My chip acts weirdly when I cut the contact of the PCB from the ESD Mat or if I lift the box that surrounds the die

[doenisz]

I have an RF energy harvester chip that I am testing at the moment. To control the charging (sleep) of a storage capacitor and the active period where it is connected to the communication/digital blocks; I use an hysteresis comparator. I test it currently without connecting the rectifier output to the rest of the system to verify the latter. So, I provide DC voltage by myself as if it's the rectifier output, which powers up a charge pump that's connected to a storage capacitance.

Under normal operation, the voltage on the storage capacitor looks something like this (yellow on oscilloscope screen):


Normal operation

As you can see, after it reaches a certain voltage, the communication signals (green and blue on the above figure) gets triggered, discharge the storage capacitor, and after a while, we return back to charging.

Now, I have observed two very interesting phenomena during the testing:

1. The discharge/communication period was initially very short. It was only after I connected the ESD mat itself to the ground by hooking it up to an oscilloscope probe that it started acting as it should.

My thinking for this is that the ESD mat was somehow not properly grounded initially. The cord going to the outlet is fine but the cord comes to a black sort of box, and at the bottom of it there is a metal circle that touches the mat. However, the mat is a little torn there so the connection was loose. That's why I had to hook an alligator to the mat myself, coming from the oscilloscope ground.

2) If I lift my board and cut its contact with the ESD mat, the voltage on the storage capacitance keeps rising but the hysteresis comparator does not trigger; so communication period cannot be activated.

Bottom of my board looks like this, so the solders do touch the ESD mat during the operation.


Bottom of the test-boards

3) Same behavior in (2) happens if I lift the box in red in the below figure, even when the board is in contact with the ESD mat:


Box surrounding the antenna and the die

This box is just for protection because the die is wirebonded with chip-on-board process.

My thinking for (2) and (3) is as follows:

In this current testing, the output of the rectifier, which will normally be connected to the rest of the chip is floating. This rectifier output is one pad of the chip and another pad of the chip takes a DC voltage that powers up the rest of the chip. For testing and debugging purposes, they are different pads, and in normal operation, I simply short them with a jumper; but in this particular test, I give a DC voltage externally to the second pad, while the rectifier output is floating.

The antenna is designed to be matched to the rectifier when there is no chip cover, so when I lift the cover, I may be establishing the matching, which would increase the rectifier output even more; hence disturb the operation, even when the board touches the mat.

So, maybe this accumulated voltage on the floating pad is what is disturbing my operation? When the board touches the MAT, this accumulated charge can simply go away. I cannot see physically, how the presence of that signal would disturb the rest of the chip, though.

What may be the reasons for these things that I am observing?

Any input is appreciated.

 

[dick_freebird]

An antenna often likes a ground plane, which the mat may provide?

 

[doenisz]

An antenna often likes a ground plane, which the mat may provide?

My antenna is a simple loop. It does not radiate with a ground plane, so maybe when I lift the board, the antenna starts radiating or receiving. However, as I said, it goes to a rectifier whose output is floating so why would it affect the rest of the chip?

 

[Akanimo]

Let's get some things out of the way. place an insulation tape on the solders at the bottom of the board, place the board back on the mat and operate it. Please, let's know the outcome.

--- Updated Oct 25, 2022 ---

However, I think the issue is EMI. Other than for protection, that box acts as an EMI shield. ESD is a significant source of EMI, so the ESD mat actually does help with EMI attenuation.

In addition to the ESD mat, are you using a wrist strap?

 

[Akanimo]

@doenisz Reviewing your posts, I realised the box can't possibly be an EMI shield which I thought it to be.

--- Updated Oct 27, 2022 ---

However, It seems like the box is affecting it's impedance. Does the antenna not need a matching network?

 

[doenisz]

Let's get some things out of the way. place an insulation tape on the solders at the bottom of the board, place the board back on the mat and operate it. Please, let's know the outcome.

I did that and a few other tests.
I have interesting results to share.

