Here are some comments I received from a design engineer:
- The PCB must be the proper thickness otherwise it won't plug into the M.2 connector, so stating min = 0.8mm isn't correct; its thickness must fall within the proper min/max tolerance range for the connector.
- The design must be 4 layer at a minimum to ensure proper ground referencing for the PCIe lanes. The high speed PCIe signals should be routed with a trace width to achieve 50Ω impedance to the reference plane and spacing for 85Ω impedance differentially. What that width and spacing needs to be is entirely dependent upon the PCB stack up and must be calculated based upon the stack up from the PCB manufacturer.
- These rules are important for good performance, I've seen poorly routed adapter cards that down train to gen 1 speeds.
Hope this helps,
Like always is Scott great with these answers.
But to be honest, I had wished more informations. Like a typical 4-Layer PCB stackup with the corrsponding thickness" and associated ground planes
Or something like a Design guideline.
For example I had done something like this.
1.Layer : Foil+plating
4 Bottom Layer (signal)
There is already existing PCIe-design guidelines. But it is not applied to M.2 slot. There is a lack of Information regarding this.
There also a lot of criteria to consider like differential impedance, maximum length, L/W. Target Z
I expected something like an application note or strong advises to do the best design.
But I think it's ok
Thanks Scott for the answer again