thermal relief for SM reflow - Page 1
Forum section: electronics / general

author: phil from seattle
author_rank: Super Contributor
timestamp: November 06, 2020, 04:25:01 am
content: I'm trying to layout a small board that has a linear VR on it. The large tab is Vout (not ground) so I can't just use a ground plane as the heat sink but my bigger question is how much thermal relief do I need. While the current draw is fairly small - 10 mA, I have a spec of up to 24V in with 3.3V out. This means the linear drop will contribute about 207 mW worst case and the ground current of the VR, 125 mW for a total of around 332 mW. Theta JA for this package with no heat sink is 140 C/W which gives me 46.5C above ambient. I expect the device to live in ambients up to 35C which means I will be pushing the 85C max operating temp. So, I want to have a relatively large copper area to mitigate this worst case scenario. But this gets me into competing requirements.The image shows about 200 mmsq pour area - I may make it bigger. And, a first (and rather ugly) stab at thermal relief. Obviously, the more copper between the tab and the pour area I keep, the higher the efficiency of the heat sink. I keep seeing conflicting information on this for reflow. Realistically, how much relief do I need? I don't want to cook my little regulator but also want it to reflow correctly.

author: ogden
author_rank: Super Contributor
timestamp: November 06, 2020, 06:00:38 am
content: You shall find what thermal reliefs are for.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2020, 07:06:07 am
content: You didn't mention what the package is? SOT-223? DPAK?Thermal relief is a win in most situations. Soldering isn't fiddly, while the increased Rth is minor.You should only need to consider very heavy thermals, or none at all (what you show above, isn't much different from direct connect -- I wouldn't bother with doing it as shown), when demands are much stronger (several watts, or much less temp rise headroom). In that case, the sacrifice in production yield may be worth the saved cost of larger devices or heatsinks.You can calculate approximately how much Rth it is. Take the thermal resistivity of copper, multiply by spoke length, and divide by total spoke area (foil thickness times total spoke width). It's just that easy.It's approximate, because the spokes dissipate some heat to air, and into the board, and the heat spreads out around the spokes so that a straight rectangular box approximation like this doesn't quite capture the real situation. But you'll find that a modest "spoking" only adds a few C/W and the real situation won't be grossly worse than that. So we aren't worried about more accurate approximations.BTW, you can still harness the dissipative value of a plane, by sandwiching it with thermal pours. Add vias to the back side, and add a pour there. Now both outer laminates conduct heat into the core, in parallel. The increased area of the pours also helps. Again, you can calculate the approximate thermal resistance, using the FR-4 thru-plane resistivity and the dimensions of the stackup and pours.P.S. Note that I mentioned production yield. If you're doing this by hand, solder it however the hell you want. It literally doesn't matter, just put heat into it until it's fully soldered.Tim

author: phil from seattle
author_rank: Super Contributor
timestamp: November 06, 2020, 04:51:23 pm
content: Quote from: ogden on November 06, 2020, 06:00:38 amThanks. Avoiding trial and error was the point of my post.

author: ogden
author_rank: Super Contributor
timestamp: November 06, 2020, 05:46:23 pm
content: Quote from: phil from seattle on November 06, 2020, 04:51:23 pmThermal reliefs are mainly for small components - to avoid tombstoning. Thermal pads of high thermal mass component do not need thermal relief unless you want to solder it using small soldering Iron which would beanyway, yet you mention reflow. So don't bother. Thermal relief works against main idea of thermal pad. If in doubt - check many reference designs of hi-power DC-DC supplies to hardly find any thermal reliefs. Look at CPU VCORE regulator transistors on motherboard of your PC, if any.Relevant doc:https://www.vishay.com/docs/79325/measurement_comparison.pdfhttps://assets.nexperia.com/documents/leaflet/75016838.pdf

author: phil from seattle
author_rank: Super Contributor
timestamp: November 06, 2020, 06:06:00 pm
content: Thanks, That is a very helpful post.SOT-223. It's a 2 layer board and I want to have a large ground plane. Unfortunately the chip's (LM317, ancient, I know) tab is Vout, not Gnd. So I could do a plane underneath and use thermal vias though they would be outside of the footprint area and thus the thermal flow would still be through the spokes. And it will be reflowed (think I said that earlier).I think what I heard is there is still a fair amount of flow with just a couple of spokes. Will try the calcs you suggest and see what happens.Quote from: T3sl4co1l on November 06, 2020, 07:06:07 am

author: phil from seattle
author_rank: Super Contributor
timestamp: November 06, 2020, 06:32:32 pm
content: Quote from: ogden on November 06, 2020, 05:46:23 pmThanks. So, I'm hearing don't bother with thermal relief for a 223 package. That makes it a lot easier.

author: asmi
author_rank: Super Contributor
timestamp: November 06, 2020, 07:13:11 pm
content: Quote from: phil from seattle on November 06, 2020, 04:25:01 amYour main thermal path is likely going to be the board itself, you've got to look at JB (or JC bottom depending on how manufacturers measure it) parameter as opposed to JA. It should be significantly smaller if package is designed to sink heat into the board though contact pads.Also, check out thess short appnotes from TI:https://www.ti.com/lit/an/spra953c/spra953c.pdfhttps://www.ti.com/lit/an/slua566a/slua566a.pdfI think they give you enough information to work with, while avoiding going too deep into the woods. For TI parts, they have a calculator, which will show you graphs of board temperature depending on the area of a pad given the part parameters and expected power dissipation:https://www.ti.com/design-resources/design-tools-simulation/models-simulators/pcb-thermal-calculator.html

author: phil from seattle
author_rank: Super Contributor
timestamp: November 06, 2020, 08:29:00 pm
content: The part is designed with a tab so that means it doesn't sit on thermal vias.For what it's worth, it looks like I can get almost 300 mm sq on the top layer. That should be enough to avoid pushing the thermal limits of the part.

author: asmi
author_rank: Super Contributor
timestamp: November 06, 2020, 08:47:42 pm
content: Quote from: phil from seattle on November 06, 2020, 08:29:00 pmIt doesn't matter. Conduction is conduction.Quote from: phil from seattle on November 06, 2020, 08:29:00 pmWhen I'm unsure if it will be enough, I usually remove a solder mask from a part of the pad so that I could stick radiator to it if cooling performance will not be adequate. Something like this (note the gold pad on a right side of a board):In this case it's under device because there is no space right next to device on a top side, but it could be either way, as long as there is enough thermal vias.

author: wraper
author_rank: Unknown
timestamp: November 06, 2020, 09:31:57 pm
content: Quote from: phil from seattle on November 06, 2020, 04:25:01 amThere should be none. As it only makes heat dissipation worse with no advantages whatsoever. But tombstoning for avoiding which thermal reliefs are used for is impossible to happen for such package.

author: wraper
author_rank: Unknown
timestamp: November 06, 2020, 09:33:22 pm
content: Quote from: T3sl4co1l on November 06, 2020, 07:06:07 amIf it is minor, then thermal relief would have barely any effect for soldering as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2020, 09:43:13 pm
content: Quote from: wraper on November 06, 2020, 09:33:22 pmThe difference is the heat flux available. A couple watts, hardly anything. Soldering, upwards of 50W. Instead of a minor 10C drop you can have hundreds.Tim