever kind of film material art we saw before on the other ones. Not entirely sure there may actually be some purpose behind it, but of course it's very important to actually protect these fin film resistor networks from moisture.

so this glass is hermetically sealed. You can see the brown like a silicon type glue around the outside. It is complete that would be forming a nice her medic seal inside. keep out the moisture sand.

The moisture doesn't affect the resistive elements because that would defeat the entire purpose of a precision resistor network. Now it looks like these parts here are actually manufactured by Fluke as you can see all they were manufactured for fluke. But considering that Fluke have a long history of making precision resistors themselves, it wouldn't surprise me if they are manufactured by Fluke specifically. And it's not surprising to find it in a Keithley instrument because the data her group own Keithley Fluke Tektronix you name it.

The way these are typically manufactured is as you can see on a ceramic base. that's that white back in material you can see there. Now they do this for temperature stability reasons. Ceramics are a very highly temperature stable base because you don't want any thermal expansion because that can change the properties.

And then with thin film resistor networks, what they do is they add a layer of exactly what it says, a thin film of resistive material. In this case, it could be one of several different types. Nickel Chromium is the most stable and the most pretty much the most popular for this sort of application. There's silicon Chromium, there's tantalum, Nickel, Tantalum Nitride, and there's each.

manufacturers of these. Precision Networks is going to have their own little special formula and their own name for their you know, trademark material or whatever it is, But you can probably bet your bottom dollar it's some sort of Nickel Chromium. Nichrome, as it's typically known, is what's probably going to be used here. maybe with a little some added extra secret sauce in there.

And as you can see on this graph here from Vishay, the different types of materials going to have different properties in terms of their load stability, their aging, and of course, there are resistive value itself. ie. they're ohms per square area. so Nichrome is pretty much sort of.

you know, the best material available, maybe with some added extra like a V shake or there one time locks go figure. And of course, there's a lot more to this with precision and resistant networks like this. Because their networks, it's all about the resistor divider ratio, not about the precise value itself in most cases. So because these are all the same film, the same material, they're on the same back in their in the same environment.

They're going to be basically the same temperature, thermal expansion, everything else gonna have very similar properties between them. The absolute values can actually drift, but as long as that ratio remains the same, that's what's critical with these tights are resistant networks. And in the case of this particular Keithley instrument, you know we're talking about a one-year maximum stability. much maximum worst-case spec of like 16 ppm on the best range.

So you know your resistor network has to be better than that. And it's important to briefly explain the difference between thick film and thin film resistors in terms of not only a regular SMD resistors you've seen, but also these are precision resistor networks. In fact, they're manufacturing them. Basically, they're exactly the same way.

except these resistor networks. they're just more stable, more precise, just real, much higher quality than your standard. SM DS Now, apart from the actual thickness of the materials, use the conductive and the resistive materials themselves. Yes, the thick film ones are thick, generally thicker than 10 microns or so, and the thin films, as their name suggests, are much thinner.

They're less than 1 micron thick typically, or thin, however you want to call it, but that's not really the main difference. The main difference is that the thick film ones use an additive process where they screen the materials on rather than a thin film, which is entirely different, which is a subtractive process which is very similar to how they make out silicon chips of course, with their photographic mass and how they make art. PCBs Now, in the case of thin film resistors, that resistive film of that material that we've talked about they produce that look like using a traditional evaporative deposition process, for example, to produce, you know, a really consistent layer, and then that is that photo exposed to produce the required patterns that we can see here on our precision resistor network to allow us to do the further laser trimming further on. And then of course they can do a similar thing with the conductive layers as well, which go to the outside pads.

So after they've laid down their resistive and conductive patterns like this on the ceramic substrate, they go into laser etched the exact value required and this is usually done in an active way. ie. they attach test jigs to the test pins down here and they actually measure the value as they laser edge. You can see some laser etched marks in here.

looking see a traditional L-shaped laser cut here. That's a very traditional method to do it. There's different reasons that they do different shapes as Ill patterns, serpentine shapes and curves and all. Lots of things.

