EEVblog #1347 - SMD Thermal Jumpers could be GAME CHANGING! - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: November 13, 2020, 09:31:25 pm
content: New SMD Thermal Jumper parts could radically change your PCB thermal designs!https://www.vishay.com/docs/60157/thjp.pdfhttps://atceramics.com/userFiles/uploads/pdfs/Q-BRIDGE.pdfThis is a follow-on from the original SMD Thermal Design video:

author: Elasia
author_rank: Frequent Contributor
timestamp: November 13, 2020, 10:14:06 pm
content: Yes please! I'm immediately going to use these.

author: nctnico
author_rank: Super Contributor
timestamp: November 13, 2020, 11:08:35 pm
content: That looks very interesting! I have quite a few designs with large isolated pads on top & bottom to create a heatsink areas. Maybe I'll give these a try in a new design.

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2020, 11:39:35 pm
content: I wonder how this compares to gluing an aluminum block on top. 60C is a impressive differential, I am surprised you can do that much with a lateral heat sink. I also wonder how this effects the thermal reflow profile, because the part may cool much more quickly. I would like to see failure watage also (smoke point, spot heating, internal boiling).

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 13, 2020, 11:53:39 pm
content: love the Strictly Ballroom reference

author: tszaboo
author_rank: Super Contributor
timestamp: November 14, 2020, 12:29:31 am
content: I've seen these around 6-7 years ago from a smaller company.But here are a few examples:https://www.avx.com/news/lambda-bridge-conductors/https://ims-resistors.com/products/therma-bridge/I've evaluated for some application, and concluded that it is 95% marketing wankery going on with these parts. If you look at the test images, they are using single sided PCB and there is a huge gap between the heated copper and the ground. So if yo just flood fill that, your thermals will be considerably better to start with. In fact most resistors will have a power defined in their datasheet that is very conservative, with a good PCB flood, you can easily push them 3-4x their rated power.And the prices of these are just insane. For this amount, you could buy a couple of stick on SMD heatsinks, a larger PCB, larger components, that can dissipate heat better. Or depending on your board size, doubling the copper thickness or adding extra two layer could be cheaper than 1-2 of these things. As Mike said in the YT comments, probably a 10Meg resistor would also do the job, these are probably just some resistors, that someone forgot to place in the coating machine.

author: Renaud
author_rank: Newbie
timestamp: November 14, 2020, 01:53:23 am
content: I think a technical video review might be in order to verify the marketing claims.Maybe some test PCB to check their ability to evacuate heat in characteristic circumstances, like a LDO that has to drop too much and overheats and check the cooling effect of various configurations.It would be interesting to see it in action and confirm the actual limits of these devices.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 14, 2020, 02:56:57 am
content: If you do it you might want to compare them to a potentially inexpensive ~25x25mm land to the 2nd copper layer flood directly beneath, which normally goes through a much thinner insulation layer than the 3rd.Renesassays ~1.3 C/W for this so it might be a useful comparison.

author: westfw
author_rank: Super Contributor
timestamp: November 14, 2020, 11:10:29 am
content: OK, now I'm curious. What's the thermal resistance of the base material of a standard SMT resistor? Aren't those usually Aluminum Oxide, which is pretty conductive? I can go out and buy 10M 1206 resistors for a fraction of a cent each, and for many applications, they might as well be insulators (say, a typical LDO regulator with Vout on the tab....)

author: WizardTim
author_rank: Contributor
timestamp: November 14, 2020, 11:36:40 am
content: Interesting idea I'm certainly going to add a couple to my next DigiKey order but I don't know if I'd actually use them on anything considering the price compared to other solutions.Also I made a video a while ago that demos some thermal images of how rubbish fibreglass PCBs are at transferring heat across even a small gap between two copper fills, I suspect if those bridges interest you this video might also.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 14, 2020, 12:43:39 pm
content: For a 4 layer board the connection between the 2 upper layer or 2 lower layer should not be that bad, with a little more area. Of cause it would still also give quite some capacitance.I would consider the main purpose or the thermal jumpers in avoiding the extra capacitance from the heat sink. This may still be a big plus for SMPS or RF circuits, but less of an issue with a normal linear regulator or a LED, where capacitance is less of an issue.It is interesting that the thin gap in the ground plane makes such a big difference. With a little more space one could at least use a more zig-sag line to get a better contact without an extra thermal via. The shorter distance would be to the close by layer in a 4 layer design.For a heat sink area, there is nothing bad with having the solder mask, the transfer to air is not that great anyway, unless in a very fast air flow. It is more of a problem having a gold plated copper layer, as it emits very little IR. In this respect the solder mask helps, increasing the emission factor.

