Why does Samsung include useless capacitors? [closed]

Question

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 -- 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 -- This is on the reverse side of the PCB from the USB commutator IC. It's right next to the battery connector. 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.

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*.

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

Answer 1

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 "TANJ 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.
https://i.sstatic.net/Fl5fV.png

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

Answer 2

At first I thought it might be purely mechanical, maybe a way to keep people from bumping the BGA part off the board, but the other two pictures suggest that it is not as the caps are surrounded by many other parts.

There is some commonality between all three designs:
1) They are placed next to circuits. One of them is a boost\buck DC to DC circuit.
2) They are all the same size.

They do not have the same thermal relief to ground

I'll bet these are test points, they are always located next to circuits and it would be easy to probe. If you were cheeking different components with a tweezers probe you could always know which component was the ground reference. It may also be useful during EMI checking to see what that top ground plane layer is doing, and if it's really ground.

They may also serve some other RF purpose but I seriously doubt that, if they did the thermal relief would probably be similar to produce a similar result with parasitics. At very high frequencies a cap such as this would change the ground plane impedance, for what end I can only speculate.

EDIT

I decided to simulate the parasitics of the board and the nulled capacitor, for this I estimated 0.25oz copper (for that many layers it would have to be very thin and no need for most circuits on the board to have a great current carrying capacity)

I estimated 3 mill traces for the leads into the capacitor, a 0.1uf capacitor in an 0402 size, which would have about 0.7nH of ESL and 30mΩ of ESR.

I also threw in an estimation for the copper around the outside of the cap, which will not be super accurate because ideally this would need to be simulated by Finite Element Software (FEM) to really find out what is going on, but the bulk resistance and inductance can give an idea of what is going on.

The results were surprising, I probed the points directly on the other side of the capacitor, and got a high pass filter, but it does have a 10dB of blocking. In conjuction with the vias this may be useful for passing EMI tests. This is just an example of a best case situation, to really model this you'd need to use an FEM

Answer 3

That might be a feed trough capacitor, it's not clear from the pictures. Feed through capacitors are usually used in RF circuits and are designed to be connected to ground on the edges and have a center pad for the other terminal of the capacitor.

EDIT

The new picture reveals no pad under the capacitor so it's not a feed through capacitor but I'm leaving this answer because it might help others to identify feed through capacitors.

Answer 4

Possibly a crazy thought, but it could be process control. The caps all are near large metal objects, which are can prevent the board heating properly during reflow. The double grounded caps are larger than their neighbours, and with 2 connections to the ground plane make them the most likely candidates to not solder properly if your pushing the limits of your line speed. You can use them as a single point of automated optical inspection to check rather then checking every component therefore increasing throughput.

Just an idea, I have not seen anything like this before

Answer 5

Under high frequency conditions, a metal plane is not a continuous, equipotential conductor, but instead acts as a resonant structure due to distributed R and L, as well as geometry, i.e. fringing fields. This is how microstrip antennas function.

The result is that the fields and impedance of the ground plane varies spatially. See, for example, Page 16 of this presentation. The only way to accurately see this is via FEM simulation because of the non-regular shapes in the PCB.

The capacitor is analogous to a tuning post or a varactor in a waveguide. By linking the fields between two points on the ground plane, the resonances will shift spatially and in frequency by some desired manner.

Normally, this is done by a decoupling cap between power and ground. I suspect the purpose of this capacitor instead is to shield the nearby circuit from any RF signal induced on the ground plane by the wireless transmitters.