Helium Bombing Test

The bombing test is a special leak test for small and previously hermetically sealed parts which have an internal cavity like transistors, diodes, microprocessors or small relays. It cannot be used on plastic covered semiconductors (permeation). To get helium into these test pieces which are already tight, a large quantity of parts are placed into a pressure chamber and this chamber (bomb) is filled with helium to a certain pressure.

On a leaking part, helium penetrates into it but not into leak tight parts. After a certain bombing time, the parts are placed (also in large batch) into a test chamber and this chamber (bell jar, test fixture) is evacuated by a helium leakdetector. If there is a leaking part in the batch (as indicated by the response of the leakdetector), the batch is divided and each division is retested. After several divisions, the leaking part is located.

The measured leakrate is not the real leakrate. During the bombing time, the part may be not filled to 100%, also, after taking the parts out of the bomb, there is a waiting time, until the part is being tested. During the waiting time helium is escaping already, and if the leak is large, all helium may have escaped before the part could be tested.

This shows that, with the bombing test only small leaks can be discovered. A second test for larger leaks is necessary. This second test in most cases is a bubble test. The parts are placed into a hot liquid. The air inside a leaking part expands and a bubble escapes.

The filling process of helium into a leaking part is not linear. As the pressure inside increases (i.e. the lower the differential pressure), the slower is the filling rate. So the filling rate as well as the outflow rate follow an exponential function.

The formula, which descibes this process is shown below:

q_A = \textup{measured leakrate}

V = \textup{internal volume of the testobject}

p = \textup{bombing-pressure}

L = \textup{the real leakrate}

T = \textup{bombing time}

t = \textup{waiting time}

(* the value, which would be measured, if the testobject would be filled with 100% He at atmospheric pressure).

This formula cannot be solved to L (the real leakrate). L can only be found by trial and error (Iteration). Therefore leakrate specifications in most cases specify the bombing time, the bombing pressure and the maximun waiting time for the quoted maximum leakrate.