Back migration in vacuum systems can be caused by several factors:

1. Leaks: One of the main causes of back migration is leaks in the vacuum system. Even small leaks can allow gas molecules from the external environment to enter the vacuum chamber, leading to an increase in pressure and reduced vacuum effectiveness. Leaks can occur at various points in the system, including seals, gaskets, valves, and connections.
2. Temperature changes: Changes in temperature can also cause back migration. When the temperature inside the vacuum chamber increases, the gas molecules inside gain energy and can escape back into the external environment. Similarly, when the temperature decreases, gas molecules from the external environment can enter the chamber.
3. Outgassing: Outgassing refers to the release of gases from materials used in the vacuum system. Some materials, especially those with high vapor pressure, can release gas molecules over time. These gas molecules can migrate back into the chamber, increasing the pressure.
4. Residual gases: Even after thorough evacuation, there are often residual gases present in the vacuum system. These gases can be difficult to remove completely and can contribute to back migration.

To prevent back migration, it is important to design vacuum systems with proper seals, gaskets, and valves to minimize leaks. Regular maintenance and monitoring of the system are also crucial to identify and address any potential sources of back migration. Techniques such as bake-out and degassing can be used to remove trapped gases from the system.

Source: https://www.lesker.com/newweb/vacuum_technology/vacuum_technology_handbook/vacuum_system_design.asp