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I've come up with a clever hack to save space and reduce # power adapter eye sores by bolting my cable modem (Netgear CM500) under the Wifi router (WRT 3200ACM).

enter image description here

The heat sink over the Broadcom baseband processor is very hot to touch. I measured ~70C for the processor and ~55C for the RF (de)modulator enclosure (had to set mode to glossy or else readings for shiny surfaces are wildly underestimated). Modem consumes 8W max.

enter image description here

As clever as it seems, I was worried if there's any fire hazard or other problems if I were to let the heatsink lie on top of a a wood surface or carpet.

I had a read some of this guide,

https://incompliancemag.com/article/electrically-caused-fire-in-multilayer-circuit-boards/

and the main take away was that for fire, it had to generate a temperature beyond the ignition temperature. But I didn't see any way to calculate the temperature. That would be a function of the power density and thermal resistance of the heating element.

Also, it said to distinguish between normal conditions and fault conditions. Normal condition of ~70C already feels worrisome.

But most developer boards for embedded platforms are unenclosed. I guess they would've thought about fire hazards also?

Yale Zhang
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    Place a small fan down there. Make the Broadcom IC last many years longer. – analogsystemsrf Sep 09 '18 at 22:20
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    Unless you set the emissivity to correctly match the heatsink, the indicated temperature will probably not be correct. – user57037 Sep 09 '18 at 23:05
  • @analogsystemsrf Yeah, I don't get why Broadcom chips run so hot. Intel usually gets bad rep for power usage, but apparently Broadcom's MIPs based chips are even worse than Intel's Atom based ones. My previous CM400 (340 MBit) modem using Intel DNCE2530 doesn't even have a heatsink. I also opened a 1.4 GBit Arris SB6190 and the heatsink on the Intel DHCE2672 was much smaller than on the Broadcom. – Yale Zhang Sep 10 '18 at 00:32
  • @mkeith I don't get why IR temperature readings are inaccurate for shiny surfaces. Clearly, shiny surfaces will *reflect* incoming rays non-uniformly. But for emission, shouldn't it be the same in every direction (solid angle)? – Yale Zhang Sep 10 '18 at 00:37
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    @YaleZhang different materials when heated to the same temperature, give off different amounts of thermal radiation. Those that eimit the most have high emissivity, and those that emit the least have low emissivity. In order to calculate a temperature, the thermal imaging camera MUST assume some emissivity for the surface whose temperature is shown. If it assumes wrong, temperature will be wrong. You can read more here: https://www.fluke.com/en-us/learn/best-practices/measurement-basics/thermography/how-emissivity-affects-thermal-images – user57037 Sep 10 '18 at 00:44
  • I never used the word "shiny." I merely pointed out that emissivity is very important in thermal temperature measurement. And not to put too fine a point on it, but if you don't know the emissivity of the heatsink and you don't know the emissivity setting of the camera, then you basically have no idea what the heatsink temperature really is... – user57037 Sep 10 '18 at 00:48
  • Fire hazard from normal operating temperature is very remote. Fire hazard from component failure is a possibility, but it is hard to say more without knowing what the failures are or might be. – user57037 Sep 10 '18 at 00:56
  • your `clever hack` is not as clever as it could be, unless you mount the board with the heatsink facing upward so that heat can escape properly ..... if you are running the router upside down, then the router may overheat – jsotola Sep 10 '18 at 01:12
  • Without a case, you also now have to worry about conductive things bumping into it and causing a short. – Blair Fonville Sep 10 '18 at 02:51
  • With the PCB outside of the box there is now a hugely increased surface area for static discharge to damage components. –  Sep 10 '18 at 14:24

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70 c is not enough to spontaneously ignite paper let alone wood. The carpet depends on the material. But there is no reason you have the heatsink touching anything. Use stand offs.

That said, of you do catch your house on fire, I'm sure your insurance would rule that cable modem as voiding your insurance.

Passerby
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I doubt that it is a fire hazard but it will likely put both the cable modem and the router in a higher temperature environment than they were designed to handle. This will potentially increase error rates, cause random crashes, or even cause the devices to fail entirely.

The modem is normally vertically oriented so that convection currents help cool the heatsink. With the modem sitting under the router, the heat can't escape as easily. The trapped heat will cook both the modem and to a lesser extent the router.

If you really want to do this, sit the whole thing on top of a mini laptop cooling fan.

crj11
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It is easier to mechanically damage the circuit board. This could cause a short circuit, and thus fire.

You have removed the fire retardant plastic casing, meaning you can place other flammable materials near it.

Yes, you have increased the fire hazard.
It is not an direct fire hazard, it's still a low power device using an AD/DC adapter.

Jeroen3
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