Even though fans may be “12V”, they can sometimes still run at lower voltages. I have a 12V 80mm fan sitting on top of my board, and powered off 5V or even 3.3V pins on the GPIO header it does a good enough job at cooling, and is pretty quiet too.
Introduce ROCK 5B - ARM Desktop level SBC
thats right its why i wanted to connect this fan also to the fan header because i know that this fan works at 5v and it pretty quiet at this voltage but sadly its not compatible with the mounting holes
12v 40mm and an adhesive 40mm heatsink works great.
PWM is a way to control fan speed whilst minimising torque loss.
Lowering the voltage is just another way but has less range as too low the fan will just stall.
All the 12v fans I have used on 5v work great and @ 41% speed are super quiet but provide much additional cooling over passive.
You do not want to put a 5v on 5v if no PWM as @ 100% 100% of the time will likely sound like a mini vacuum.
as far as i know the rock 5b on the fan header has pwm at 5v and is software controllable
If Your 12V fan gets enough airflow it should be ok, some will not even turn on.
I still think that it’s just easier to get any 40x40 good quality (meaning not too noisy) fan and mount it via two side holes and adhesive heatsink as @stuartiannaylor pointed. Such fans are easily to find on most of pi 4 cases.
Mount holes are about 40x55 something can be mounted on extension arms to that size.
A quick ebay https://www.ebay.co.uk/itm/304627003246 as had some thermal tape anyway and No2 (2.2mm x 16) just screwed in between the vanes.
Standard 40mm 12 on top
40x40 heatsink may be too big, here it’s 20x20 covering only rk3588:
white spacers are for 40x40 fan (which is way too big here, but this was quick setup).
I think that 30x30 heatsink should be ok unless there will be released some fitted large heatsink like the one for rock 4b
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yes thats right but if you dont center the cpu on the 40X40 heatsink then it would maybe work
Yeah its just off centre and underneath that is a 20x20 copper shim that to be honest did not make any difference.
Should of twisted both heatsinks 90’ with hindsight so push/pull through both, but when I saw the temps I was getting with 100% stress running for extended times what I realised is not a lot of cooling thought is needed for this SBC as it seems to run super cool.
The additional ram heatsink I don’t think is at all nessacary, but is product of having tape and there was one on the desk…
All you need is a tiny bit of airflow and it makes huge difference to the surrounding area.
Whatever you find easiest and low cost is my advice and yeah whatever from passive to just a fan or the specific heatsink fan header pwm.
Depends on environment but this summer in the UK in open air passive likely would of been fine.
I got a roll of double sided adhesive thermal tape off aliexpress a while ago as always seem to try a range of quick and easy heatsinks dunno what adhesive they use on those tapes but its mighty strong if you never break the bond.
It just runs so relatively cool that you could do much worse and likely it will still be fine.
i think i wiil use some old pc motherboard chipset heatsink with a little bit of thermal paste and lots of hotglue
Please note that You don’t have production version - with different layout,
On Your photo one ram chip has heatsink - i’m not sure if that can cause any issue, also if its worth to share heatsink with cpu (maybe two bit smaller are better solution). Sure rock5 is not overheating too much so it’s fairly easy to fit something. 40mm fan is easiest to find and mount IMO, with heatpads anything will work 
hotglue?
maybe just use some thermal pads? they comes with different sizes, thickness and they stick quite well
but i have none and i have a hotgluegun and some hotglue sticks (it comes off easly with some rubbing alcohol)
Just use tape or pads as the difference you will see with thermal paste will likely not make a jot of difference.
thats right but i have no thermal pads if i glue it on with hotglue and use thermalpaste i dont have to order something extra
Guess you could, I would just do as I have done, but prob would be adverse to daubing qty’s of hot glue over my nice and shiny new sbc
it wouldnt be as pretty but it isnt a artwork it is a computer (not a mac)
Yeah as said each to there own, I am just not into daubing a load of hotglue over the components either and trying to keep it in sensible realms of warranty.
I am a champion bodger but that is veering to what my ocd would consider a nasty solution.
Is on its way in form of a metal enclosure passively dissipating the heat away. Since using a fan with RK3588 is so weird…
cool i think i will buy that case when it comes out