Join our next Ask Me Anything about PCB stack-up where you can ask us questions on:
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Post questions before: January 8th
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I’m not sure if I’m allowed to ask about FPCs, but if so…
Can you cover topic on GND plane between analog and digital circuit? More layers give option, but lower layer count save $$ for volume design.
What is the typical tradeoff when using a material with a higher permittivity? Of course, the width has to decrease and/or the substrate height has to increase to keep Zo the same, but what usually happens to the loss at a given frequency? How about on dispersion?
In a mixed-signal circuit design where both analog and digital circuits exist on the same PCB, a “ground plane between analog and digital” refers to a shared ground plane that connects both analog and digital sections, usually with a single connection point called a “star ground,” to minimize noise coupling between the sensitive analog circuits and the potentially noisy digital circuits while ensuring they share the same reference voltage level.
Key points about the ground plane between analog and digital circuits:
The primary goal is to prevent interference from digital switching noise affecting the delicate analog signals by providing a separate return path for each circuit while still maintaining a common reference point.
Most commonly, this is achieved by connecting the analog and digital ground planes at a single point, known as a “star ground,” to minimize ground loops and ensure a clean reference voltage for both sections.
While sharing a ground plane, careful PCB layout practices are crucial to physically separate analog and digital traces and components to further minimize coupling.
In some very sensitive analog designs, a completely separate ground plane for the analog section might be necessary, but this is generally considered a last resort due to potential complications in managing the shared reference level.
The primary tradeoff is the cost versus performance issue. Higher dielectric constant means higher loss. For a higher dielectric constant width would increase or height would decrease. Dispersion would likely be worse.
Hi, it’s difficult to talk cost without seeing the design and Gerbers. The cost will probably increase based on the number of bends but I’m told it shouldn’t be much. Finally, a 2-layer FPC will likely be more expensive than an FFC.
Is your design ready?
Hey Lucy! Thanks for responding!
I don’t have a design ready yet, but on that note, is there an FFC design guide? I’ve seen one for FPCs, but haven’t found one yet for FFCs from Sierra.
No, sorry. But thanks for the idea. We could add a section on FFCs in our Flex Design Guide.
Have you reviewed your answer with the answers of an AI? Can you explain the contradiction?
Hi Lucy,
Can you provide some general guidance on the trade-offs between broadside coupled and edge coupled differential pairs?
Thanks,
Jim
Weird question coming from a TTM RF/EMC engineer…
Could you clarify what is the contradiction you’re referring to?
Hi Lucy,
Differential pairs can be structured as edge-coupled or broadside-coupled.
I was wondering if you had any general guidelines as to why one would be selected over the other.
For example:
Any other general guidelines that you can add would be appreciated.
Thanks,
Jim
For a beginner, is it the chicken or the egg? In my experience whatever I spec when it comes to stack-up, gets changed by the Fab vendor. Even if I copy an existing design that was already successfully built. So, should I expect the fab house to give me an initial stack-up if I give them my requirements? My understanding is that they would need the order placed before they give me this info. Also, I get lost in all the material choices out there. Is there a place to see a list of the common materials used for specific needs and their approximate pricing info?
I’m going to give you a few resources that will help you for sure.
Let’s start with the stack-up part. Sierra offers stack-up design reviews even before you place an order. You can meet with our experts who will discuss your requirements and give you options. We will just ask you to provide this:
We also have a free Stackup Designer tool that builds your stack-up.
As for materials, we have this great blog on PCB Substrates: Knowing PCB Dielectric Materials | Sierra Circuits
We also have a free tool that gives you material options based on your requirements, the Material Selector.
For pricing, I’m afraid I can’t help much. That requires quoting but if you speak with your fab house, they will be able to give you an idea of how much it’ll cost you.
The dissipation factor(Df) of the dielectric plays a major role than the Dielectric constant(Dk) in signal loss. lower dissipation factor means low loss at high frequencies. typically you will find materials have both low Dk and low Df.
Assuming the same dielectric height. to obtain the same impedance for the low dielectric constant a bigger trace width is required. for a compact design and low pitch ICs big Control Impedance traces are not optimal. So if the trace width is getting too big some material options are available with low Df and high Dk.
Trace width plays a role in loss as well, high trace width gives low conductor-related signal loss, and as trace width decreases loss increases.
Finding the right balance in all is important.
Do you have “preferred” or “standard” stack-ups that you would like customers to use say for:
50SE,90Diff,100Diff,75SE ohms
I have tried to incorporate these requirements into your “No touch” stackup and find that most would would require not ideal trace widths for a moderately dense board.
Are there reasons to use different types of prepreg in the same design?
I have an engineer that likes 1080 and 2113.
Yes, broadside diff pairs are more used for lower differential impedances .
Nowadays, broadsides are hardly used or preferred.
