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I accidentally knocked off a capacitor from my laptop motherboard (Lenovo Thinkpad S540, VIUS6 LA-A171P board). According to the circuit diagram, this capacitor (C2434) was something to do with the USB 3.0 circuit (perhaps the VBUS). See the pics attached. The C2434 capacitor is toward the top-right of the second image

enter image description here

enter image description here

My question is should I be worried? Everything seems to be working fine right now, but I'm worried this will come back to bite me in weeks/months from now. I don't really care if the USB port stops working or something, but don't want a more serious issue to arise.

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    \$\begingroup\$ knocking off a smoothing cap rarely causes serious problems (partially because any decent design should have other caps somewhere on the same rail), some devices might reset if the voltage drops too much, but these devices should have their own smoothing caps anyway, it's not going to make the laptop explode if that's what you're thinking (I've got an exploded IC on one of my usb3 ports and it's now bypassed with some 30AWG hook up wire, port works fine now, most PCs are built pretty well) \$\endgroup\$ – Sam Jun 27 '16 at 0:38
  • \$\begingroup\$ Losing a host-side decoupling capacitor could put the port outside of USB spec for host capacitance. \$\endgroup\$ – user2943160 Jun 27 '16 at 2:17
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    \$\begingroup\$ should I be worried? If you find yourself in a position where you random knock components off of the motherboard of your laptop, yeah, you should - in general - be worried. :) \$\endgroup\$ – pipe Jun 27 '16 at 2:24
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    \$\begingroup\$ How do you have a schematic for your motherboard? \$\endgroup\$ – immibis Jun 27 '16 at 10:19
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    \$\begingroup\$ @immibis I bought it for a few dollars from notebookschematics.com \$\endgroup\$ – fpghost Jun 27 '16 at 11:17
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It's a smoothing capacitor. They are used to smooth out transients in current draw by USB devices plugged in to the port.

If you plug in a device that uses large peak currents - such as a Wi-Fi dongle or USB HDD - then without the smoothing capacitor the 5V rail of the USB port may drop with any spike in current. Depending on the amount at which it drops you could potentially see USB devices reset if the transient is large enough to lower the voltage below what the device can run at.

If I were you I would replace it - a 220uF 6.3V capacitor is called for by the schematic.

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  • \$\begingroup\$ Thanks for the reply. Is there any risk of damage occurring to the laptop itself or would damage be limited to USB devices plugged in? If I just didn't use this USB port I wouldn't risk any damage right? \$\endgroup\$ – fpghost Jun 27 '16 at 0:33
  • \$\begingroup\$ I'd consider replacing it, but I don't have much soldering experience and am scared of making things worse especially if the work isn't essential. If I were to go ahead, would a fairly basic soldering iron be ok for an SMT capacitor? as long as I used rosin flux and some non-lead solder? I really know very little about how to do this, so any advice appreciated. \$\endgroup\$ – fpghost Jun 27 '16 at 0:35
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    \$\begingroup\$ @fpghost It is probably not going to do any damage. At worst you may end up with corrupted data on a memory stick if the voltage drops during a memory write. \$\endgroup\$ – Tom Carpenter Jun 27 '16 at 0:41
  • \$\begingroup\$ OK, thanks. IF I did decide to replace the cap, could you advise on what to buy? The markings on the old were "3D5, VLPS, 220, 6.3v". I know it's 220uF and 6.3V and surface mounted, but do I also need to replace with VLPS series or will anything do? \$\endgroup\$ – fpghost Jun 27 '16 at 0:55
  • \$\begingroup\$ For example would this do the job? \$\endgroup\$ – fpghost Jun 27 '16 at 1:05
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While the accepted answer addresses the possible functional deficiency, there is another matter to consider. When you "knocked" that cap off, the cap pads and thus the PCB tracks might have been damaged (cracks, lifts, etc.) depending on the physical impact. In most PCB designs, the power tracks first come to capacitor pads and then to sockets. Since it is working now, there appears to be no immediate problem. But in time, that USB port may become unusable.

But no, it would not affect the rest.

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