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What does the process of physically removing the wifi/bluetooth module look like for a Raspberry Pi 4? How hard it is for a beginner & for someone experienced with soldering? And how likely it is to damage some neighbouring component?

I don't care to save the module for re-use later on, just want to get it off even if that means damaging it.

This is for an air-gapped device, so physically unavailable hardware is much better than hardware disabled by software. I read this answer on a similar question but the one of RPi4 looks bigger, not sure how it looks under that metal shield. Couldn't find a guide on the same.

Evaluating RPi4 over other models which lack WiFi/Bluetooth capabilities purely on the basis of performance. CPU even though is only a little better, but memory seems to be a lot faster than any other models.

Ashfame
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    @Milliways So everyone who has ever done anything had the equipment for it beforehand? – Ashfame Jul 24 '20 at 00:06
  • It seems unlikely that you'll find a "tutorial" on this :) The old phrase "Just do it" comes to mind. A few days ago I might have suggested that you consider a device like the "pocket beagle" - but my short experience suggests it sucks. – Seamus Jul 24 '20 at 00:24
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    If you try it and it goes wrong your experiment will have cost USD$50 as you'll need a new RPi. If the WiFi chip is SMD use a sharp Xacto knife to cut the pins close to the chip and leave it mounted otherwise. If it's a BGA chip you'll need to use a hot air gun. – Dougie Jul 24 '20 at 07:02
  • Thanks Dougie! As per https://www.cypress.com/file/348636/download it seems to be 140-ball WLBGA I am going to attempt Dmitry's short-circuit method below. Thanks Seamus! – Ashfame Jul 24 '20 at 10:03
  • Before you go nuts attacking your Pi have you tried putting the Pi in a metal enclosure? That might be enough to block any wireless signals. As has been mentioned you will be lucky to find a tutorial specifically on the process of removing the Wi-Fi/Bluetooth module. Having a look it appears to be all surface mount components. So any tutorial covering the removal of those would be a starting point. – Darth Vader Jul 24 '20 at 12:45
  • You haven't mentioned what your exact implementation is but the Pi Compute Module might be worth a look as it does not include any wireless modules; and does give you some good flexibility on IO compared with a normal Pi. – Darth Vader Jul 24 '20 at 12:48
  • @DarthVader My use-case requires the absolute certainty of WiFi & Bluetooth being unusable, so can't rely on any software disabling functionalities. Can't reply on a faraday cage either as a screen will sit on top of it, rendering the cage useless. Pi Compute Module isn't suitable either to be portable or have its bootable media ejectable, which is what I require as a feature. Thanks for your input though! I wasn't looking for help on architectural decisions hence why I didn't detail out the use-case except summarizing it as an air-gapped device which conveys security is paramount. – Ashfame Jul 24 '20 at 13:28
  • @DarthVader I explored Compute Module lite versions which might be a good fit as well, and will be costlier but considering no hardware mods would be required, this can actually make sense. I will get one in future and explore that route. Does it behave exactly the same as regular RPi3 as far as operating systems are concerned? Cuz I have a custom one with strict behavioral properties. If this doesn't require extra work, this would be a good option as well. – Ashfame Jul 24 '20 at 16:29
  • @DarthVader CM IO Boards are quite expensive, which makes this option economically infeasible at the moment. Cheapest one that I could find was ~ $100. – Ashfame Jul 24 '20 at 17:09
  • I've never used the compute module but I don't see why it wouldn't support the same software as a regular Pi. The underlying hardware as far as I'm aware is the same. The idea behind the compute board is you can use it as daughter board within a larger integrated system. In that sense it is best suited to industrial applications with the idea being you would build your own board for it. For a large quantity this might work out significantly cheaper than buying an IO board off the shelf, especially if you don't require all the features those offer. – Darth Vader Jul 24 '20 at 17:20
  • @DarthVader I did notice one extra software configuration bit about kernel needing different device tree or something. My project is essentially an open-source project, not really an industrial use-case prototype but I understand the market they are trying to target with it. I just hope to make it smooth sailing for the end-user who chooses to use the project. – Ashfame Jul 24 '20 at 18:08

2 Answers2

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If you want to physically disable Wifi/BT, the easiest way is to disconnect the antenna, or short the antenna output to ground via a 1nF capacitor.

enter image description here

The capacitor is there to avoid having an actual short to ground, which could blow the chip up. 1nF is only 0.06 Ohm at 2.5GHz, so for RF purposes it's as good as a short. A typical antenna impedance is 50 Ohm. Having a 0.06 Ohm path to ground means that only about 0.1% of the transmitter signal will reach the antenna, while 99.9% will be lost. That's about 60 dB attenuation.

Considering the WiFi signal already starts at about -30dBm, attenuation by another 60 will bring the signal down to -90 dBm, which is barely above the thermal noise. This means a few meters away from the device (another -10 dB), you won't be able to receive the signal even theoretically.

Removing the WiFi/BT module is trivial with the right equipment (heat up the board and lift the shield and the IC up with a vacuum pen), but without a schematic I would not give you any guarantees that the rest of the Pi will work once the WiFi chip is gone. And even if the schematic is released, it would be quite a task to analyse it and create a modified device tree for the kernel to correctly understand the new hardware configuration.

