What is STPM3x's max SPI clock? I got 5MHz working but not 10MHz. Anyone get a clue? Cheers

What is STPM3x's max SPI clock?

The datasheet (always a recommended read) specifies 50ns clock period, meaning 20MHz clock frequency.

These boards are connected by flying wires(5cm). The max SPI clock could be influenced by the flying wire?

Are you familiar with Laplace/Fourier transformations ?

Rectangular signals like the SPI signal can be expressed as a series of sinusoidal waves of increasing frequency.

Your cabling needs to support a bandwidth of about 100MHz for you signals, else the distortions make it increasingly unrecognizable.

or Do I need a impedance resistor?

Would probably help. And twisting each signal line with a ground wire, to get a consistent impedance.

You can check the signal quality on the (signal) receiver side - if you are familiar with RF measurements.

Probe impedance should match signal/cable impedance as well, else you measure rubbish ...

It seems the max SPI clock period by STMPS is 50ns ==> 20MHz?

The quality of the signal maybe important, check out the SPEED value for the STM32 GPIOs

Make sure the ground is well connected (current = ground loop path = inductance area)

Not a specialist on this slave device.