Conformal technology delivers breakthrough in RF shielding
There are multiple ways components and modules can be shielded on printed circuit boards. Conformal shielding, labeled MicroShield, is a new technology that outperforms others by reducing radiated power signicantly and minimizing EMI/RFI interference. Plus, it eliminates sensitivity to board placement.
By Ulrik Riis Madsen and Carsten Hinrichsen
O ver the years, mobile phones have undergone dramatic changes in form, functions, performance, and cost. Evolving new technologies have brought smaller, more energy efcient, and highly inte- grated 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, handset makers have added complementary radios to mobile cellular phones like FM radio, MP3 players, as well as digital cameras. All these features are coming in a form factor that is delivering considerable challenges to handset designers and hardware engineers alike.
Plated vs Can shielding
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Consequently, handset designers working at the printed circuit board (PCB) level en- counter 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. Figure 1. Radiation measurements on a test vehicle using RF3178 Tx module. too many holes, trenches or other openings in the shield can reduce the effectiveness since an induced current can only ow 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 nd another way around the opening, leading to an induced eld that does not completely cancel the incident eld. 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. One area that was identied early in handset design to improve these central issues was the widespread implementation of shielding. Shield- ing reduces electromagnetic interference (EMI) and radio-frequency interference (RFI), greatly diminishing the levels of unwanted radia- tion 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. 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 us- ing 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 use- 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) eld induces currents in the conductor surrounding the eld lines. Thus, in a perfect conductor the induced currents generate an EM eld that opposes the incident elds, resulting in a cancellation of eld lines inside the conductor. Therefore, ful 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, the cut-off frequency for a rectangular waveguide is: L L Port Port L1 L2 C C P2 P1 Num = 1 L = 1.8 nH Num = 2 C1 C = 8 pF C2 C = 2.2 pF 2 1 f c m m⋅ a n⋅ b 1 + ⋅ 2⋅ m = Figure 2. Simplified circuit schematic of the output match covering the frequency range between 1710 MHz and 1910 MHz. Port 1 is matched to the collector of the PA and Port 2 represents the 50 Ohm termination. ⋅ Where “m” and “n” represent the mode, “” the permeability and “e” the permittivity, equation1 demonstrates that if dimension “a”