author: Elasia
author_rank: Frequent Contributor
timestamp: November 14, 2020, 01:39:14 pm
content: It's specifically good at two things.. one is a super low clearance vs a heatsink standing up and the second is for EMI and SI considerations.. mega ohm resistors are great.. less you are pwm driving or the like and they turn into a noisy cricket.I actually have some boards that only have maybe 8mm of height between pcb and snap in shell so being able to dump heat into the main ground planes is huge.. normally i'd have to carve out a chunk of space, isolate the stackup, and via punch it together so now i got this big blob in the middle of my HDI

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 01:42:16 pm
content: can this mean something for induction heaters when used with SMD foil capacitors ? In a highly parallel organization with active cooling, double sided, to make a big tank capacitor. Or is the voltage rating not there with SMD parts?

author: Unixon
author_rank: Frequent Contributor
timestamp: November 14, 2020, 02:51:11 pm
content: It is quite typical for 2-layer PCB to have a large ground plane on the bottom layer and most of signals on the top layer, for this type of board my idea for thermal jumpers is illustrated in attached picture.BTW, regular SMD resistors substrate has way worse thermal resistance that these thermal jumpers at least according to datasheets.Al2O3 has thermal conductivity about 25W/(m*K) and it depends on temperature 12..30W/(m*K) in 25C-300C range,AlN has thermal conductivity about 285W/(m*K) and Vishay claim their devices do 170W/(m*K) including terminals and so on,pure copper has thermal conductivity about 385W/(m*K).So, THJP are about 10 times better than conventional SMD ceramic substrates and 2 times worse than copper.I'm curious to see what performance they show in real application.

author: aix
author_rank: Regular Contributor
timestamp: November 14, 2020, 03:05:52 pm
content: Nice calculator collection there Dave!

author: joeqsmith
author_rank: Super Contributor
timestamp: November 14, 2020, 04:09:53 pm
content: One way we would combat temperatures was to laminate the PCB to an aluminum substrate (which was part of the case). The case could then use additional methods to draw the heat away.See attached.

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2020, 05:35:10 pm
content: Quote from: Paul Moir on November 14, 2020, 02:56:57 am25x25 is also about the most that makes sense, with the large the thermal spreading resistance of the thin copper anymoredoesn't really help

author: westfw
author_rank: Super Contributor
timestamp: November 15, 2020, 02:49:01 am
content: QuoteThanks. 0.81W/(m*K) for FR4, according to wikipedia?

author: Feynman
author_rank: Regular Contributor
timestamp: November 15, 2020, 09:15:55 am
content: Hmm, what kind of schematic symbol and reference designator to use for these

author: VK3DRB
author_rank: Super Contributor
timestamp: November 15, 2020, 01:02:39 pm
content: I did a design recently where I had to have 4kV galvanic isolation for the USB power and data (medical device). That means an 8mm creepage distance and a 5mm clearance distance. The circuit is pretty straight forward except the allocated real estate was insanely tight around the USB input side, considering a charger detector IC, voltage protection, ESD protection, optocouplers, a switcher and a transformer were needed. Ground planes are completely isolated and there was no metal around the region to couple heat to. At 1A current from a USB charger, the switching IC case temperature got to 85 degC when ambient was 21 deg C - not ideal, especially since the environmental temperature could get to 32 degC. The fix was simple: decrease the input current to 700mA, which also made the switcher/transformer pair much more efficient as well (per the efficiency curves). Battery charging is slower, down to 2 hours for a charge which is OK from a user perspective, but the whole thing runs cooler. Incidentally just doing the track laying out the USB input took around 1.5 days. It was like playing chess. I also had to have copper clearance under a common mode choke and other important issues to deal with.Thermal jumpers would not be suitable, unless I tried some in series which would get messy and the real estate from just one side or a short fat pad thermal jumper would have taken up too much real estate, believe it or not. But I can see these thermal jumpers would have merit, especially when the case a a device should not be grounded.Another solution not often used (but I have used it once in an isolated power supply in an RF transceiver), is thermal conduction pads. These are typically like blue plasticine sheets that might squash between a TO-220 type case or whatever and the metal case. They work well. I found a reliable vendor in China where the price was small fraction of the Digikey prices.

