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. As a requirement the crypted drive's passphrase has to be the same as your user's linux password.
- 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 and 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.