What is SOT223 Rth(j-case Top)? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: January 04, 2024, 09:38:26 pm
content: The TLV1117 datasheet from ti.com, on page 4, states that SOT223 has 54 degC/Watt for Rth(J-case top).Do you agree with this?...it sounds too high.Eg plastic TO220FP FETS are only 6degC/Watt Rth(j-c)TLV1117 page 4 gives Rth(j-case top) for SOT223 (DCY package)https://www.ti.com/lit/ds/symlink/tlv1117.pdf

author: ConKbot
author_rank: Super Contributor
timestamp: January 05, 2024, 02:10:35 pm
content: It gets 5.5 K/W to for junction to board (though metal) and you're wondering why the thermal path though thick plastic (die soldered/glued to the leadframe) is 10x higher and can't get the same performance?

author: AnalogTodd
author_rank: Regular Contributor
timestamp: January 05, 2024, 02:15:46 pm
content: Absolutely agree with the numbers. The plastic used in creating these packages is a horrible conductor of heat. That's why manufacturers make parts with exposed pads on the back, or in the case of a SOT-223 package, you use the tab to connect to a large copper plane on the board. That tab is a solid piece of copper that runs through the center of the package (to have the die mounted on it) and becomes the pin on the other side.I designed products that went into these packages and made thermal measurements on them for years, so I know from experience that those numbers are real. If you want to point to the plastic FETs that have lower thermal resistance, give a data sheet to compare against, I'd be willing to bet you're not getting the direct comparison you think you are.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 05, 2024, 03:40:11 pm
content: It's really surprising they even give this number, and highly useful, you don't see that often. Usually we just assume that such devices are not intended to be cooled through the top side plastic, at all, regardless of Chinese Ebay module manufacturers doing that shit. With this number, you can at least calculate and see if it's OK - it sometimes is.

author: Faringdon
author_rank: Super Contributor
timestamp: January 05, 2024, 06:00:41 pm
content: Thanks...Thermal resistance to the plastic case top of this fet is 31C/WIRFH7440...Thermal resistance on page 2https://www.infineon.com/dgdl/Infineon-IRFH7440-DataSheet-v02_05-EN.pdf?fileId=5546d462533600a40153561f0e821eedHowever, thermal resistance for Rth(j-c) for a plastic TO220FP FET is only 3.6 degC/W.Plastic TO220FP FET...thermal resistance on page 3..https://docs.rs-online.com/fbb0/0900766b8135f861.pdfWhy are these figures so different?They should be the same.Plastic is plastic.I guess the plastic tab of a plastic TO220FP FET is some special kind of highly thermally conductive plastic? And a SOT223 plastic case simply cant be made from this?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 05, 2024, 06:16:23 pm
content: Quote from: Faringdon on January 05, 2024, 06:00:41 pmPlastic is plastic, and approx. 0.3 W/(mK). The latter one is specifically engineered to be cooled through the plastic; maybe it means a more thermally conductive plastic, but probably not - just cutting the thickness does great job. 100µm thickness really is good enough for the voltage rating; for some safety against dust particles penetrating the plastic maybe they use 200-300µm. But the plastic on the top of the component is easily 1-3mm, depending on package. Another factor is surface area. If you use a heat spreading copper tab inside the package, then you have more plastic surface area to conduct the heat, too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2024, 08:51:10 pm
content: It's not quite plastic. It's mostly fumed silica IIRC. This is why the expansion is so low, and softening very minor even at soldering temperature. The ceramic fraction raises thermal conductivity a bit.Which can also give a high CTI, hence the low creepage distance along component bodies, perhaps.Also as plastics go, not that the conductivity of the plastic fraction is all that different, but the material itself is somewhat unusual, a novalak epoxy something or other. Which gives the high softening point.The quoted numbers sound very reasonable to me. Top plastic is thicker compared to fullpack backside for example, and a '220 is wider besides. You don't get much out the top of a D(2)PAK or TO-220-like, either (but you can if you need a few extra points for some reason, I suppose).OTOH, I have tested at least one FWB, of the wide body full-pack style, that is about equally capable of dissipating heat front and back -- a bit less front, mainly due to the smaller contact area I think (there's a bump along the bottom edge that prevents heatsinking the whole front side).Tim

author: Faringdon
author_rank: Super Contributor
timestamp: January 07, 2024, 12:27:03 am
content: Thanks, i see your points...in that case, we now need a SMD 40V NPN BJT which has no more board area than a SOT223, but has lower Rth(j-case Top) than a SOT223.Current rating to 300m or more.Can't be a SOT23. That'd be too low power handling even with the gap pad we'll put on it.I am not sure digikey etc allow sorting on this, so this may take a few hours.

