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Thread: Capacitors (resistors?) on the bottom of CPUs....

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    Capacitors (resistors?) on the bottom of CPUs....

    Hi all,

    So after testing some CPUs that I got sent as free-bee extras when I bought some computer bits off Ebay, I have a question, that isn't REALLY answered from what I can find on the net....

    On the bottom of CPUs are a bunch of SMT devices - some CPUs have more than others. I've always assumed these are capacitors to "smooth out" voltage to the actual CPU silicon. Although, please correct me if they are not.....

    On a number of these CPUs (from several different generations / era, ranging from soc 775 Core 2, up to soc 1155 i5's), they had some of these "capacitors" missing, ie they had been broken off at some point. As I had the kit out and was testing, I tested them anyway.

    Pretty much all of them seemed to work flawlessly. They were stable, passed stress tests and were fine. One (a core i5 3570) had 3 of these "capacitors" missing, and functioned 100%.

    Are these capacitors? If some were missing I'd have thought the circuit would be broken, or the chip would have fried due to too variable voltage?

    Or are they placed on the bottom of the CPUs in such a redundant fashion, as long as a few remain for each section, it stays functional?

    Any light appreciated!

    Cheers!

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    Capacitors.

    Getting power into a modern CPU is hard, hence something like half the pins are for power. Some capacitance as close as possible to the CPU helps smooth things out.

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    maverick77_uk (30-10-2021)

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    Hi

    Thanks - yep what I thought. So they are just in massive redundancy, that if a few are missing, no great deal? If this is the case, why not, design and run them closer to the mark, ie needing all the capacitors, so that it is cheaper / easier to manufacture, and if some silly person knocks one off and breaks his CPU, that was his problem?

    Cheers!

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    Indeed, its not the end of the world, in many of the phones I have seen some have had the odd capacitor/resistor vacant and sometimes you actually need to remove one to get it working, strange indeed but still
    Jon

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    Are you sure the capacitors were actually ripped off? Can you see damage and bits of capacitor left behind?

    There will be a cost in metal and processing, but very small, to having pads there (fraction of a penny each) so it is common to have pads for optional components. So it is likely that a carrier is used for multiple parts, but only fit the supply caps where needed. Low clocked CPU with most of the GPU disabled might need least, but a Xeon or unlocked part probably having all fitted.

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    Hi,

    Thanks. Yeah, definitely ripped, off - I can see the remains of the capacitor is some instances (not just a pad). Also, I've got identical CPUs in most instances, and can see the pads are populated on those - so definitely were there and been ripped off, rather than just a pad and never attached.

    Cheers!

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    Re: Capacitors (resistors?) on the bottom of CPUs....

    If I remember correctly, and I'm thinking of the same components, it's not so much about smoothing voltage .... or not directly. It's about reducing impedance. There are multiple votage paths to different parts of the CPU and (obviously) CPUs are very, VERY fast devices. Impedance rises in a circuit with the rate of change of flow of electricity (frequency), and those capactors. being about as close as they can be got to he CPU die, are supposed to reduce impedance and so smooth out power delivery to different internal sections of the die as power draw changes rapidly. I think.

    It might well not stop the CPU working if they're missing, but could well hamper peformance.
    A lesson learned from PeterB about dignity in adversity, so Peter, In Memorium, "Onwards and Upwards".

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