Over the years, mobile phones have undergone dramatic changes in form, functions, performance, and cost. Evolving new technologies have brought smaller, more energy efficient and highly integrated semiconductor devices, leading to new levels of portable (mobile) phone integration. While operators are offering additional new services, such as short message service (SMS), multimedia service (MMS) and GPS, manufacturers have added complementary radios to mobile cellular phones like FM radio, MP3 players, as well as digital cameras. All these features are delivered in a form factor that is creating considerable challenges to handset designers and hardware engineers alike.

Consequently, handset designers working at the printed circuit board (PCB) level encounter central issues such as unwanted coupling between integrated devices, line coupling and crosstalk. All leading to increasing handset development costs resulting from a greater number of design iterations, lack of design portability between handset form factors, and prolonged design cycle times. Under the pressures of today's highly competitive marketplace these factors play a critical role in the success of mobile handset manufacturers and the designers that create them.

One area that was identified early in handset design to improve these central issues was the widespread implementation of shielding. Shielding reduces electromagnetic interference (EMI) and radio-frequency interference (RFI), greatly diminishing the levels of unwanted radiation and the havoc it causes. Today, shielding and RF frequencies go hand in hand as all RF communications standards have some form of requirements with regard to minimizing unwanted radiation.

A shield's effectiveness is characterized by how much it attenuates radiated signals over a wide frequency range. For instance, a shield made up of a metal “can” have a removable lid or the can itself may be directly soldered to the PCB. Using a lid is practical for tuning purposes and is often used in applications such as TV tuners, but the shield's effectiveness depends heavily on electrical contact between the lid and can. This is based on the basic concept behind RF shielding in that a time-varying electromagnetic (EM) field induces currents in the conductor surrounding the field lines. Thus, in a perfect conductor the induced currents generate an EM field that opposes the incident fields, resulting in a cancellation of field lines inside the conductor. Therefore, too many holes, trenches or other openings in the shield can reduce the effectiveness since an induced current can only flow in areas of the conductor where free electrons are available. An opening in the conductor (the can) represents no free electrons causing the current to find another way around the opening, leading to an induced field that does not completely cancel the incident field. Another important factor is the skin depth, which is determined by the EM wave's ability to penetrate a conducting sheet. Especially if lower frequencies are of importance, a thicker sheet will be needed to effectively shield the emanating RF signals.

The focus of this discussion relative to shielding will be around a common RF semiconductor element in today's handset designs, the cellular transmit module (TxM). In brief, the TxM is constructed using a substrate, similar to a PCB, with bare die and passive elements mounted to it. The resulting assembly is then overmolded and ready to be mounted on a handset PCB. This example is particularly useful as it generates the most radiated power of any element in a handset resulting in a great potential to create EMI and RFI. Additionally, a TxM, in general, resembles the dimensions of a rectangular waveguide and according to Pozar^[1], the cut-off frequency for a rectangular waveguide is:

Where “m” and “n” represent the mode, “µ” the permeability and “e” the permittivity, equation1 demonstrates that if dimension “a” is greater than “b,” the dominant mode is TE10. Hence, equation 1 is rewritten to:

Where “c” is the speed of light, “E₁” represents the relative permittivity, “µr” the relative permeability and “a” is the opening.

Equation 2 shows us that the cut-off frequency, as expected, increases with a decreasing opening dimension, “a”. When several holes are present in the shield, the formula becomes more complex, further emphasizing the importance of not having any openings at all.

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