I am reviewing a design with a .4 mm pitch, .2 mm ball WLCSP (9x9) on a 4
layer .031 FR4 pcb.

These will be used like a remote control, so there will be force applied,
however there are supports for the PCB.

Due to the presence of switches with cleaning restrictions, underfill is
not possible.

Looking for any comments on potential reliability concerns. The design
could be changed to .064 if that would help.

Thanks in advance!

    Curt


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