SMD on normal heatsinks - Page 1
Forum section: electronics / beginners

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 29, 2014, 07:55:37 am
content: i think this might be of use to hobbyists looking to find ways to stick power SMD devices (DPAK? D2PAK?) on normal ALU heatsinks. they are not exactly commercial grade level of work, but the level of heat that can be disspated is definately way more.i use a small piece of brass 30mm x 12.7mm x 1.6mm, the SMD is soldered on the brass piece, and then entire brass heat spreader goes onto the ALU heatsink. the thermal pic shown is a TO252 device loaded at 5.7w, the brass is adhered to the ALU by using a lousy gasket silicone (ha! ran out of thermal grease!). actually the original idea was to drill 2 holes on the sides n bolt it on, then i saw the tube of old silicone lying around. nevertheless, this is a very non standard method, and it sort of showed a somewhat a large conductivity gap.i also tried the brass spreader directly soldered on PCB, and the SMD device solder on the brass, the pic with the larger heat signature is the PCB direct sample (also loaded at 5.7w).the brass on the PCB is 2 pieces : 75mm x 15mm x 0.8mm + 50mm x 15mm x 0.8mm. the smaller piece is soldered onto the larger piece which is also solder onto the PCB.the brass solution could be a nice workaround for builders who do not have a choice of plating copper, and wish the power SMD devices to disspate more heat than the usual 1-2watts. it could also be a nice solution to situations where the shop only has that SMD (esp logic level mosfets)*edit : the interesting thing about the brass on ALU heat sink is that, it sort of elevates the SMD to same level of the PCB if the PCB is also going to be flat mounted on the ALU block. but it will require a larger cut out than a TO-220

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 29, 2014, 07:57:34 am
content: both heat sink samples side by side

author: SeanB
author_rank: Super Contributor
timestamp: October 29, 2014, 05:49:26 pm
content: Nice idea, and if you use a silpad you can get isolation as well, just use the regular TO insulating bushes and the M3 hardware, and drill the holes in the brass 4.2mm to fit the sleeve of the bush.

author: Mr.B
author_rank: Unknown
timestamp: October 30, 2014, 01:41:19 am
content: Thermally conductive epoxy.I have used it before.http://www.masterbond.com/properties/thermally-conductive-epoxy-adhesives?gclid=CjwKEAjwoMKiBRDHwoaQ0dzn0UwSJAAUk5JiCCTajqw8brT8NxKWGmjn54ERqxaryRNtYp37gHjakRoCcbnw_wcBNot really "instant", but I have seen 5 minute cure versions.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 30, 2014, 02:31:59 am
content: here are some other brass thingys i tried on PCBthis version is 2 plates 50 x 25 x 0.8 mm brass soldered direct on PCB 2oz single sided blank. most of the solder seeped around edges, cant really reach the middle. but it still spread the heat very well. the heat load is about 5.3w. i purposely botched the soldering on the top right edge to see what will happen thermally if the soldering is bad.the black matte paint that help stabilise flir image actually help dissipate more heat. before painting, the temp was at least +5 extra degrees

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 30, 2014, 02:37:36 am
content: this piece is a single 50 x 25 x 0.8mm brass. same heat load 5.3w (again, w/o the paint, the temp is well over 81 degrees)@ MrB, hmmm yea i thought of epoxy. but its one of those 3 seconds stick it there moments lol, but its ok, most of the time the common super glues i used before did quite well, some lasted for years surprisingly

author: SeanB
author_rank: Super Contributor
timestamp: March 05, 2015, 06:47:27 pm
content: Pretty close to that, and I have seen commercial equipment ( Nemtek electric fence energisers) where they use a LM317 in SMD, and use a soldered on brass tab as a heatsink. Those run very hot, I would have used a TO220 package device instead and a clip on heatsink, which they also do depending on the board revision you get. not good to rely on the thermal shutdown to protect the charge circuitry, if it fails you will cook the battery pretty thoroughly.

author: SeanB
author_rank: Super Contributor
timestamp: March 05, 2015, 07:09:08 pm
content: 70C die temp is a good aim point. Not into insane heatsink size and still going to have a long life with minimal thermal stress on the chip die and package.

author: krivx
author_rank: Frequent Contributor
timestamp: March 05, 2015, 07:20:08 pm
content: Quote from: Mr.B on October 30, 2014, 01:41:19 amTake note that thermally conductive often means electrically conductive as well, a lot of those epoxies use metals suspended in the paste to transfer heat.

