Electronic – Why does Samsung include useless capacitors?


I do component-level repair of tablet mainboards, and I have seen this puzzling situation on two different models of Samsung tablet mainboards so far (SM-T210, SM-T818A). There are ceramic chip capacitors on the PCB that are clearly connected to the ground plane on both ends. Resistance checks confirm, plus it's pretty obvious just looking at them.
SM-T210 (first location)
SM-T210 — This looks like signal conditioning of some sort. It's on the reverse side of the PCB from the SD slot but SD uses more than two signal lines so I dunno.
SM-T210 (first location, part removed)
SM-T210 (second location)
SM-T210 — This is on the reverse side of the PCB from the USB commutator IC. It's right next to the battery connector.
SM-T210 (second location, part removed)
SM-T818A — This is the AMOLED power supply. The mystery cap is actually located at the edge of an EMI shield (removed for the photo) and the shield frame had to include a cut to clear the cap. So they went to some trouble to have the cap right here.
SM-T818A (part removed)

The only scenario I can come up with is that during Capture the design engineer placed a bunch of caps for eventual use, but connected both ends to ground so the DRC module wouldn't complain about floating pins. Then they ended up not using them all but didn't delete the extras from the design. The design gets sent to a Layout engineer, who simply places and routes the design they've been given.

I'm willing to allow for somebody doing something so intelligent and wise that it's beyond my ken (filtering terahertz-band noise from the ground plane?), but I don't think this is an example of that*.

*Of course, that's exactly what I'd say if it was an example of that.

Best Answer

There are four comments on this reddit thread that may be on to something:

By silver_pc:

could it be a form of 'paper towns' on maps - AKA fictitious entry to identify direct copies?

By toybuilder:

Not that they are necessarily doing this, but I've heard it said that mass manufacturers will keep removing capacitors until their product stop working. (Certainly, it was common to see PC motherboards with unpopulated decoupling cap pads all over the place back when I used to hand-build PCs.)

If you have a mass-production setup to stuff boards and do automated visual quality inspection, maybe you don't want to take the downtime hit to reprogram your production line as you introduce and monitor ongoing production changes with the ultimate goal of removing the capacitors. If so, you could nullify the capacitors by stuffing them as before, but with both pads on the same plane.

Samsung manufactures capacitors, so maybe they're a bit more willing to burn through a short run of boards with wasted capacitors if, in the long run, they can more definitively get rid of them.

Keep in mind that large companies like Samsung have the ability to test their products for certification purposes in-house, so it's probably cheap enough to run a small batch to test and accept/reject. And if accepted, to just release it into the market.

At least, that would be my guess.

By John_Barlycorn:

I believe this has more to do with manufacturing process than it has to do with electrical purpose. Modern electronics manufacturing is bat-shit insane with regard to speed.

We're talking about robotic movements that are so fast, that air resistance and machine vibration have to be considered.

The position of parts that feed the pick and place machines is critical to the speed of operation. So they spend a lot of time on setup. Then press "Start" and watch her whirl. So if they end up with 2 products that are similar, they have to go through this expensive setup change run by an expensive engineer to switch them out. But these caps are so cheap that after you consider this setup change, it might actually cost them more money to remove them during different runs. They might just say "Fuck it" and let them populate them despite not needing them.

My father worked in the industry for years, and had some experience in smaller volume stuff. In manufacturing this sort of backwards logic is not uncommon. You do what's cheapest/most profitable which is not always the least wasteful option.

By CopperNickus:

There are other planes in a tablet: the display and case. Maybe the answer lies in the third dimension. Might there be a brush/spring contact or some other connection on another layer of the device that completes a circuit when the tablet is assembled? That technique is used in their cellphones to mate various internal boards to the back and case.

In the phones, it's spring contacts mating to gold or silver contacts when the device is assembled.

Or perhaps just some proximity based RF control related to the display?