holes on shielding case ?

[xinxin]

I found some dimensions of holes are different on shielding case of mobile phones boards, and their locations are different . I know it deponds on wavelength of interested , but why some are a little biger and others a little smaller? Could u tell me the reasons?

 

[flatulent]

Generally, the leakage through a hole is dependent on the largest dimension. The placement of holes has a second order effect on the total shielding. The holes you saw may have been different sizes for reasons other than shielding. Air flow is one of these possible reasons.

 

[r-snake]

But what the purpose are these hole for? SMT, thermal, or some other?

 

[flatulent]

I would suspect that they are either for thermal air flow or to get to test points on the circuitry underneath.

 

[xinxin]

i already check the holes, it should not for test purpose , maybe for thermal dissapation , but its location is mostly around the chip. Is there anyone tell me how to decide to a size of hole if as a shielding purpose? How much should it compared to a wavelength ?

 

[vfone]

The reason of the holes (actually they are vias) with different diameters and different distribution on the board is mainly for shielding. Usually the thermal vias are under the chip. The vias under PA for example, shall be very careful calculated and distributed, using a thermal simulator.
Random distribution of the vias on the ground edge will improve the wide-band shielding. If you keep the same distance and the same diameter of vias, is possible to have some high frequency resonances (is like when you design a microwave filter). This is happened mainly due to parasitic inductance of vias.
In a handset design also have to have in mind that bigger through vias are more expensive to make than smaller ones.