author: motla
author_rank: Newbie
timestamp: November 15, 2020, 05:17:24 pm
content: Thanks for your video Dave, it's very instructing as always.However at 13:00 you say "if using a ground plane as a heat sink then you wouldn't cover it with solder mask generally, that's just less efficient"https://youtu.be/q3RhWuXFixU?t=782Intuitively I thought the same, as I previously saw a lot of designs with no solder mask on copper planes around power components, but I was unsure to explain why (maybe just letting the possibility to solder heatsinks afterwards, in case the thermal design is bad).So I posted the question on StackExchange and other people seem to say this is not true (that's even maybe more efficient with the solder mask).https://electronics.stackexchange.com/q/532338/141397Have you (or somebody else) any documents or studies on this subject that we can rely on?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2020, 05:45:00 pm
content: There's no reason to remove soldermask for most heatsinking purposes. The layer is thin, and its contribution to Rth, calculated easily.Which, hmm, I can't seem to find thermal conductivity in any datasheets. Interesting.If we assume a fairly pessimistic 0.1 W/(m.K), typical for lightly filled epoxy blends -- then over a useful say 10 cm^2 area, with a generous thickness of 0.025mm, then the thermal resistance is h / (A K) = (0.025mm) / (10 cm^2 * 0.1 W / (m.K)) = 0.25 K/W. You're looking at at least 20 K/W over that board area, so going up to 20.25 is a very low priority indeed.It might be something to worry about when using thermal pads, highly conductive ones at that -- but you still have to contend with copper spreading out the heat, and vias conducting it through the board (the PCB laminate itself has very poor thru-plane conductivity).Indeed, in still air, at modest temp rises, radiation is more significant, and you completely defeat it using a metal surface -- in this case soldermask is better!Tim

author: floobydust
author_rank: Super Contributor
timestamp: November 15, 2020, 09:52:10 pm
content: I think SMD sucks at dissipating over a few watts. I'd tried the D2PAK heatsinks and they are a basic fail.What I found is they don't really work because the heat path from the D2PAK to the heatsink is PCB copper, which is a poor conductor of heat at high power. You would need 2-3oz copper. The few mm space of PCB fill from the tab to heatsink, I saw it didn't conduct the heat well enough.The heatsink is basically unworkable, the heat required to solder or desolder one is very high, beyond a soldering iron and in reflow its (thermal mass) is high so it takes a long time to heat up, then bakes your part with the longer cooldown.An application needing decent heatsinking is automotive ECU's. Look at how Bosch and others do it, just basically glue the pcb to the diecast housing, using thermally conductive glue. It works very well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2020, 02:31:22 am
content: Yes, this is also easy to see. Take thermal conductivity of copper, 385 W/(m K).If the path length is 2mm, width (part or heatsink length, more or less) is 10mm, and thickness 0.07mm (2oz), we get:RthCH = 2mm / (10mm * 0.07mm * 385 W/(m K)) = 7.4 K/WCompared to a heatsink rated for 10 or 20 K/W, this is pretty considerable -- the difference between 2-3 or 4-5 watts, say!The exact number may be higher due to the thermal path not being the straight rectangle calculated here, or a bit lower due to heat spreading of the laminate (in-plane conductivity isn't terrible: comparable to a pour of 0.5 to 1oz copper). And anything you can do to improve heat spreading (vias to pours on other layers, inner planes, heat-conductive chips) is helpful on top of that.It's very easy to see how advantageous going to 2oz, or 2-1-1-2 oz multilayer, is. Or even more layers still, with the added bonus that 8+ layers of 1-2+ oz. gets very attractive for planar magnetics, for compact and high performance power converters.And yeah, those heatsinks aren't the easiest to use. May even be hard to hand-solder, even with a powerful iron. The first kind (extruded aluminum with tinned leads crimped into the face), forget about it. No way to get under it. Maybe a huge ugly blob of solder? The second kind (formed copper or brass sheet?), easy with enough heat.Tim

author: agtrbt
author_rank: Contributor
timestamp: November 17, 2020, 02:42:13 am
content: I have seen a slim chinese set top box with a silicone "chewing gum" tightly inserted between cpu and aluminium case.