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2024, 02:08:40 am
content: Why do you withhold problem details for this forum but not other forums? It's grotesque behaviour and I think you just want to troll here for something."Don't worry, the bad design can be salvaged" have a hug. Now it's changed from an IC to an NPN?if you can change the PCB, you can change the design and fix it."A customer has sent us a product PCB to modify....""SOT223 dissipates 1.1W. Trouble is, there are eight of these linear regs, and the eight SOT223's are "shoulder to shoulder" on the PCB. (no other room).""Board is 4 layer but there is no room""they want a quick fix with as few as poss changes"https://www.edaboard.com/threads/sot223-with-1-1w-dissipation-and-gap-pad-to-its-plastic-case-top.409534/#post-1766227

author: AnalogTodd
author_rank: Regular Contributor
timestamp: January 07, 2024, 04:17:57 pm
content: Trying to get heat out through the top of a plastic package is near impossible. Plastic hasHORRIBLEthermal conductivity. Just... don't. You won't get there from here.Go back to your post on the TO220FP package. The plastic on those packages is THIN. That gets coupled with a thick copper tab internally and the Rth(j-c) to thebackside(which should be mounted to a heatsink) can be fairly low. Normal thermal resistance on a TO-220 is one-third of this, but you don't get the electrical isolation.Anything surface mount, you get your heat out through the board. Go look at DD-Pak heat sinks and see what they look like. They don't attach to the part, but instead to the copper of the board around the part. Your best bet in this case would be to add large copper planes under the SOT-223 part with lots of thermal vias, and add a heat sink to the back side of the board if really needed. The more board layers and the thinner the material between the conducting layers, the better the thermal response will be.

author: temperance
author_rank: Super Contributor
timestamp: January 07, 2024, 04:46:18 pm
content: It seems Faringdon doesn't know how to approximately calculate the thermal resistance of some material. So let's perform a sanity check.Thermal resistance in R= t/(k x A)R= thermal resistance (K/W)t= material thickness (m)k= thermal conductivity (w/m²k)A= surface area (m²)For a SOT223t= +/- 0.8 mmA= 22.75 mm² (6.5 mm x 3.5 mm)R= 54 K/Wk= t/(R x A)k= 1 mm / (54 K/W x 22.75 mm²)k= 0.65 W/(m²K)A value of 0.65 W/(mK) is very close to the values one can find for semiconductor packaging molding compounds.You're mistake when comparing different packages is:ignoring mold thicknesssurface areaEdit:if, as you state, you develop power electronics as a professional you would know how to perform the above calculations or better, you dream in W/(mK). Or do you always have the luxury to just slap some off the shelf heat sinks into your power supplies? I don't think so if you are a power electronics developer.Send me a PM so we can discuss a pay check to solve this problem without modifying the board.

author: Faringdon
author_rank: Super Contributor
timestamp: January 07, 2024, 05:08:15 pm
content: Thanks temperance that looks really good....b4 i get distracted i will check out the assumption made about plastic thickness in there.Ill step the value between max poss and min poss, and see how Rth(j-case top) varies.Also, the assumption made about the mold thermal conductivity.Since there are assumptions made, i am afraid i will tend toward the actual datasheet stated value of 54K/w, rather than an albeit kindly calculated value, which was based on assumptions made.Much appreciated though, the sanity check. I like the way you use W.m/(m^2.K) which is far more intuitive than w/(M.k) BTW, youre very kind in your description....im actually a complete numbskull!......but i can design SMPS to customer specs. I used to work in a co where i was "slave" to two PhD would_be power engineers......who used to bombard me with ridiculous questions all day....(eg "whats a relay" etc etc) so i understand how nauseating it can be.One of them used to sit on the lab bench and take the space so nobody else could work on it...then go on his mobile phone after 5 mins of working..either that or start makiing solder pools on the ESD mat, and stirring the hot solder round and round.........and this all under the lab_cam!....the boss used to let him do it, he was lauded as the new messiah....though i suspect his father had payed our gaffer a small fortune to employ him and "give him experience".If you have time, and you want to go in on some SMPS design contracts feel free to drop me a line....there are many where you need to be "more then one person" to get the contract...ie a registered company, or at least two engineers....they often dont like giving contracts to "lone wolfs", as ive often found out. Wouldnt need to work under the same roof so to speak....you can hire a workspace for a lab etc etc. If its ok to ask, where are you based?I think if i can find four more SOT223 datasheets and they all say 54K/W for Rth(j-case top), then that will be the winner.....not yet though, doing other work for the time being unfortunately.Finding 4 more sot223 datasheets which give Rth(j-case top) will likely take several hours.Amazing when you consider how, in the real world, gap padding to plastic case top is so common....simply because its often so convenient.Only the other day i took apart a Chinese Drone Inverter and saw DPAKs cooled via plastic case top to metal above them...There's another good few hours....finding Rth(j-case top) for a DPAK or DDPAK.With the SOT223, we will have to measure temp at the case-to-tab interface, (on the metal bit), where it is known to be 15c/w.We will then work back from that, to get the Rth(j-case top). (ie, with another temp measurement on the plastic case middle top.)We will have to use a bit of kapton over the metal tab to isolate the thermocouple from the voltage on the tab.

author: temperance
author_rank: Super Contributor
timestamp: January 08, 2024, 01:01:22 am
content: The W/(m²K) is typo.(Wm)/(m²K) cancelling the meters you are left with W/(mK) which indeed looks confusing if you don't know where it came from.What I calculated is just a sanity check.-The chip itself is of course much smaller than the top area of the component.-The metal tab size inside the mold is unknown. Just cut one open if your curious.The calculation is good enough to perform a sanity check and the obtained value doesn't look strange. Molding compounds with better thermal properties do exist but I don't think you will see those in use for a a low cost SOT223.Besides, FvM, an in my opinion knowledgeable person active in power electronics on EDA board alluded to the same without giving you some approximate calculations.Quote