Seeking advice for retrofit heatsink to QFN in network switch

[bernroth]

Dear all!

Soon I'm going to get some special Netsys NS-280FX SFP switches on my repair desk.
Models which have been manufactured before 2023 will die in the field due to a bad PCB design.

Attached a picture of the PCB. This one has been repaired aready but you can see the baked PCB from the previous regulator

This is a small QFN-14 case and I seek for advice about how to cool that thing properly.
It's sitting in the airflow of a fan and maybe a simple yet small glue-on heatsink will do the trick.

Any product advice or real life trick?


Background:

The switching regulator MP8712GL-P has been used in this design to convert the 12V from the main PSU to 3.3V for all SFP modules.
Fully equipped with VDSL modems, the switch will draw significant current on the 3.3V rail.

Unfortunately the designer of the PCB was obviously skipping the "thermal design" section of the datasheet.
This will cause the poor regulator to operate at the upper thermal limit, bake & break with time and sometimes even switching off during operation.

Customer unhappy, public transport agency unhappy, travellers unhappy due to train departure terminals going offline... 

Have a great week-end!
Cheers,
Bernhard

[SeanB]

Well on top a stick on small heatsink will work, and underneath what will work is a large thermal pad to allow the heat to be conducted to the chassis it is mounted on. You would need to see how thick the pad will have to be, and possibly have a small square of aluminium sheet under it as well to reduce the thickness needed, with that aluminium pad then dissipating the heat, held to the chassis with some VHB foam tape.

This kind of thermal pad is a good start for the base of the board, will have to be around 30mm by 30mm to cover the copper under the chip properly, and give good transfer as well. Also clean the flux off the board as well it will help thermal transfer.

https://eu.mouser.com/datasheet/2/987/THR_DS_TFlex_600_07_2_14-1590672.pdf

 Heatsink for the top likely is this.

https://eu.mouser.com/ProductDetail/CTS-Electronic-Components/APF19-19-13CB-A01?qs=7H2Jq%252ByxpJLu1pwCdE5jLQ%3D%3D

[robert.rozee]

from the datasheet (https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP8712/):
"[pin] 8
[function] PGND
[description] System power ground. PGND is the reference ground of the regulated output voltage. PGND requires special consideration during the PCB layout. Connect PGND to the ground plane with copper traces and vias".

the device has several enlarged pads underneath, esp pin 8 (PGND), which appear to have significance wrt heat transfer. have these been handled correctly in the design, as per the above quote?

if not, then i'd be inclined to suggest removing the MP8712GL-P and associated parts completely from the PCB and create a new daughterboard to hold them, which includes the requisite heatsinking arrangements. as for simply gluing a heatsink on top of the QFN package where it currently is, i'm not too sure this will have sufficient effect - it looks like the part really needs a good heat transfer path from the PGND pin.

another option: after removing the existing part, can you pick up ground, 12v and 3v3 nets elsewhere on the board and add an auxiliary power supply module to take over its function? i'm picking that cost isn't a big issue, as the cost of your time doing the rework to each unit is going to far exceed the value of any components (or PSU module) used.


cheers,
rob   :-)