Amira,

You may wish to look at IPC-2221/2222 Design Standard for Rigid Printed
Boards. It describes the issues that should be considered when developing
through hole lands, such as the relationship between the hole, land,
fabrication and assembly processes. It also provides industry standard
tolerances.

You may find more information on these standards as well as other standards
at: www.ipc.org.

Regards,

Gary Ferrari
Executive Director
IPC Designers Council
860-350-9300
fax 413-771-5386