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Hello all together,

I'm working on a CmodS6 board for my university course right now. It'd be helpful to now something about the layer design of the Board. I assume it's a multilayer PCB, so at least 1 layer between front and back side, but can anyone tell me something more about it? e.g. common ground plate yes/no?

Hope these information are not somehow confidential or protected, then never mind my question :)

Thanks for any input!

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Hey @TireV

What your are referring to is called the stack-up and refers to the layers of non-conductive prepeg material and copper plating that makes the planes and signal traces. You are correct that as a user of the CMODS6 ( or any small form module for that matter ) has an interest in this part of the board.  The thickness of the module is not necessarily proportional to the number of layers. What is important is how much and how thick the copper on the prepeg layers is because large ground or voltage planes thermally connected to the IC substrate are the only route for thermal energy to be 'wicked' away from the active devices on the module. More layers is not necessarily better.  PC motherboard manufactures for a while there saved cost by using 2 layer boards but there is a lot of engineering and expensive tools required to do this right.

I responded to your question because it's not just a good question but a nice segue into this comment that you brought to mind.

FPGA timing performance is temperature dependent. There are industries that like miniaturized high performance modules and take extreme efforts to minimize and or manage heat. These are very expensive. These modules also [should] come with very detailed testing results that indicate what the upper level of performance the modules can handle and how much heat it dissipates into the system. When someone offers you a very cheap and tiny module you can assume that thermal issues are something to keep in  mind when you use it. It's not just performance that heat has on and FPGA but longevity as well.

I happen to very much like and uses the CMOD-A7 modules, as well as other modules from other vendors. I also keep in mind that it's possible to run into thermal problems if I expect too much out of them. But, like a speciality tool; when you need it it's nice to have around. I have no expectation that a vendor of cheap modules tests much less would specify in their advertising performance, environmental, or thermal information. If I was buying a $100K module with an FPGA then I would insist on this information.

So I offered a narrow slice of a proper reply to your question. I do encourage you to lean more... the information is out there, often as documentation on EVMs that will supply all of the PCB design information as well as the guidelines that were followed in making them.

Edited by zygot

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Consider that the FPGA on your module has 196 balls. The A7 versions have 236. You can answer your own question by thinking about how one gets all of those surface mount pads to ground, voltage and signal traces.

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Hey @zygot,

first of all, Thank you for this detailed answer!
I'm aware of the thermal aspects in electronics in general, but the fact that FPGA-timing is temperature depending was useful!
In addition to thermal issues, I'm interested in EMC, so that's why I was asking about the stack-up and common groundplate. A huge copper plate will likely cause much more Trouble under external Radiation than just single ground lines. On the other side it's a bigger heat sink for distributing thermal gradients equal.

The amount of Connections (balls) is obvious, but unfortunately I didn't check for this in the datasheet... Thanks!

So by now I'm on research but still grateful for any additional comments or Inputs (links, datasheets, ...)

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1 hour ago, TireV said:

I'm interested in EMC

Uh Oh... you're prying open a can that a lot of people would rather not see opened.

When I think of EMC I'm usually concerned with electromagnetic transmission by the product. If you buy just about any evaluation, demo or development board from major vendors, like Xilinx, you'll find some verbiage somewhere that the product in question does not conform to FCC rules and regulations and is only to be used in a lab setting ( let me say that this is a very poor paraphrase here ). You won't see that kind of verbiage anywhere in the packaging or user's manual of FPGA development boards like the ones discussed on this forum. My guess is that there's no such testing and it's gone unnoticed by the relevant organizations. Of course EMC has victims as well as aggressors ( forgive me if I get the lingo wrong.. it's been a while since I spent time in an EMC test chamber ).

