CNC milled RF shield

[breelizhong]

Hi All,
I'm working on CNC milled type RF shield with aluminium, it is perfect for small production and prototype, as the setup cost is almost zero but unit price is high, like a few dollars. There will be no solder needed, as they will be fixed to PCB by screws.

The partition walls will sit on the soldermask removal area, and they contact very good from seen.

Is any one who also working on this type of shield? We may share some experience.
Bree

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[pinout]

I'm not an expert on this type of design, but from my experience this is an expensive solution and not a good RF shield when considered as a whole including the pcb.

It will suffer from long term problems where it is in contact with the solder due to oxidisation and possible corrosion.
It is probably mainly an electrostatic screen as you do not have continuity around the circuit, needs to include pcb traces both sides of board.

It really depends on what your circuit is but I would suggest investigating pressed steel (tin) enclosing the pcb.
If it it just to screen electrostatically consider laying out the circuit to allow a rectangular enclosure and investigate screen options for this.

Let us know what the screening/shielding relates to for a more informed discussion. Hope that helps.

 

[jiripolivka]

Similar complex-form shields are mass-produced of plastic and covered with copper or aluminum layer. Good contact is often made from a braid laid in grooves in the shield form, to optimize the contact between the shield and a PCB or the other half of the shield. Open a cell phoe to see the details.

 

[breelizhong]

**broken link removed**
I have taken a few pics but not clear, sorry.
Observing from that, the shield touching the soldermaks copper not bad, but for some area without screw, there looks like a very thin gap there, do you guys think this will affect the shielding performace?
About the oxidization, I'm not sure how this will be after years later. The material chosen is Aluminium Alloy 6262, this is a corrosion-resistant type, but how to understand this corrosion resistant? does it means this material can still be conductive even after many years?
Another issue is, the contact is hard to hard contact, so this contact is just some point to point contact. This is difficult to solve, but from seen, they are mating nicely.

 

[pinout]

You will find that it is the length of the slit (lack of contact) that determines when the slits passes the higher frequencies, so your connection points or mounting points will provide (as a minimum) good contact points and determine the high pass nature of the 'gap'.
Conductive rubber of the right material can be a good solution as can beryllium copper fingers.

My concern with aluminium is the surface oxide layer (insulates) or the anodising protection which insulates, I think it is 'passivation?'

But my other concern is that it has to 100% cover, not just on one surface of a pcb for good EM screening. Even the PCB thickness will be a 'gap' for RF.
Electrostatic screening is a different issue, but then even a thin bit of foil will be good.

Hope this helps....

 

[jiripolivka]

A good screen may be tricky.Contact to PCB can be best soldered but a braid helps, too. Over time, corrosion is a problem not only with aluminum. Gold-plating is costly but stays much longer.
Openings must be treated as waveguides, best closed. Better, soldered. Good coaxial connectors and capacitive feedthroughs.