You'll see a hook pattern in here, one in there, one in there. You'll see some laser-cut marks in there and also all these trim elements which I'll talk about in a minute and your different fill materials are going to have different sheet resistances. for example, a Nichrome which I think this one most likely is some form of Nichrome type alloy. It's going to have about fifty to maybe up to 200 ohms per square.

and note that's per square. that's not per square centimeter or per square millimeter. it's per square. So if this was a square like this for example, which it almost is, it's kind of a rectangle, then it would have say 50 ohms per square.

That's regardless of the size of the square itself. So to get an increased resistance, you have to actually create tracers. So little squares like this add up to form to increase your resistance like that as you snake through the tracers. So this resistant network, especially these ones here are going to be quite low resistance.

Let's assume it's like 50 ohms per square, shall we? And then you can. you know basically drawing squares like this and you can see that because we've got this big laser cut mark through here. it separates that and then you've basically got squares like this, which you add up. Please excuse the crudity of my drawing here.

But basically you'd expect this if it's 50 ohms per square to be like a couple of hundred ohms. And well, let's see what happens when we measure that. So let's measure that and see what we get. Bingo! 584 Ohms, That's in the ballpark.

So with this resistor network here, what we've basically got is a resistor from here. Buh-buh-buh-buh Hand drawn my resistor going down to here because it snakes its way up through all these elements down here and laser trim as I'll explain in a second back to here. Then we've got another resistor element. What? Ah man, drawing these on the computer is hopeless.

Anyway, you can see what I'm doing. A resistor boom down there. and then this one has got to have another resistor through here like this because you can see this one is going to be much higher value though, cuz look at all those squares you've got to add up all the way through here. all the way through these elements, Boom and way back down to here.

Now this is where these are all joined up. but these ones over here are isolated. So we've basically got one from here over here like this. going down to this pin down here.

Trust me, it does go down there. and then we've got this one going up here. and bingo down to that pin down there. But you do have to be a bit careful analyzing these because as I said, there are two layers in here.

There's going to be the resistive layer, which is this sort of gray, you know, steel coloured one that we can see here and then this other color I Don't know what you'd call that. These are the conductive layer. You know, extremely low resistance. They're not designed to have a, you know, an ohms per square figure.

It's not a resistive material, it's conductive material. hence why they're going down to the solder pads down there. But you can add conductive layers on top of your resistive layers as well. And that's maybe what they're doing here.

With these different colors in there, you can see that one big block there, that's going to block in there. So yeah, you can't always be 100% sure when you see those different colors in there, that it's going to be. you know, an addition of the squares. Now you might be wondering why some of these laser marks look different.

I Mean these l-shaped ones look really very clean. Look, there's no burn marks around there, but sort of. You know you go over to these ones here and look it looks like it's You know it's almost been burned in and that's not surprising. You'll notice that the the burnt looking ones are over the ones that are a different color.

so there looks like probably a conductive layer, so they probably have to use I Don't know I'm guessing here a higher power laser to do that to actually cut through them. so that's probably what we're seeing there. I Don't know what this circle here is I got no idea what's going on there. mm-hmm and then to set your resistive values, you can see that these ones here in particular.

this is like the most interesting one on this particular arrangement. Here you'll notice that yet said before. thin traces going up here. these are what's known as the course part of the network and that sets the course of value.

And then when you get in here and you get say this one's actually called a ladder that's going there and try and get it you'll notice that this value goes up. This trace goes up here. Let's just assume it goes down like this. snakes its way back down here, right? So it comes down here like this and then it goes across like that.

so it's almost like no point going up here like this. Up this ladder. this is a ladder network which it's called in one of these things because it looks like a ladder and this is a medium trim value. So they put that in there.

but it's like having all little resistances all stacked up in parallel like that and they've done the same thing here. Of course, when this trace comes in here like this and goes along here, then you've also got these medium trim values in here. and then you can go in and laser cut some of these values and you can see that that's actually happened here. Actually, they've gone in and laser cut straight through the center are there.

This one's actually really interesting. See where this trace comes through like this and it has to Now go up here and around like that. So they've done some fine trimming, but this one has to go all the way down here. That one's a bit far so that one's got to go all the way around like that.

