Using PCB as heatsink! - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 21, 2020, 01:40:48 pm
content: Hi,I want to know if there is a calculator for calculating thermal design for PCB, In one of my designs I have added some Copper fills to act as a heat-sink, so the question is where can I find some tool for calculations? I'm using LM1117 in SOT-223 as the Vcore regulator for my Spartan 6 device (XC6SLX9) so, I want to make sure everything is fine.

author: tszaboo
author_rank: Super Contributor
timestamp: January 21, 2020, 01:43:39 pm
content: 555:You can dissipate something like 5 W in a 50 x 50 mm PCB if it is 4 layer with high copper on all layers.

author: thinkfat
author_rank: Unknown
timestamp: January 21, 2020, 02:02:10 pm
content: Quote from: NANDBlog on January 21, 2020, 01:43:39 pmWith ample thermal vias, of course.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 21, 2020, 02:37:19 pm
content: Quote from: ali_asadzadeh on January 21, 2020, 01:40:48 pmThe best you can realistically hope for with standard pcb construction (2-4 layers of 35um Cu) is a thermal resistance of 40C/W. Allotting a bigger area to the heatsink doesn't help because the lateral thermal resistance of the thin copper is too high to make use of it; anything beyond 15mm per side as a heatsink for an SOT-223 component will likely end up as wasted area, in fact. You can get marginally lower thermal resistance with a larger SMT package (e.g. - D2Pak) but if you have to allot, say, 50mm per side of board area just to dissipate 5W then you really need to rethink your approach (ie - a PTH component with a small aluminum heatsink or a, perhaps, a buck converter [pre-]regulator instead of a linear one).

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 21, 2020, 02:57:13 pm
content: What MagicSmoker said.A common 1 oz copper is only 0.0347 mm thick(*), so it will not conduct heat readily.I have used as a rule of thumb, for 1 oz copper, I've used 1 sq-in or roughly 25 x 25 mm, as the largest practical copper area to be used as a heatsink.(*) Sourcehttps://pcbprime.com/pcb-tips/copper/

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 23, 2020, 07:12:51 am
content: Thanks, Is there any calculator that could accept area and copper wight and tell thermal resistance?Also how much current would the FPGA use? assuming all logic in there used with the fmax? The FPGA is XC6SLX9

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 23, 2020, 11:46:00 am
content: Quote from: ali_asadzadeh on January 23, 2020, 07:12:51 amAn online calculator that can solve 3D heat flow problems (and you'll want it to be free, I bet)? No.I'll just advise you to read TI's AN-1028 and work things out for yourself. Given that all of the graphs of thermal resistance from junction to ambient vs. copper area will be rectangular hyperbolic curves, with the knee somewhere between 100 to 500mm2board area depending on heatsink tab size, R_theta[j-c], and copper weight, there's little point in throwing lots of board area at the problem.Quote from: ali_asadzadeh on January 23, 2020, 07:12:51 amXilinx publishes a power management guide (UG394) for that chip family with a direct link to download it in the family overview datasheet itself.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 23, 2020, 11:56:50 am
content: Yes, MagicSmoker's general assumption of heat transferring sideways approx. 15mm is a good one. Let's round it down to 10mm for now, for a bit of extra margin: assuming imaginary component of 10x10mm, this means you have dissipation area of approx. 30x20mm, assuming one side of the component is needed for tracks.Now, adding a large bunch of thermal vias - certainly within that 10mm, but the closer the component, the better - allows you to continue heatsinking on the other side. Then, you can go 10mm both ways from the vias. If your vias are right say 1mm from the component, now you have 30x30mm fill on the bottom side, as well.There are web calculators that calculate the thermal resistance of the via for you! So you can just divide by number of vias, to get an estimate how much cooler (and hence, less effective) the bottom side fill runs.If you need more heatsinking, this bottom side 30x30mm fill is the place to couple the actual heatsink to. You have a surface area approximated (say 30x30mm), you have the thermal resistance value and thickness of whatever thermal pad material you are going to use, so you can calculate the total dT.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 23, 2020, 01:10:34 pm
content: FWIW, a handy rule of thumb is to multiply the material thickness by the thermal conductivity of the material to determine how far heat will practically flow. Copper has a thermal conductivity of about 400 W/m-K so heat will travel about 15mm on a 1oz/35um thick layer, or 30mm on a 2oz/70um thick layer, etc. This thumb rule breaks down at much higher thicknesses, but that's why it's a thumb rule and not a physical law.

author: thinkfat
author_rank: Unknown
timestamp: January 23, 2020, 02:34:23 pm
content: Has anyone tried to use the free "Energy2D" thermal simulation software for this kind of stuff? Granted it's only 2D, though...

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 25, 2020, 06:58:42 am
content: QuoteNo, Have you used it?

author: thinkfat
author_rank: Unknown
timestamp: January 25, 2020, 09:19:22 am
content: I've played around with it. I think the biggest problem is that it lacks a database for materials so you need to enter all the parameters by yourself. Specific heat, density, thermal conductivity, to name a few. You'll spend a lot of time with search engines and then another lot of time to enter the data. For aluminium you find the data readily in e.g. Wolfram Alpha, but try the same for FR4? How do you model a "Silpad" if you only know its total thermal resistance in K/W ?The next thing you need to figure out is how your model should look like. Then, the editor to capture your design is also quite cumbersome to use, I guess the best way is to sketch out the model on paper and then enter numerically.So, it's probably doable, but it's in no way easy and quite disruptive to an iterative workflow.

author: schratterulrich
author_rank: Regular Contributor
timestamp: January 25, 2020, 02:28:08 pm
content: I have made a tool to simulate the temperature of a PCB.It may give you some numerical results. You can find it athttps://leiterplatte.jimdo.com/thermal-pcb-sim/Kind regardsUlrich

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2020, 02:58:03 pm
content: Quote from: schratterulrich on January 25, 2020, 02:28:08 pm

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 26, 2020, 06:42:13 am
content: QuoteThanks for sharing