We have a component with dimension 1.5 cm x 1.5 cm on the board which will be vertically mounted in the rack. We need to attach a heat sink with dimension 1.5 cm x 1.5 cm of height let's say 5 mm or 10 mm. How do we normally attach the heat sink to the hot components ? by thermal paste or some other way if there is no place to attach the heat sink block mechanically to the PCB.

Would the thermal paste also work for vertically mounted boards ? I was looking into some heat sink blocks. Some of them have a value of thermal resistance like 20 degC per Watt or 25 degC per Watt. I am trying to understand what does that means.

Does it means that if we attach these heat sink on the hot component then temperature will be increased by 20 degC or 25 degC per every 1 Watt power dissipation. Is that correct ?

 

hi engr,
Check this link for Cdeg/Watt heat sink rating
E
https://www.google.com/search?client=firefox-b-d&q=what+does+a+heatsink+of+20c/watt+mean

 

Heatsinks for small parts can be attached with thermal epoxy, paste isn't an adhesive.

A heatsink rated at 20C/W means the temperature difference between ambient and the mounting surface will be 20C for each Watt dissipated.

Say your part has a Tjc (thermal resistance junction to case) of 15C/W and a max junction temp of 125C, and say ambient is 30C.Your part dissipates 2W.
Then the heatsink base will be 30C + 2W x 20C/W =70C and the junction will be 70C + 2W x 15C/W = 100C! Note that the thermal epoxy will have a thermal resistance as well, maybe as much as 1C/W

Also be aware that many SMDs expect to be cooled through the PCB. Cooling through the case may be marginal.

What is your part and how is it mounted?

 

Thanks. I understand the calculations.

We have Raspberry Pi Compute Module 4, and we need to attach the heat sink block to the processor mounted on this module. The dimension is 1.5 cm x 1.5 cm. How can we attach the heat sink block to the process of this module is a question. Any suggestion please ?

 

This is still open for discussion.

The RPi CM4 is vertical mounted in the rack.

How can we attach heat sink block to the processor of RPi CM4, given that the module is vertical mounted.

We don't want the heat sink block to fell down.

 

Many of the RPi CM4 heatsinks on offer, like the one below, use the same mounting holes as the CM4 itself; this is only 5mm thick but gives much better cooling than the small stick-on heatsinks. Whats wrong with using this one??



The 14 x 14mm stick-on heatsinks, like this below, normally come with some 3M thermal adhesive tape:



If your one doesn't have the adhesive tape it's probably just a generic one and you'll have to get some 14mm thermal tape or use some thermal epoxy as I already said.

 

I will have a look at both solutions.
I will also check if we can get thermal epoxy or thermal pate from Mouser, Digikey, Farnell or RS.

 

I have used 3M epoxy 2216 before. It comes in two parts. We need to mix them and use within 5 - 10 min. The cure time depend on temperature. At 60 degree it should be taking few hours to cure.

Can we use 3M epoxy 2216 to attach heat sink blocks ?

 

Thermal tape is easiest and simplest solution - from Amazon

Epoxy 2216 is not thermally good so not suitable.

 

Thanks for the suggestion.

I have attach a thermocouple on the processor chip and measure the temperature on the logger. It shows around 60 degC.

At which temperature we must have heat sink or fan attachment ?

 

hi engr,
What was the Ambient temperature when you measured the chip temperature?
E

 

Why do you think 60C is a problem?

 

The ambient temperature is around 30 degC.

The ambient can be more then 30 degC when we put all the electronics together.

This is a question at which temperature measured on the top of the processor chip we generally need to think for a heat sink ?

 

Why do you think 60C is a problem?

Because it appears that at a case temperature of 80C it starts throttling...

 

Because it appears that at a case temperature of 80C it starts throttling...

My guess is as the temperature goes above 70 degC, (there is a temperature sensor on the professor module), we should better put a heat sink on RPi CM4.

 

My guess is as the temperature goes above 70 degC, (there is a temperature sensor on the professor module), we should better put a heat sink on RPi CM4.

Ideally, if you have the space, you should use a fan-assisted one that covers the whole board. If not, then the passive whole board (armor) heatsink. The little stick-on heatsink is unlikely to be sufficient if you are already hitting 70degC