So they've trimmed in those values like that. so that's these are the lasers are what does the fine trimming in the end and you can't actually subtract resistance from here. Or when you do the laser cuts or you can do is add add resistances in. So when they design these things, they're going to be smaller resistance values than what they need and then they trim them up to the value that's actually required.

They've done the same thing over here. Look, this is going to go up here like this. just through the resistive material. Then they've decided, oh, let's make a small little cut there so it goes down here.

but this all this part in here adds extra resistance as well. It trims it to a certain value so you can put laser anywhere you like and really get ultra fine adjustment. Likewise, they've cut through this like here right up here so this has to come up here and then up. can't can't go up there, but that's all part of the trim network.

otherwise they would have cut the whole thing out. So it's got to go up here now and then sneak around like this and then all the way back down before it can get to this ladder. Network and you can see that they've only cut sort of like 2/3 up the way of the ladder networking. See that laser mark right down there? so it's gonna now go up here.

Bingo! But all of these extra ladder positions on top they will all add up to trim the values. So think of all those as all resistors in series. parallel all those silly little resistor problems you used to get in Electronics 101 and then this other resistor network we've got over here, which is physically wider and bigger. This is the input while the high voltage input divider resistor and it's essentially it's got five pins, essentially only using three.

So there's only a resistor between here and here like this and here, which then snakes all the way up here or back and forth, back and forth. Goes all the way through this all the way through this, all the way through this all the way through this bla bla bla bla bla all the way through here like this. Crazy until it comes out this pin down here. So that is one huge resistor that's typically going to be a, you know, a nine point nine Meg Nine Point nine Meg resistor or something like that and then a smaller value down the bottom.

So I tackled it. It taps off the high voltages down to a usable level. You can see we've got a whole mix of stuff on this particular one. We've got our ladders down in there, we've got our intermediate range trims, and our coarse trim there.

but most of this is all course trim because it just loops back and back and back. But we do have some elements in here that loop back and then we can just increase. sort of. like you know, more intermediate course kind of stuff like that.

and there's up the top here. We've got all sorts of jazz happening around here, a combination of some ladders plus some other stuff so weird and wonderful shapes. You can see this coming around this, sneaking back out. But effectively, it's just one huge big resistor like that.

and that's why that's the only way that you can get the huge values like you know, the 9/10 made you need for these sort of our resistor dividers. So let's try and measure this one. and this is the big serpentine one. Bingo! About Nine Point nine Meg That's what you'd expect.

and then if we check the other one here, we expect a very low value. Bingo about six odd. K So a very big resistor divider here, and the reason why they're actually going to use a mix of different geometries inside this thing, as we've seen the ladders and the serpentine traces and all sorts of other weird and wonderful treatment techniques, is that so it reduces the sensitivity of the final value to the amount of material that gets laser trimmed out. It just means that they can, you know, get progressively closer and finer towards the individual laser valued that they need in the end.

and they can do this on an individual resistor basis and a ratio basis as well based on these resistor networks. So in the end, all this is going to be a trade-off between size and the resistance value you want and the stability. Want the ratio stability you want between the network? all that sort of thing. And that's why they have to use these quite large geometries.

You just can't get these on a small scale. Can't get the a precision adjustment and laser trimming required on like little tiny things. That's why you always get these huge things even inside, like multimeters where you spec size to be an issue. Not, they still have to laser trim these networks and they're physically large single inline packages.

So there you go. I Hope you found that interesting. This was a lot longer than I expected it to be I Tend to waffle on a bit, but that's how these thin-film precision resistor networks work. And as I said before, basically exactly the same way that other a your regular thin film resistors work your standard you know, o 805 surface mount parts that you used to and also if you look in the center there that dodgy-looking one standing up off the board that's actually a precision vishay resistor.

It's got to be manufactured in a very similar way. Here's a photo of Um in internal shot of one of those, and once again you can see the serpentine traces in this thing and the course values. and they're going to have laser trimming in this as well. It looks a fair bit different to what we had before, but it's essentially exactly the same thing happening there as what we got on our theme filled ceramic ones.

So if you liked that video, please give it a big thumbs up. And if you want to discuss it, please leave. YouTube Comments: I try to I read them all and reply to them where possible and the Eevee blog forum also linked in down below catch you next time you.