ST STM32F4 Series Application Note page 43

AN4488 Recommended PCB routing guidelines for STM32F4xxxx devices
A better way to route this package and the fan-out signals is to use a through microvia
technology. Microvia will route out internal bumps to a buried layers inside the PCB. To
achieve this, the WLCSP package pads have to be connected to this internal layer through
microvia. In case of four layers PCB, the first layer is WLCSP component, the second layer
will be used as a signal layer, the third layer as the power and ground and the bottom layer
for a signal layout. Figure 27 shows an example of the layout for four layers PCB.
AN4488 Rev 7 43/50
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