Engineering Question - Details of the bottom of a CPU

Foxie3a said:

I'm specifically talking about CPUs like LGA771/775 and LGA1366.

I need to know everything about the "caps" that are on the bottom of the CPUs.

I've found that when missing some, about 80% of them still POST just fine (I don't test further). I get a lot of CPUs that are missing the "caps" on the bottom, and I don't get the original missing parts, so I need to figure out exactly what they are.

I'm told the larger ones are gap resistors that connect each layer of the core together... While the smaller ones really are capacitors. But there's many different sized ones and different looking ones on CPUs now. I need to know what they are so I can buy the right ones to put back on them, and get the CPU to work fine.

Another thing, is I need to know how to test each component on the bottom of a CPU. They are not all caps, and if some of the bottom chips were to stop working, it would render the entire CPU useless. I get some dead CPUs that are still $500-1000/each server chips that just don't work for some reason, despite no damage... and it would be wonderful if I could test the bottom components and maybe realize a resistor was no longer working, or whatever the case.

Not many of u will probably know much about this, but I'm just starting out learning about it and anything u can contribute would be great. Thanks!

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yzmij said:

Those capacitors combined with the resistors, they form a RC circuit. Their purpose is mainly for noise filtering, dc/ac couplings, which is why the chip may still work without some of the passives.

It's very rare that a processor would 'miss' a cap or a resistor, they just don't break off that easily. A possible explanation to the 'dead' chip you are getting is that they are defected chips that are failed during post silicon testing, or they are of an earlier stepping that requires special scripts to bring up, or they are simply test vehicles made for IO, thermal designs.

Back to the subject, the most common way you can damage a cap or a res is to burn them through high current, and that will leave a very obvious mark on the bottom of the die.

Lastly, you should never measure the capacitance and resistance of the passives in the circuit. Always dis-solder them then measure the components individually.

For resistors, it's easy, measure it with a DMM (de-solder it first)

For caps, build yourself a RC circuit with known resistor, you also need a function generator, a DMM or a Oscilliscope that can measure RMS values up to the frequency your function generator can generate.

the circuit will be something like this (the periods are place holders):
.................................... _________________________________________
....................................|...............................................................................|
....................................|...............................................................................|
....function generator (~) .............................................................................|
....................................|...............................................................................|
....................................|_____[known resistor]_______[unknown cap]____|
...........................................^..................................^...............................^
...........................................A...................................B...............................C

you can calculate the capacitance using this formula:
C = (Vr / Xr) / (2 x pi x f x Vc)



1. very the frequency and amplitude of the output from your function generator, not sure what frequency and amplitude to use ? see below step:

2. use your DMM or O-scope, measure the RMS Voltage across the known resistor (between point A and B), this is you Vr, unit is in Volt. The amplitude and the capacitance will influence the Vr you are measuring, so feel free to adjust the output of your function generator until you get a good reading (between 1 to 2 volt)

3. measure the RMS voltage across the unknown capacitor (between point B and C), this is your Vc, unit is in Volt. again, adjust the frequency and amplitude of the function generator to get a good number for your calculation.

Note: Vr + Vc may not equal to func generator's output Vrms.

4. once you get a good number for Vc and Vr, you are good to go, Xr is the resistance in Ohms of your known good resistor, f is the func generator's output frequency in Hz. Put all the variables into the formula to get your Capacitance in ferad ( this will be a very small number, times 1000000 to get uF which is very common unit in the industry).

Next, find the right cap, they come in different sizes, but for CPUs, the most common sizes are 0402 and 0201.

The real challenge is soldering the caps and resistors and interfacing the circuit with your func generator.

Good luck.

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