author: SeanB
author_rank: Super Contributor
timestamp: March 05, 2015, 07:27:01 pm
content: If you do not have thermal epoxy use a regular epoxy. I have had good results using Pratley quickset steel, a grey epoxy. Araldite steel works as well, but the slower cure is an issue, the Pratley is cured to a non sticky state in under 10 minutes, the Araldite takes about a half hour. I use it to stick heatsinks down to chips that run hot. It works well there.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: March 05, 2015, 07:30:14 pm
content: guess what i use for the motherboard? silcoset 153 ... i dont think it has fantastic thermal qualities, but for that 1x1cm surface area attachment ... i think it was fantastic, because at the point where the SOT223 joins to the brass, the "heat pipe" is only about maybe a 2x5mm area, the brass piece is only about 1.6mm thick, i didnt think it would bring down the overall temp by more than 10C, but it did. (similar to silcoset could be loctite 596 iirc)i want to get my hands on some hysol C1 ... or was it call S1? i would love to try this next

author: tszaboo
author_rank: Super Contributor
timestamp: March 05, 2015, 08:38:01 pm
content: Quote from: 3roomlab on March 05, 2015, 05:59:24 pmWell, according to this very rudamentary 1117 datasheet, that 80K/W is closer to 15K/W, so that chip runs at 75 degrees. Probably gonna work forever and some more at that temperature:http://www.ti.com/lit/ds/symlink/lm1117-n.pdfI'm just curious, if you are so desperate to get the temperature low, why arent you jut using the TO220 package? Or even a TO247. Those are awesome, you can dissipate 200W with them if you have a heatsink big enough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2015, 02:24:44 am
content: Another idea:These are RF MOSFETs, which look like a mix of an SO-10 package with a bad case of webbed feet, and a D2PAK or something like that, for the size of body and heatsink tab. Or an SMC diode with a slightly wider body, much wider leads, and a huge heatsink pad.The construction is like so:First, carve a slot in the board. The heatsink tab is a hunk of 1-1/2 x 1/8" copper plate, about 8mm long (to match the heatsink pad on the devices). The hole is tight enough to provide a friction fit (yes, cutting and filing the hole was tedious -- I have no CNC, so I did it by hand). Tap it in place until the component side is level with the surface of the PCB, then solder support straps across (the extra hunks of copper clad straddling the top and bottom ends of the bar). This takes a lot of heat, but mainly it takes a lot of time (and a heat resistant surface) waiting for the copper bar to heat up.When the plate and devices are soldered in place on top (also, a piece of copper foil, slathered in solder, shorts across the top side copper), go around the bottom, soldering a fillet. This both shorts the plate to the bottom side ground, and provides additional mechanical mounting.Also, before the copper plate goes in, take the time to cut at least a few traces. I carved out the 6mm wide (10mm length, 1mm gap) pads for the transistors (gate and drain, respectively) before soldering in the plate.This is going to be a ~20W push-pull RF amplifier, which should dissipate about 20W at ratings. I'm still designing the transformers for source/load matching. Taking it methodically, since it should be capable of ~500MHz bandwidth. (The transistors themselves roll off somewhere in the GHz!)Tim

author: 3roomlab
author_rank: Frequent Contributor
timestamp: March 06, 2015, 11:08:27 am
content: yea i know some SOT223 dies have better ratings. im not familiar with this SOT223 which is now under the heatsink. so i have to assume a "worse" case average, very worse. i should have gone to find the PDF ... but then again. i am kind of like shooting hot spots only, so that didnt quite matter, as long as something looks "white" on the FLIR ... it has to "die". actually i didnt think to mod the dissipation, famous brand of motherboard, it shouldnt fail. but what the heck ... the hot spot is really a eye(flir)sore. but in the end -15oC is not a bad start. i managed to try to move some air, and that SOT223 went right down to 36oC.@tim, that is a very nice construction ! a slot to sit a large heat spreader! with more time and a good metal file, i think my brass bits wont be spared, BUT ... i cant really put a slot in that motherboard... i have not seen any DIY RF board so far on the web that use such a fixing method (or maybe i have not seen enough) . i saw alot that just solders on direct without a slot! are you at the stage being able to measure some operating temperatures?

author: SeanB
author_rank: Super Contributor
timestamp: March 06, 2015, 07:49:51 pm
content: I have a pile of RF transistors where you cut a hole in the board to mount them, ad then solder the flat plate leads to the board. Very common, and commercial equipment uses the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2015, 01:22:25 am
content: Yeah, traditional RF transistors are screwed to the heatsink (either with an integral threaded stud, or with screws through a flange). The transistors on the board above are fully board mounted types, intended for via-in-pad type heatsinking. I merely made a ludicrously thickly plated via in my board.Tim

author: Evil Lurker
author_rank: Regular Contributor
timestamp: March 07, 2015, 06:33:00 am
content: I use junk TO-220 devices like those negative voltage regulators and no-name Schottky diodes in old power supplies whenever I can. Throw a torch to them or set them on gas burner outdoors till the case disintegrates and falls apart, hit them with a bastard file a few times to remove whatever solder and silicone bits that remain followed by sanding them smooth, and a quick dunk in some plating solution and you are good to go. Main advantage is that you can use the same nuts, screws, isolation washers, and sil pads without any modifications to most heatsinks. If the pins are too short to reach the PCB, I take a IDE header that I have removed off a junk motherboard and simply hold it up next to the device's pins and tack them on. This makes it ridiculously simple to adapt things to 0.1" or even 2mm lead spacing. In fact I use these "extendapins" for all sorts of things with lead spacings that are too short or not the correct pitch for use on breadboards. .