MTBF = 1 / FIT.
FIT for this part is 4FIT(accroding to PSoC5 LP Device Family qualification report https://www.cypress.com/file/126316/download)
4 FIT is 4 x 10E-9/hour. So the MTBF = 1/FIT = 1 / 4 x 10E-9 (hours) / 24 (hours/day) / 365 (days/year) = 28539years.
However, I don't quite understand how to use the microcontroller MTBF to define the reliability of the PCB.