First of all, as I said, when I lift the chip and there is nothing underneath it, it doesn't work. However, when I put it on top a book, it worked again. Not as good as when it was on top of the ESD mat, but still. I would say it's a middle-ground between touching the mat vs floating on air.

Something very weird happened while I was playing out with different setups. When I lifted the right area of the board on the top figure (where there is the antenna and the die), it did not work. However, when I kept the right-hand side close to the mat and lifted only the left portion, it worked.
Also, when I placed the entire board on top of a book, the chip also worked, even though as good as the case on mat.

This makes me think that it's about a ground plane issue. The mat (or even the book) indeed provides a ground plane.

 

[PlanarMetamaterials]

What frequency range does this RF system operate at?

 

[doenisz]

What frequency range does this RF system operate at?

3.5 GHz

 

[Easy peasy]

So you are trying to couple to a weak 3.5GHz RF source ? can we see the source please?

All of your problems are easily explainable by an expereinced RF engineer - a lot of them are related to the wavelength of the RF energy

 

[PlanarMetamaterials]

This makes me think that it's about a ground plane issue. The mat (or even the book) indeed provides a ground plane.

ESD mats typically contain conductive fibers to shuffle any high-potential charges away. At these frequencies, many traces on your board are more than half a wavelength long -- most likely you are exciting a parasitic mode that exist between the mat and the PCB. I can't comment on your book, but I think your data suggest you are exciting some sort of mode that has a relatively large modal volume and extends far from the PCB. This is usually a sign of inattentive EMI design (which you may not have even been considering).

I'm not sure what "floating" means exactly in the context of this circuit, but in general, it's a bad idea at high frequencies. Many EMI issues are solved by keeping as many conductors as possible at the same potential, usually by some form of bonding.

 

[doenisz]

ESD mats typically contain conductive fibers to shuffle any high-potential charges away. At these frequencies, many traces on your board are more than half a wavelength long -- most likely you are exciting a parasitic mode that exist between the mat and the PCB. I can't comment on your book, but I think your data suggest you are exciting some sort of mode that has a relatively large modal volume and extends far from the PCB. This is usually a sign of inattentive EMI design (which you may not have even been considering).

I'm not sure what "floating" means exactly in the context of this circuit, but in general, it's a bad idea at high frequencies. Many EMI issues are solved by keeping as many conductors as possible at the same potential, usually by some form of bonding.

Thanks for your input. By floating I mean that the rectifier output is not connected to the rest of the circuit, but rather I supply a DC voltage to the rest of the circuit by having these two at different pins - which will normally be short-circuited in the standard operation.

I discussed with some of my colleagues and arrived at the rather uncertain conclusion that because the PCB does not have a ground plane, i.e., no conductor underneath the PCB, it somehow makes the chip malfunctioning. When the PCB is on the mat (or even a book) it somehow provides that and makes the chip working.

I arrived at this conclusion based on my Nov1, 2022 message.

--- Updated Nov 21, 2022 ---

So you are trying to couple to a weak 3.5GHz RF source ? can we see the source please?

All of your problems are easily explainable by an expereinced RF engineer - a lot of them are related to the wavelength of the RF energy

No, in this particular testing scheme, I only connect a DC voltage as if it is the rectifier output. I do not connect any RF source to the circuit. Perhaps the antenna itself picks up some radiation which I doubt so. Even if it does, the rectifier output is not connected to anywhere. In the standard operation, it will be short-circuited to the pin that acts as the DC input for the rest of the chip but for testing purposes, I connect a source to the latter and make the former floating.

 

[Easy peasy]

"My chip acts weirdly when I cut the contact of the PCB from the ESD Mat or if I lift the box that surrounds the die "

not much to go on - as you will agree - no mention of what the chip is - or what "weirdly" means - not to much help to readers is it ?

As the chip appears to be energy harvesting - what are you really trying to do - show a ckt with your " DC power " attachments - then readers may be able to make useful answers ....