Heat dissipation from PCB to housing - Page 1
Forum section: electronics / projects

author: mgwalker95
author_rank: Contributor
timestamp: January 27, 2021, 04:20:42 am
content: I am currently having a problem where my PCB is generating too much heat and getting too hot. I am trying to fix this issue by transferring the heat to the case quickly. I saw that the saleae Logic used an interesting way to transfer heat from the PCB to the case by having the edge of the PCB be bare cupper and pressed to the metal housing. Has anyone tried this technique on a project? What is this technique called? would it cause issues to connect GND to your housing or would that cause ESD, EMC/EMI issues?https://www.eevblog.com/forum/testgear/saleae-logic-pro-8-quick-teardown/

author: bob91343
author_rank: Super Contributor
timestamp: January 27, 2021, 07:18:17 am
content: Generally, you can't get rid of very much heat that way; it's a sort of band-aid.You can calculate the thermal resistance or you can measure it but I don't like the idea.

author: hans
author_rank: Super Contributor
timestamp: January 27, 2021, 09:15:48 am
content: In theory a full metal enclosure that's grounded will create a Faraday cage, so EMI should not be compromised (as in, the EMI that's emitted from inside the device, you can still have a problem with the systems or cables you attach to the device)Generally you want to carry over ESD spikes to chassis GND. A discharge may offset the GND potential though and disturb the device. Again, you would need to know how you connect your chassis GND and system GND together.In terms of cooling it will probably be only very marginal effect. You may carry out some heat that's stuck inside the device, however the terminal resistance from heat generating components (like a FPGA or MOSFET) is likely quite high. The surface area of a case is also quite low compared to a heatsink.The Saleae has quite a small circuit board with no space for large copper planes, so I wouldn't really expect this case substitute to perform significantly better.I don't know the ins and outs of the Saleae design, but it could have helped them get the extra little bit of temperature range they needed for their design.

author: nfmax
author_rank: Super Contributor
timestamp: January 27, 2021, 10:15:10 am
content: I've seen this technique in avionics, where direct air cooling of the PWA is a no-no. The PCB was designed with a thick internal copper layer, as complete as possible. At the edges of the board upper layers were cleared away (i.e. half the layer stack finished 'early') and the exposed thermal plane wasclampedto a card guide slot milled into the housing, using a wedge lock type card clamp. The other side of the housing was a heat exchanger receiving air from the cooling plenum. A similar technique should work with surface layers: you could use an inner thermal layer, and connect it to surface layers at the edge with plenty of vias. Mechanical clamping helps a lot, I believe

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2021, 10:47:25 am
content: It's hard to tell without any details of the size of the PCB or enclosure, but you could try some of that compliant thermal transfer sheet.e.g.https://uk.rs-online.com/web/p/thermal-pads/9156095/P.S. I didn't say that it was cheap!

author: Psi
author_rank: Super Contributor
timestamp: January 27, 2021, 11:58:26 am
content: It's quite effective having solid inner copper layers linked with via's directly to the heat source/IC.It will pull a lot of heat away from a tiny IC and spread it around the entire PCB.And you can make the inner layers out of 2 or 4oz copper if you want even more thermal performance.I had a project recently where the 4x3mm monolithic switchmode IC was getting up to 100degC on a 2 layer PCB.But as soon as I went to a 4 layer pcb with 2 solid inner layers the IC temp dropped to 62degC

author: Northy
author_rank: Regular Contributor
timestamp: January 27, 2021, 01:46:48 pm
content: I've had decent success in the past with a thermal gap pad to an aluminium case.The thinner the pad the better.G

author: jmelson
author_rank: Super Contributor
timestamp: January 27, 2021, 04:12:19 pm
content: Bergquist has "gap pads" that are squishy elastomer filled with some heat conducting material. It comes in various thicknesses.You put it between the component side of the board and the case, and it conducts the heat to the case. This requires a (usually) aluminum block or thick sheet to be very close to the board surface, and bolted to the case, if the case/cover is not close to the board.We use it on some gear that runs in vacuum, to conduct heat from chips to aluminum plates, which then coduct to the outer case which is water-cooled.Jon

author: mgwalker95
author_rank: Contributor
timestamp: January 28, 2021, 06:40:06 am
content: Thank yall for the help and ideas! I will try out your suggestions!

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: January 28, 2021, 11:37:54 pm
content: If remaking the PCB as to be single sided (if it isn't already) is an option and you are getting it from a PCB manufacturer, than you might consider an aluminum pcb which acts as its own heatsink and also can be put agianst an enclosure for more heatsinking. I have found this is what they do in 120V LED Bulbs with aluminum PCB attacched to larger case/heatsink with screws and thermal grease. At the very least, thou, in cheap knockoff brand bulbs (the only ones I have deconstucted much) since there is a ~0.1" layer of pastic with the pring and whatnot between the metal case and free air, they seem to defeat much of the purpose of the heatsink.Thanks!

author: bootloader9800
author_rank: Contributor
timestamp: January 28, 2021, 11:58:34 pm
content: How about having a computer fan installed in the area where the heat is high?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 30, 2021, 09:47:34 am
content: If you could place the heat-generating components near the edge which is coupled to the case, then this is a really good strategy. If the heat needs to travel a long way through the PCB first, then not so great.My favorite is doing all-SMD design with one-side load, with a lot of mounting holes spaced out as evenly as possible, then screw the whole board to the flat aluminum board with some 0.5mm thermal pad in between. Said aluminum board can be the case directly, or it can be a heat spreader that is thick enough to transfer the heat somewhere else where it is connected to the case.Sometimes just using metal to spread the heat internally helps dissipate it through the plastic case. There are also graphite films available which employ lateral thermal conductivity many times better than copper; these can spread the heat from a single hotspot to a larger area, so that it can then conduct through the case better, even if the case is of poorly conducting material.