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Case Studies

Case Studies

Masach Drawn EMI/RFI Shields – Case studies #01

Every now and then an unusual application for EMI/RFI shield configurations can arise which might require some lateral thinking to solve the problem.

 Case Stadies #01 - image

Fig.1

 Fig.1 shows an example of a circuit working in a 3.2GHz environment with multiple strip array filters, coupled together with a variety of processing circuitry. In this particular application, engineers needed to isolate discrete processing circuitry from each other and yet maintain a reasonable level of accessibility to each circuit module.

The most efficient mode of isolating modules in a tight circuit would be to design a tuner type integrated shield (Multi cavity). The problem with this solution is that partition seams between the circuits would have to be soldered or welded. Furthermore, the cover of an integral shield at such frequencies requires significant contact between partition and screen.

These shields yield excellent results in many applications, however in this series of circuits for this particular customer the results were not good enough. EMI/RFI engineers specified for a shield with absolutely no apertures and with availability of multiple access to each shielded module. This required an extremely easily removable cover from each module and yet provide adequate EMI/RFI integrity upon closure.

 
 

 

Masach Drawn EMI/RFI Shields – Case studies #02


Does one design for functionality, manufacturability or accessibility? This is the traditional argument between the design engineer and the fabrication workshop. Try desoldering 5 or 6 small EMI/RFI shields assembled onto a tiny PCB and the answer to the above would be obvious.

 

Two-piece shields (Fig.1), that is a frame soldered to the PCB and a cover which can be snapped on and off easily, are a godsend for field service engineers.
Cover Only shields (Fig.2) provide cheap and functional solutions to interference problems, however they may end up becoming an expensive headache. Any repair or rework under the shield requires a messy and expensive solder wicking.

One usually finds these solutions on cheap and proven reliable mass produced circuits, such that when a fault is traced to the area under the shield the solution is simply to replace the print. Conversely, should the print contain expensive components what could be more convenient than flipping off the cover of the shield and replacing once the repair is done.

Two-piece shields are indeed double the cost of a Cover Only, however during assembly they can prove themselves very cost effective. Consider a fully assembled print with a shield ready for reflow. The frame of a Two-piece shield allows the heat to reflow into the circuit area soldering all components within the frame, as well as fixing the frame itself to the circuit.

With Cover Only shields, it is often that components are soldered first, with covers being passed through a secondary reflow. These covers also must have holes (Fig.2) to enable gases to escape during the solder cycle as well as providing access for flushing and cleaning. The simplest and most effective way for snap-on lids are via dimples on the inner side of the cover, seating into respective holes on the frame (Fig.1).

Other solutions are possible, but invariably more expensive. Internal dimension of covers are exactly the same as external dimension of frames, hence the interference of the dimple height provides enough occlusion to create a snug fit. \\

 

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