To have an encrypted home partition mounted automatically when logging in, you can use pam_mount. It will mount your /home (or whatever mount point you like) when you log in using your login manager or when logging in on console. The encrypted drive's passphrase should be the same as your linux user's password, so you don't have to type in two different passphrases to login.
- Install pam_mount from the AUR: 
- Edit /etc/security/pam_mount.conf.xml as follows:
Insert a new line at the end of the file, but before the last closing tag, </pam_mount>. Replace
- USERNAME with your linux-username
- sdaX with the corresponding device.
Add mount options, if needed.
Login Manager Configuration
In general, you have to edit the corresponding file in /etc/pam.d . After adding some lines, pam_mount will be called on login. The correct order of entries in each file is important.