You are interested in what kind of  effect an EMC aggressor might have on, say a CMOD6. I'd say you and a lot of other people; some with 'friendly' agendas and some not. Certainly if you wanted to send your CMOD6 on a mission to the moon you'd have to consider this. You'd also have to consider those very high energy Alpha particles flying about. For a while now IC vendors have been accommodating, to a limited extent, the Apha particle situation, mostly for high altitude aviation applications. FPGA devices and a lot of MCU devices have an SEU to detect such events. I'm quite sure that there are a lot of other radiation types an astronaut using your CMOD6 would need to be concerned about. Generally, for extreme environment applications these types of concerns are dealt with on a system as well as component level. If you scrounge around there are application notes on this topic. Start with Actel (Microsemi) as they are big in the 'non-terrestrial' market.

Hmmm.. you're on to asking questions about a class of very interesting related topics for investigation.... 

[edit] One thought that just occurred to me... is that you might want to use an FPGA platform where the gerbers are available as this would provide a wealth of information.

Edited by zygot

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7 hours ago, TireV said:

I'm interested in EMC, so that's why I was asking about the stack-up and common groundplate. A huge copper plate will likely cause much more Trouble under external Radiation than just single ground lines

You might go to Texas Instruments' site (or AD or both) and find documentation for some $500 high frequency ADC or DAC eval board as example to study..

There's nothing wrong with copper planes, generally. Free-standing structures (such as non-connected filler polygons) can be bad, if they resonate. So are loops if the driving wire spans an area together with the GND return wire (for which the ground plane is an obvious solution).

The worst resonators have high quality factor meaning loose coupling meaning it can be surprising how the energy managed to couple in. There's no such thing as too many ground vias...

Note, your ground plane can do very interesting things in combination with the metal box it's in (resonant cavity) but that's a different story.

 

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It's easy, even for well trained people to develop mistaken ideas about PCB design. I've been involved with PCB design for digital and analog signals up to about 3 GHz. The analysis and rules for UHF or microwave is very different than that for 100 MHz. I'm very reluctant to make any broad generalizations that might be interpreted as guidance. I'm not countering the advice of @xc6lx45; just making a comment.

Yes, I agree that you can find very good guidance from IC manufacturers who want their customers to be happy with the performance of their devices. Often very good general rules are available as application notes or EVM board design files. Some take great care in explaining why they chose to place parts, panes, traces, etc. the way they did. I never ignore such help.

Designing a PCB to mitigate radiating energy in controlled spectrum is a bit different than thinking about designing a PCB to be less susceptible to external radiation. Often susceptible circuitry is shielded by a cage. You can find these on many a Xilinx development board.

From the perspective of a PCB as a victim of electro-magnetic radiation I couldn't put a boundary on the frequency range of interest.

Warning, long old man tale follows:

I may have mentioned this in another post but many years ago I interviewed for a job with a high-end modem company. One of the engineers who took me for a tour explained how he had changed the whole philosophy of board layout by eliminating all planes and just having return traces to provide a path for return to ground currents from every component. After a long discourse he asked what I thought of his idea. I had to tell him that it really didn't make much sense or conform to what I had already learned. We had a brief discussion about this but who wants to get into a brawl with some nut interviewing you for a job? Later interviews with higher level managers confirmed that everyone in the company was grateful for the new direction for a number of reasons. Needless to say that, since I didn't choose to drink the cool aid , I wasn't considered a suitable candidate for a position with that company. Many years later I had an interview with another company and during a discussion with one of the engineers I found myself relating this prior experience. The guy interviewing got all red in the face and after a bit of silence told me that he was the engineer responsible for that change in design theory and that it ended up being a fiasco. He mentioned that he missed a few things in his analysis. I didn't make a suitable candidate for that company either....

If you are going to design a PCB or use a high performance component then you better avail yourself of all the reliable information you can find. Better yet work for a company that has years of success doing verifiable products that meet specifications under rigorous testing. For most of us general rules of thumb will do for the kinds of boards that we can afford to make. There are a few good texts but IC and microwave and RF component manufacturers still supply good advice if you can find it.

Edited by zygot

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