Linksys smart WiFi routers like EA6300 EA6350 EA6400 EA6700 or other similar design routers usually heat up very fast and have high temperature issue. CPU temperature usually stay over 90°c after short run even idle. Although Broadcom chips are said to have an operating rating up to 130°c, it is not recommend to let routers that run 24/7 stay in high temperature since it will shorten its lifespan. Some consumer routers are designed to fail within a certain period of time, but we can make some tweak to make it stay longer.
Here are two options to resolve the overheat issue:
Option 1, add a fan to the router. It will cool down the router to around 60c without without any other steps being taken. Not recommended because adding a fan will also add noise and space. If you have issue following option 2, do option 1.
Option 2, replace factory thermal pad with thermal paste plus additional steps explained below.
Linksys use heatsink on routers' CPU. However, some people report that the heatsink is not contacting verywell with the CPU. We unscrewed the heatsink from EA6400 and found that the CPU has only about 1/3 area contacting with the heatsink based on the footprint on the 1mm thermal pad attached to the heatsink. If the heatsink was well contacted with CPU, we would see a square visible of place of contact. because there is a large gap between the heatsink and the CPU so even Linksys add 1mm thermal pad it still have some space left.
CPU in picture left labeled 1. The chip next to the CPU should be the RAM by Hynix labeled 2. Note: RAM in default has not contact with heatsink at all.
Below are what we need (you can get all of them from Amazon):
Step 1, remove the screws to under the feet of the router. Use plastic tool like credit card, open from two sides first and remove the cover. Unscrew the heatsink.
Step 2, remove the thermal pad. Clean the heatsink and CPU and RAM surface. Because the thermal pad has less thermal conductivity compare to others so we remove it.
Step 3, apply Arctic Silver 5 High-Density Polysynthetic Silver Thermal Compound to CPU surface.
Note: because there is a gap between the heatsink and CPU plus we removed the thermal pad. We need to add some high to the make sure CPU and heatsink are well contacted.
Step 4, place one clean Copper Shim Thermal Pad to the CPU surface on top of the Arctic Silver 5. Apply Arctic Silver 5 High-Density Polysynthetic Silver Thermal Compound to other side of the Copper Shim Thermal Pad.
Cooper Shim Thermal Pad has better thermal conductivity than the aluminum heatsink, so this will also help to spread the heat from CPU to heatsink since the contact area is increased.
So here is how the heat flow now: CPU - Arctic Silver 5 - Cooper Shim Thermal Pad - Arctic Silver 5 - heatsink
Step 5, Arctic Silver 5 High-Density Polysynthetic Silver Thermal Compound to RAM surface.
Step 6, place one clean Copper Shim Thermal Pad the RAM surface on top of the Arctic Silver 5. Apply LARGE amount of K5 PRO viscous thermal paste to other side of the Copper Shim Thermal Pad. Because the RAM surface is lower than the CPU, so we use K5 PRO viscous thermal paste to add more high to the make sure the RAM's heat will flow to the heatsink. Arctic Silver 5 has better thermal conductivity than K5 PRO but Arctic Silver 5 is more liquid like. K5 PRO is better in filling the GAP between RAM and heatsink.
So here is how the heat flow now: RAM - Arctic Silver 5 - Cooper Shim Thermal Pad - K5 PRO viscous thermal paste - heatsink
Step 7, re-install the heatsink and put everything back to normal.
All done. Plus in the router and let it with normal load for couple hours. Temperature will slowly increase from ~40°c to 70°c and become stay around 71°c to 73°c. Compared to the CPU's temperature of around 90°c this is a great result.
If you need the temperature lower than this, now you can consider option 1 and add a mini fan like the below one. You can click the image to get it from Amazon too.