Dmitry Grigoryev
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  • I was trying hard to find that antenna on RPi 4, was about to ask you if you are sure the image is right :) – Ashfame Jul 24 '20 at 08:26
  • This is 2-fold problem, as this being an open source project, I am not just doing it for myself but also asking others to do it as well. So whatever is the easiest and gets the job done with certainty is preferred. But how can removing a component without physically damaging nearby components even affect the rest of the board? – Ashfame Jul 24 '20 at 08:28
  • Hopefully it gets clear with the right picture. I bet cutting that trace with an exacto knife is by far the easiest way. And yes, removing components can affect the rest of the board, electronic is not really like Lego but rather a bunch of loops in which current is flowing, and removing parts breaks those loops, so the current has to stop or go elsewhere. – Dmitry Grigoryev Jul 24 '20 at 08:33
  • OK, I trust you to know this a lot better than I do. So cutting with an exacto knife or shorting antenna output seems like 2 easy choices. In your screenshot, are you red arrows pointing towards those blobs of solder or the path? – Ashfame Jul 24 '20 at 08:36
  • No, arrows are pointing towards the antenna trace and the ground plane. There is one solder blob on the antenna trace (which allows you to connect an external antenna), but the ground plane has no solder blobs at all. You can make one though, if you scrap off the paint. – Dmitry Grigoryev Jul 24 '20 at 08:39
  • Like this? https://imgur.com/a/dl7pqaL (please see description as well) Also, can you confirm both WiFi & Bluetooth uses the same antenna and this disables both of them with 100% certainty? – Ashfame Jul 24 '20 at 09:24
  • @Ashfame Yep, that's exactly what I had in mind. 1 nF is certainly enough to kill both WiFi and BT (and yes, it's the same antenna for both), but of course you should try it out for yourself. Also, I hope 1nF will not destroy the output stage of the radio, so that if you remove the cap, WiFi / BT will work again, but that's just a hope not founded by any proof from my side. Again, you'll have to try. If you don't need WiFi/BT to work ever again, just destroy the trace. – Dmitry Grigoryev Jul 24 '20 at 09:55
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    Thanks a lot for your guidance! I will circle back in a couple of weeks when I would have received my order & attempt to do this :) – Ashfame Jul 24 '20 at 09:59
  • My experience is that cut antennas can still communicate. Can you explain the need for the capacitor? What happens if we just bridge the traces? Ashish, how did your project go - do you have a link to it? – fuzzyTew Jul 28 '21 at 11:37
  • @fuzzyTew The capacitor is there to avoid having an actual short to ground, which could blow the chip up. 1nF is only 0.06 Ohm at 2.5GHz, so for RF purposes it's as good as a short. – Dmitry Grigoryev Oct 26 '21 at 09:46
  • Thanks; i understand now it's intended as a short. I don't know yet what 0.06 ohms to ground does to a radio, but i infer the signal will still be there, but that it will be too weak for normal equipment to communicate with. – fuzzyTew Oct 27 '21 at 13:47
  • @fuzzyTew A typical antenna impedance is 50 Ohm. Having a 0.06 Ohm path to ground means that only about 0.1% of the transmitter signal will reach the antenna, while 99.9% will be lost. That's about 60 dB attenuation, if this helps. – Dmitry Grigoryev Oct 27 '21 at 13:54
  • @fuzzyTew Considering the WiFi signal already starts at about -30dBm, attenuation by another 60 will bring the signal down to -90 dBm, which is barely above the thermal noise. This means a few meters away from the device (another -10 dB), you won't be able to distinguish the signal even theoretically. – Dmitry Grigoryev Oct 27 '21 at 14:16
  • @DmitryGrigoryev I'm not sure if anyone tested this (and reported back). Now the idea seems great, but after testing this on a new RasPi, we've seen that this actually brings the signal down about +- 10dBm (which is +- -60dBm), therefore we could still establish a wifi connection. – eDonkey Aug 29 '22 at 08:06
  • @eDonkey Hi, thanks for testing this! I don't understand your comment though, is the attenuation -10dB or -60dB? Or is the signal at -60dBm after the attenuation? – Dmitry Grigoryev Aug 31 '22 at 09:44
  • @DmitryGrigoryev Sorry if I was not clear. I meant that the signal was at about -60dBm after the attenuation. – eDonkey Aug 31 '22 at 10:35
  • @eDonkey And what did you do exactly? Disconnected the antenna, or shorted it to GND? And where exactly? A photo of the mod would certainly help. – Dmitry Grigoryev Aug 31 '22 at 10:45
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    @DmitryGrigoryev As I didn't do it myself (because I simply don't understand it enough) I currently can't provide that information. I will give you an update as soon as I know more about it. – eDonkey Aug 31 '22 at 10:53
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I desoldered the wifi chip with hot air gun and Rpi seems to be working just fine. Also disabled wifi and BT in /boot/config.txt (maybe not necessary).

luki
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  • Can you be a bit more clear on how you did that? A normal soldering iron probably won't work. – PMF Nov 04 '21 at 07:30