we would like to design ADuM5401, but I confused the layout.
as checked datasheet the air distance is 8.0mm here, I saw the ADuM5401 package is 7.6mm
what can I do to increase the distance ??
I assume you need a higher creepage and clearance like 8mm, not the 7.6mm? If so, the ADuM6401 has an RI - 16 package that is longer at the ends for increased creepage of 8.3mm. The ADuM6401 works like the ADuM5401, but has higher isolation capability and as a result can not supply as much output current as the ADuM5401, so check the specification tables for the input output voltage of the application you are using.
If you use the ADuM6401 please note that it is not entirely pin to pin, it has a pin 7 that is VDDL and needs to be connected to power, same as the VDD1 pin. On the ADuM5401 pin 7 is an output called RCout (used for synchronizing the PWM switching frequency of the Viso output of multiple isoPower devices).