How to ID SMD components and know the correct resistance?

[Electromouse]

I’ve just started learning about electronics and recently bought a dummies book and I’m experimenting on a faulty GPU but I’m really stumped!

I was watching some YT vids on GPU repair but they didn’t go in to enough detail or show closeups of what was being checked so I’m not sure what components need to be checked or how to identify them. On the board next to the SMDs there is text like “R309”, “C168” or “CP802”. I assumed this meant Capacitor or Resistor (unsure what CP is) and when I google the text I don’t usually find anything, even when using the code on the component itself as I’d like to see what the resistance is supposed to be to help identify a short.

Is there anything I can use to get a clearer understanding of what to check and how to ID them components and know what their correct values should be when using a multimeter?

[Roverfan]

I am a learner myself but to identify components you can do a couple of things. If you have a schematic the board designations such as  “R309”, “C168” or “CP802” will be listed with the actual components used. If you dont have schematics the component itself usually has identification on top of it.  Sometimes you have to tilt the board in light or clean the component to see these markings

[Siwastaja]

A GPU is likely overwhelming for a beginner due to sheer number of small parts. Older and simpler devices are easier to reverse-engineer.

In modern electronics, quite a long time ago already, component-level repair has become not completely but mostly obsolete. This is because the reliability of an "average" single component has increased by orders of magnitude, and physical size and price gone down.

For example, a GPU, maybe ignoring a few specialized / more expensive ICs like the GPU chip itself and the memory chips, there may be 1000 components which cost together one dollar, equal to the cost of a single component 50 years ago put in today's money. Same for size; those 1000 components fit the same volume as a single resistor or capacitor or a logic IC from 1960's to 1970's.

Instead of replacing a faulty 0.0001$ resistor (which doesn't even fail ever, basically), if the GPU card fails, the whole PCB assembly is replaced.

All this introduction leads to the fact that it makes no sense for the manufacturer to support reverse-engineering or repairing the PCBs on component level.

R309 and C168 are called designators and just codes applied by manufacturer. Like in pipework, you could choose to say "this is pipe number 1, this is joint number 2, this is valve 3" and label them up. Then you look up the list to see that R309 is actually a part number XYZ128-ASDF47RPBXD. No, it's not a 47ohm resistor; that's insufficient.

So we come to components. We are taught early on that resistors are described by their resistance. This is all wrong, and applies only in the ideal world where resistors are ideal, perfect, cost nothing, and are only manufactured by one manufacturer (or if many, are all exactly identical). Then you can just buy a 100 ohm resistor!

In reality, a resistor has properties that are described within many pages worth of parameters, graphs, drawings. Maybe sometimes you can, but often can't replace a 100 ohm resistor by another 100 ohm resistor. All of this list of properties are identified by the component manufacturer's part number. For a resistor, that's typically 10 to 20 letters/digits long. So now that the designer has made sure XYZ128-ASDF47RPBXD does the job, then it's only matter of buying more XYZ128-ASDF47RPBXD.

So you could not print that part number even into a 1970's resistor unless it was a huge power resistor! Today's miniature SMD part can hold even less information. So down to some 0603 package size, they print a few digits that indeed tell you it's a 47 ohm resistor but nothing more. It's insufficient.

But who cares? The GPU manufacturer buys these components by tens of thousands on reels for example, and the reel has the full part number printed on it. Then they just need to place correct components in the correct places on the PCB. No one can afterwards see by eye what the component actually is, but why would you? Reverse-engineering and repair becomes more difficult but manufacturer has no incentive to support it, and honestly speaking, giving full schematics doens't help in practical GPU or motherboard repair. You just need to know the few typical failure points like certain electrolytic capacitors or the solder under the largest and hottest chip failing.

Manufacturer has the BOM, bill of materials list which lists that hey, R309 is QWERTY-1337-420-XYZ. But today this information won't leak out because the products are not component-level repaired, which means there will be no repair manuals published for the repair staff which would then leak out to the general public.

It still seems to be a tradition to print component designators on the PCB. This is becoming increasingly difficult because the components themselves fill the board, there are not many gaps to place the text. So manufacturers are dropping this; in my designs I have dropped the silk screen designators almost entirely because these designators serve purpose only when next to some specific parts the customer may need to know about, including connectors, jumpers, and possibly some resistors they are supposed to desolder and replace with different values, but this rarely happens.

Truly debugging or repairing a modern high-density board just requires you have the original design files so you can open it up on CAD, on the computer screen, zoom in really close and see all relevant information of the components and also cross-reference to the schematic which includes more logical information, in best case, designer even wrote a note about why they did what they did.

[Electromouse]

That's very informative. And yes it's very overwhelming to see a micro city! I ideally want to create a USB miner for duino coin, I see someone done one on YT but I'd love to solder it all myself. I'll maybe create a new post to see if anyone else has done something with Arduino.