Many electronic components are classified by a certain ingress protection rating. Parts that are prone to get in contact with water are often classified as IPX7, i.e. the part must withstand submerging in water at 1 m depth and not allow for water intake large enough to cause the part to malfunction afterwards.
For production of these components, the question arises what leak rate requirement a part needs to be tested for when classified to fulfill IPX7.
To answer this question, we first need to understand the failure mode of such parts. When submerged under water at 1 m depth, the pressure of the water at 1 m depth is 1.1 atm. The water pressure is the driving force to press water through any given leak geometry. However, for very small leaks, the surface tension of water will prevent water “flowing” through a leak geometry. For example, we have all seen a drop of water hanging at a water tap, but not falling down.
To calculate the largest hole size that will be clogged with water, but not allow any water through, one has to make some assumptions:
Housing material | Contact angle for water | Largest leak rate diameter that will not let water through |
---|---|---|
Aluminum | 9° | 5 µm |
Steel | 90° | 29 µm |
PVC | 40° | 19 µm |
PE | 78° | 28 µm |
PC | 81° | 29 µm |
ABS | 83° | 29 µm |
Under these assumptions, one can calculate the largest diameter of a leak that will be completely filled with water, but not let any water go through from the wetting angles (α) for the different material combinations.
From the largest diameter that will still clog, we can now calculate leak rates for different leak channel length (equivalent to the wall thickness of the component housing). The below graph shows the relation between wall thickness of the component housing and the leak rate equivalent for different material combinations at 1.1 atm outer pressure and 1 atm inner pressure. The corresponding leak rate for 100% helium is approx. 7% smaller (below resolution of the following graph).
The above leak rate limits are under the ideal conditions mentioned earlier. In real world applications, several differences to the theoretical analysis can be observed:
Hence, real leak testing requirements are slightly less demanding than theoretical limits. As a rule of thumb leak rate requirements for IPX7 can be summarized in two classes: