RT-AC86U Thermal Upgrade Guide

CaptnDanLKW

Senior Member
TL;DR

Re-did the thermals on my two RT-AC86U with new Thermal Pads AND switch to Copper Pads for the CPU.
Temps dropped from 90-95c to 75-76c.
This is not a message to rehash the problems, perceived design defect or other opinions about the RT-AC86U.
I just wanted a guide like this so I didn't have to guess with parts and thicknesses.


Full details below.

I've had two RT-AC86Us for years & temps have slowly crept up. I Re-did the thermal pads (not well) a couple months ago, improved one slightly, made the other one worse. (Temps 100>, CPUs being shut down by kernel). So this was a FAIL.

I decided to re-do and do it right with properly cut and sized pads, switch to copper for the CPU, add some pads where I thought it made sense. I didn't know the exact thickness I would need for the copper pads, so I purchased a pack with 1.2mm, 0.8mm, 0.5mm, 0.3mm and 0.1mm.

Copper Pad dimensions - 15mm x 15mm. Thickness ended up being (1.5mm or 1.7mm)*
Thermal Pad thickness - 1.0mm, 1.5mm, 2.0mm

*Note - For the first device (the remote node), I went with a 1.2mm and a 0.5mm stacked combo. For the second one (Main router) I went with 1.2mm and 0.3mm, so a slightly thinner stacked combo.
I'm not sure which one performs better; I can't be sure since the remote node sits in a hot building that is regularly 75+ degrees and can peak in the 90s and I didn't test side by side in the same environment.
Also, not sure if the thicker combination is putting too much pressure on the die, or if the thinner combination is not putting enough.
The realization that a 1.2mm would slip down behind the RF shield, and the need to build up a perimeter added an unexpected beneficial side effect of being a potential buffer, spreading the load if in fact, it is too thick.


I've annotated Picture with letter for clarification.

Front Side

A) CPU - perimeter - Thermal Pad - NARROW SLICED 1.5mm
(This prevents the 1.2mm shim from sliding down into shield.)
(A single 1.5mm or 1.7mm would be thick enough not to slide behind it.)
(I think it also helps balance the pressure against the CPU die, so even a single copper slab this is a good idea.)

B) Radio #1 IC Front (Inside RF Shield) - Thermal Pad - 2.0mm
C) Radio #2 IC Front (Inside RF Shield) - Thermal Pad - 2.0mm


REF #1 - Arctic Silver 5 on CPU
REF #2 - Two copper pads with a blob or arctic silver 5 before they were pressed together.
Side Note:

D) CPU - Copper Pad - 1.2mm + 0.5mm CPU - Copper Pad Placement
Screenshot 2022-07-04 184216.png

Screenshot 2022-07-04 194023.png
Screenshot 2022-07-04 184749.jpg
Screenshot 2022-07-04 184616.png

Back Side

E) Flash IC - No Pads, For reference only. It's close to, and above, the CPU. Occasional flash errors could be a result of the excessive heat in this area.

F) Antenna Amp #3 - Thermal Pad - 1.5mm
G) Antenna Amp #2 - Thermal Pad - 1.5mm
H) Antenna Amp #1 - Thermal Pad - 1.5mm

I) CPU - Backside (On RF Shield) - Thermal Pad - 1.0mm
(This is my idea, not ASUS) - Extra thermal transfer to the back plate makes sense to me.

J) Radio #1 IC Back - Thermal Pad - 1.5mm
K) Radio #2 IC Back - Thermal Pad - 1.5mm
L) 2R2 Inductor - Thermal Pad - 1.0mm (Or was it 1.5mm? - Sorry, can't remember and don't have a pic. I think it was 1.0 because I remember switching to a thinner one after inspecting the contact pressure on Antenna Amp #1 "H" and noting it was not making contact.)

Screenshot 2022-07-04 183553-3.jpg



I think that should do it. I can only attach 5 files so no temp chart here.

Hope this helps someone out.
 

L&LD

Part of the Furniture
Don't forget you can post again (to add more details).

If you documented how to take things apart, it would help immensely for those who do continue to use these older routers today.
 

CaptnDanLKW

Senior Member
I don’t have any disassemble videos, sorry.
I used a YouTube video to figure out where the two screws are behind the rear label and where to pry the case apart.
Rest of the disassembly is straight forward.
1 small screw and antenna connector for the antenna pcb
4 small screws to take off heat sink/plate. (The side exposed to you when you take the front cover off is actually the back of the PCB)
4 larger screws holding PCB to back case
Take PCB out of case, flip over
5 small screws holding heat sink down.
 

Oracle

Regular Contributor
I didn't quite get that but I'm not the smartest kid around.
@CaptnDanLKW, did you put copper and thermal pad on the CPU or just copper (2 stacked pieces)?

Also, your thickness measurements differ a bit from mine - both for the CPU and the radios. I wonder if I have a methodological mistake.
Measurement of factory pads was easy. I have no doubts: mine were 2.5 mm on CPU, 3 mm on radios.
For the distance from chip to heatsink I did the following:
1) Put a piece of soft material on the chip.
2) Mount the aluminum heatsink without tightening the screws too much.
3) Remove heatsink, remove material, measure thickness.

