I use hooks 
/etc/pacman.d/hooks/98-clear-paccache.hook
[Trigger]
Type = Package
Operation = Install
Operation = Upgrade
Operation = Remove
Target = *
[Action]
Description = Clear pacman cache...
When = PostTransaction
Exec = /usr/bin/paccache -rk2
/etc/pacman.d/hooks/98-clear-paccache-unused.hook
[Trigger]
Type = Package
Operation = Install
Operation = Upgrade
Operation = Remove
Target = *
[Action]
Description = Clear pacman unused cache...
When = PostTransaction
Exec = /usr/bin/paccache -ruk0