author: floobydust
author_rank: Super Contributor
timestamp: November 17, 2020, 03:59:55 am
content: Is it me, or are theVishay Thermal Jumperspecs out to lunch?The thermal resistance and conductance are almost the same for 0603, 0805, 2512I'm expecting larger parts to have proportionately lower thermal resistance.1.5kVAC hi-pot for an 0603, really? Does it come with unicorns and rainbows too?

author: nctnico
author_rank: Super Contributor
timestamp: November 17, 2020, 05:52:22 am
content: Quote from: floobydust on November 17, 2020, 03:59:55 amThey are also longer so that negates the gain from the extra width. Likely they offer various sizes to be compatible with the parts people use in their designs (like no parts smaller than 0805).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2020, 12:47:47 pm
content: If you keep contaminants away -- the voltage standoff is quite high, yes. As a ceramic, the CTI should be essentially perfect.In practical cases, yes, such ratings will be rather severely limited by creepage and lower CTI materials (PCB). You can rout the PCB under the component to address one of those, and fill the thing with potting to address the other.There are also some resistors rated for pulse voltages, I've seen 10kV on a 0603 -- the datasheet offhandedly noting that you need potting to meet that.As for thermal resistance, yah, they're all 0.03" thick, what were you expecting? If they were proportionally thicker you'd expect it to go down proportionally, because bulk resistance has units of m.K/W (multiply by length, divide by cross sectional area, to get Rth). With constant thickness, the area and length are proportional so the thermal resistance depends only on the aspect ratio. And the 2512 length is twice its width and the 1225 is half instead, so there's a factor of 4 between them, and indeed that's what they show (within rounding error, or give or take the electrodes). (This is nothing esoteric, like, we do unit conversions in high school--?)Tim

author: EEVblog
author_rank: Unknown
timestamp: November 17, 2020, 10:28:59 pm
content: Quote from: T3sl4co1l on November 17, 2020, 12:47:47 pmYes, a routed slot under the part is an absolute must if you wanted the full voltage rating here.

author: Feynman
author_rank: Regular Contributor
timestamp: December 01, 2020, 09:32:00 pm
content: Just to get a feeling: These parts have about 10 times LESS thermal resistance than 1oz copper occupying the same board area. Pretty nice, I think.

author: floobydust
author_rank: Super Contributor
timestamp: December 02, 2020, 01:18:37 am
content: But wait... where are you piping the heat to? Take a DPAK or D2PAK and a thermal jumper to... the nearest iceburg...For SMT builds, all I've seen are copper pours, thermal vias to a product's enclosure to deal with dissipating heat.An app note or design backed up with actual measurements would help.It could be good for SMPS primary switch which has HV pulses and EMI, where copper-pours radiate and the isolation is important.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 02, 2020, 01:30:55 am
content: Correct, you can get it closer to an iceberg without worries of shorting things out.The final barrier would seem to be, how close do you think your assemblers can get? Hand assembly, you could butt all these components (transistor, jumper and heatsink) together and let them all get real friendly, but some clearance will be needed on automated assembly. Or for strain relief, since these are rather brittle ceramic parts and they won't be so happy to have big metal parts and thick solder fillets hanging off them.Upside, even if you're doing a thermal pad-enclosure heatsinking approach, you can spread the heat out with a thick-ish copper (or other metal) tab. Far, far more heat spreading than the foil alone can do, so you can get your 5 or 10W out of the thing, even with fairly obnoxious materials like an ABS enclosure or something. Well, if it's big enough, heh, ABS sucks...Also neat for fast switching: they make HIP* pure-alumina heatsinks now! Sounds crazy or ridiculous. Well, however they manage to make 'em, yeah they're a lot less conductive than metal, but when you're only dissipating a dozen or so watts over a few inches of sink, that doesn't matter very much. Meanwhile, you havenoworry about isolation, just grease and go! (They usually use spring clamps, no screws needed.) Very attractive for high dV/dt.*Hot Isostatic Pressed. They use an extrusion die (apparently also made out of alumina, but it manages not to weld and erode for some reason), feed in powdered alumina under pressure, heat the middle to just shy of the melting point, and pull out a perfect extrusion from the far side. Very high density and strength is obtained, as well as apparently almost any shape!Tim