I got 1.9 mm for CPU and 2.9 mm for radios to heatsink. Say, if I had applied more pressure on the heatsink around the CPU that could have come down to 1.8 mm, maybe even 1.7 mm. If I get you right, you used 2 pieces of 1.2 mm + 0.5 mm. That's 1.7 mm + extra layer of thermal paste. I may have to revise my numbers and say the right size appears to be 1.75 mm, so 1.7 mm or 1.8 mm shim will be fine.

As for the radios, 2 mm pad or shim doesn't even touch the heatsink. You'll need something like 2.6-2.7 mm min and some more paste.

P. S. In any case, thanks for the post, well done. We could put a combined guide with best options and workarounds for people that don't happen to have the perfect thickness pieces at hand or the tools to make them.
 
Last edited:

CaptnDanLKW

Senior Member
I didn't quite get that but I'm not the smartest kid around.
@CaptnDanLKW, did you put copper and thermal pad on the CPU or just copper (2 stacked pieces)?
The CPU only has Copper Pads (with thermal paste). The thermal pad 'border' around the CPU is to prevent the copper pad from slipping behind RF the shield. Nothing else has copper.
Also, your thickness measurements differ a bit from mine - both for the CPU and the radios. I wonder if I have a methodological mistake.
Measurement of factory pads was easy. I have no doubts: mine were 2.5 mm on CPU, 3 mm on radios.
For the distance from chip to heatsink I did the following:
1) Put a piece of soft material on the chip.
2) Mount the aluminum heatsink without tightening the screws too much.
3) Remove heatsink, remove material, measure thickness.

Interesting, I don't recall a 3mm pad anywhere. I did measure the original pad's thickness on the first attempt, but actually I think it you that too the more methodical approach. It's these variables that make a process like this difficult, since we don't know what manufacturing changes have been made over the years. I remember when HW Revision meant something. The best we have, especially with the RT-AC86U, is Mfg Year and Mfg Location.

I got 1.9 mm for CPU and 2.9 mm for radios to heatsink. Say, if I had applied more pressure on the heatsink around the CPU that could have come down to 1.8 mm, maybe even 1.7 mm. If I get you right, you used 2 pieces of 1.2 mm + 0.5 mm. That's 1.7 mm + extra layer of thermal paste. I may have to revise my numbers and say the right size appears to be 1.75 mm, so 1.7 mm or 1.8 mm shim will be fine.
For the CPU, the OEM pads had quite a deep impression on the 2.5mm pad used, so that's why I opted for something significantly thinner. IMHO the tolerances with the shield and heatsinks vary widely, and the screws on the main ribbed heatsink don't have relief springs, so an oversized copper pad directly on the die could easily damage it, hence my hesitation and even back down to something thinner on my second attempt.


As for the radios, 2 mm pad or shim doesn't even touch the heatsink. You'll need something like 2.6-2.7 mm min and some more paste.

P. S. In any case, thanks for the post, well done. We could put a combined guide with best options and workarounds for people that don't happen to have the perfect thickness pieces at hand or the tools to make them.

The Radio ICs (Same side as the CPU), I used 2.0mm but I could see the case for 2.5mm. The 2.0mm choice was based on the original (and subsequent) grease contact patch on the heatsink - it seemed like good contact was made, but visually it's 'close'. I'm not an expert with thermal conductivity under compression, but I thought thinnest that makes contact is probably OK.

Admission - the 2.0mm pads I used were from my first attempt, and they are actually 1.5mm +0.5mm pads cut and stacked cosmetically poorly, but the thermal temps were very good, so I didn't bother replacing. If they creep up, I will go with a new one piece 2.5mm thermal pad.

I'd be Ok combining data and revising. If there is to be a real, 'official' guide, it should include opening (as @L&LD recommended), full dimensions of each pad (H/W/D) - not the hack job I did and nicer pictures to accompany it. Not sure I have much time to re-do anything though, crazy to think I've put 2-3 hours just documenting, annotating pictures and writing the post. Phew.

Thanks for the info and feedback.

Oh, Two Pics to show the temps.

First pic is the initial power on. I noticed it too a lot longer to reach nominal temperature. That tells me contact is excellent and the heat soak took longer because there was more material to reach thermal equilibrium.
ss2.jpg


Second just shows the normal operating range.
Screenshot 2022-07-04 200933.jpg
 

CaptnDanLKW

Senior Member
now get a fan blowing some cooler air past the vents and watch the temps drop a few ticks
Honestly that was my next idea but since I made it worse, needed to open again anyways. Also, both USBs are in use on the main router and it’s up on a smal shelf I built for it. It’s also dusty and I’m forgetful, so not a good solution for me.
 

Oracle

Regular Contributor
Well, you can always put a thin silk female sock in front of the fan to stop the dust.
Quick'n'dirty fix. :)
Then wash the sock periodically. ;)

Look what Asus engineering causes.
 

Darkje

Regular Contributor
I have cut out a square about 80x80. And mounted a 120mm fan power by USB. It rotates really slow. Every few months I just use some compressed air to get things dust free.
 

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