Yeah I knew this, but under warranty I didn't want to mess with it. The warranty ran out in march. But I've had health issues from around april til september, almost died. So I didn't get around to solving the heat issue until, it would seem, too late.
The pad, shield, pad, sink sandwich just doesn't cut it. During summer months that 5ghz radio would reach just under or just over 60 degrees. When I finally opened her up the 2.5mm thermal pad on the chip simply crumbled between my fingers. The pads between the shield and heat sink are slowly but visibly liquifying. Replaced the 2.5mm pad with an arctic cool pad, temps dropped around 11 or so degrees. But alas.....it was too late. The thermal design is not great, but more importantly the thermal material is even worse.
You can have a fancy high quality red aluminum heat sink with racing stripes.....but if the material between them is junk, it's all for nothing. Not only was the thermal material junk but also....2.5mm gap!? These routers aren't super weak low power like once upon a time. They are more like pc's now. In fact the AX89X is spec'd better than some walmart laptops. My ac88u has better specs than my playstation 3, maybe even better graphics.
I would likely have done something like: chip>1mm pad>1.5mm copper shim>compound>shield>.5mm pad>heatsink., and cut the plastic top differently. Simply leaving the top off drops the temps 10+ degrees. But, as I've discovered, even adding a fan (looking at you AX89X) won't help if the chip is baking it's thermal pad into apple crumble.
Reminds me of a ps4 I repaired. Super loud max fan. I opened her up and what did I find? Thermal compound was only on half the apu, and it was baked to crumb. Gave it the mx4 treatment now she's whisper quiet, even years later.
I'm debating between extended warranty or, voiding warranty day one, and redoing thermals post haste on the next unit. Haven't decided. I'm not a big fan of burying the planet in "disposable" electronics, which is why I repair what I can, salvage for parts what I can't.
Personally I know for a fact that the thermals could've been much better with some simple and inexpensive alterations in thinking pre-manufacture. I know the chip makers rate the chips for a max temp, but people don't realize that that max temp means "this is the point where it will sizzle and die", therefore the temps should be kept as far away from that max number as humanly possible, not within a stones throw. The hotter it runs the potentially shorter it's life span, the cooler it runs the potentially longer it's lifespan. If it didn't matter, the makers wouldn't include min max temps in their chip data. And bga style....which fails faster under sustained heat.....the chip or the solder balls? Red ring of death.
Too much coffee.

I have a router shopping to do.