Heat seperation on small PCB - Page 1
Forum section: electronics / projects

author: Korken
author_rank: Regular Contributor
timestamp: May 17, 2016, 06:14:05 pm
content: Hi all!I am working on a small PCB (35 x 35mm) that has a "high" power (300 mA) digital part and a low power (5 mA) analog part with sensors.What I want is that the heat from the high power side should spread as little as possible to the sensor side, so what I did was to split the ground plane and then added a few thin traces to connect the grounds (to have good electrical contact, but less heat transfer).Is there something more that I can do without actually adding mill-outs to physically separate the board?Thanks for any tips!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2016, 06:54:11 pm
content: The splitting of the ground plane part should be much wider. Without the copper there is still the carrier, which is not such a good isolator. If the analog part is not high frequency, it may not need a ground plane at all. Also only a single ground link might be enough. The digital part might not need the ground plane all the way towards its boundary.A series of drilled holes might be a step before a milled slot.

author: Scrts
author_rank: Frequent Contributor
timestamp: May 17, 2016, 06:55:33 pm
content: Is this a 2-layer board?

author: Korken
author_rank: Regular Contributor
timestamp: May 17, 2016, 09:06:33 pm
content: Quote from: Kleinstein on May 17, 2016, 06:54:11 pmAh, a series of holes and a wider split. I will try to include it!Quote from: Scrts on May 17, 2016, 06:55:33 pmIt's a 4 layer.

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 17, 2016, 09:47:29 pm
content: With such a small board the heat will still substantially flow through the epoxy. Use 2mm wide milled "air" slots and keep copper clearance according the design rules.

author: Korken
author_rank: Regular Contributor
timestamp: May 18, 2016, 06:49:30 am
content: This is surely the best action, just because of the size.And it is also the most difficult, again due to the size and density of the board.But I will try to reorder the board to make space for the slot. This would be the best solution.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 18, 2016, 09:03:36 am
content: Where is the heat from the hot component actually going to go? What's your cooling strategy?If you can, perhaps it would be more effective to cool the hot component by conduction, maybe via a thermal pad into the enclosure, rather than relying on trying to localise the heat. It has to get out of the PCB somehow, and it can only help if you can provide a path that doesn't involve going through the 'cold' part of the board.

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 18, 2016, 09:50:52 am
content: I wonder what digital part nowadays draws 500mA (of 5v or 3.3V I assume) and dissipate that amount of heat into the board. 35mm x 35mm will run pretty hot with that. You surely need to use thermal filler pads to get the heat to the enclosure or some form of heatsink.

author: tszaboo
author_rank: Super Contributor
timestamp: May 18, 2016, 11:37:22 am
content: You can also just omit the ground plane under the analog part. Most likely. Its not like you are really going to need it.Copper conducts some 500+ times better heat than FR4. But it is not that thick.

author: Korken
author_rank: Regular Contributor
timestamp: May 18, 2016, 06:39:50 pm
content: Quote from: AndyC_772 on May 18, 2016, 09:03:36 amThe board is the next version of a UAV control system that I use in my research, so it usually sits within a small plastic (3D printed) case so the convection is not very good.Quote from: Pjotr on May 18, 2016, 09:50:52 amThat would be the combination of an Cortex-M7 (STM32F746 @ 3.3V) at full speed, SD card, CAN transceiver, flash storage and a few LEDs.It's not that bad though, its around 200 mA during normal running conditions and about 400 mA worst case (CAN short, + write on both flash memories).Quote from: NANDBlog on May 18, 2016, 11:37:22 amThat is actually a good idea!The analog part is its own small region with the LDO driving it, so it does not really need the plane.Just for reference, I added the images of the design to give some context.

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 18, 2016, 10:22:46 pm
content: Thanks for clarification and the pics. Needed thermal management looks a bit more complex than the board only. I would say, put the pin header on the other side. Put some good quality soft thermal gap filler pad on top, and on top of that aluminium sheet as large as possible as a heat spreader. Mount that aluminium sheet in direct contact with the plastic wall to get the heat out of the box directly. The air in such a small box to get the heat out of it, is a very poor thermal conductor.