mounting PCB on heat sink? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: November 24, 2023, 07:08:08 am
content: I have a PCB that is basically the size of the heat sink that needs to be under neath or on top of it. There are parts on the PCB mounted to the heat sink (through hole).So the heat sink is on the bottom of the chassis with 4x standoffs to prop it up (vented). Then 4 studs stick out of the heat sink and the board gets fitted on it. So the board is essentially floating on top of the heat sink mounted to all four corners.Its really conveniant and fits well but there is some small SMD parts and also vias scattered on the PCB (0805).Am I torturing the board too much by mounting it on the four corners of a large heat sink? Does this raise eye brows? should I mount the PCB on some kinda bracket, so its isolated from the heatsink expansion? the board is 6x6 inches

author: dietert1
author_rank: Super Contributor
timestamp: November 24, 2023, 07:33:31 am
content: In legacy equipment even larger PCBs were mounted onto metal frames and supports similar to what you describe. I remember audible "cracks" when loosening the screws for maintainance, releasing mechanical stress. I would say it depends on how hot the heatsink gets. If the heatsink is for semiconductors, one could use plastic standoffs to increasy flexibility.Regards, Dieter

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: November 24, 2023, 08:12:46 am
content: Assuming the heatsink is doing its job and isn't getting hot to the touch, then I wouldn't worry about it. Don't forget that PCBs also expand when heated... As anyone who uses a machine vice to hold a board then heats it with a hot air soldering station will know, been there, wrecked a board!SJ

author: coppercone2
author_rank: Super Contributor
timestamp: November 24, 2023, 08:16:21 am
content: hmm no the standoffs are already purchased. as i want to see how long it lasts if it works at all hopefully in the deep future, god knows i spent too much time and money and fucked up a calculation (its over specified AS HELL), enough to hopefully turn it into a lab tool with some extra connectorsI think what I can do is drill some holes in the heat sink and put columns there to support the board. On the perimeter it should not matter (the part is in the center) even if its used to 100% rating. Actually I think I can just nibble out the flange on the perimeter of the heat sink for columns.cool heat sinks are a bit optimistic for compact parts IMO.I don't see a problem with a part running on 80C if it does not expect a transient. for like a regulator (i.e. short circuit condition).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 24, 2023, 08:57:24 am
content: Quote from: coppercone2 on November 24, 2023, 07:08:08 am6" / 150mm is fine for a 'normal' board span, but if you have connectors or heavy parts anywhere, those may need to be considered.

author: coppercone2
author_rank: Super Contributor
timestamp: November 24, 2023, 09:20:26 am
content: actually it does have 3 more heat sinks on it. 50mm aluminum extrusions.that is a good idea maybe I can put another support column there too.kind of thinking to make a big bent flange to put the board on with a hole for the heat sink bottom parts. i gotta start thinking with my sheet metal bender. or just a sheet metal plate with 4 posts that has a bunch of standoff to screw it down on. probobly the best idea, braze those together too so nothing annoying can get loose.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2023, 09:28:18 am
content: That's a normal design practice, but notice the heatsink mounted components should be soldered in after mechanical assembly to minimize lead strain.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 24, 2023, 09:40:01 am
content: I thought that too but recently I swear I read some where that they should be soldered before fastening the components. I was like WTF. I hope its author dyslexia but I lost the document. I think it was vishay . I think they meant bend before soldering but wrote solder before tightening.