A system-in-a-package (SiP) is a module package that contains a plurality of integrated circuit (IC) chips and/or other circuit components (e.g., transistors, capacitors, indictors and resistors) that are mounted on a system printed circuit board (PCB), which is also part of the SiP module package. Such module packages are commonly used in wireless devices, such as smart phones, for example. The module package typically includes a system epoxy molding compound (EMC) that encapsulates the IC chips and other circuit components. The module package typically also includes a system EMI shield for reducing EMI emission from the module package. The system EMI shield is typically a conformal EMI shield formed on the module package by using, for example, a metal sputtering process to form a metal coating that conforms to the outer surface of the system EMC.
While the system EMI shield is effective at reducing EMI emissions from the module package as a whole, it has no effect on EMI emissions within the module package. Some of the ICs and other circuit components contained within the module package comprise an RF module made up of RF functional blocks. These RF functional blocks emit EMI that can interfere with the operations of other RF functional blocks within the module package. For example, one of the IC chips of one of the RF functional blocks may be a multi-band power amplifier (PA) chip supporting different modes of operation (e.g., Code Division Multiple Access (CDMA), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), and Global System for Communication (GSM)/Enhanced Data GSM Environment (EDGE)). Another of the IC chips of another of the RF functional blocks may be, for example, a multi-band low noise amplifier (LNA) chip capable of supporting different modes of operation.
Without suitable EMI shielding of these RF functional blocks from one another, EMI emitted from one RF functional block may detrimentally impact the operations of another RF functional block. One known EMI shielding solution that is used for this purpose is an electrically-conductive metal “can” that is placed over an RF functional block to reduce EMI emissions from the RF functional block. However, current trends to reduce the sizes of SiPs and/or to increase the amounts or types of functionality that are included in them have made the use of electrically-conductive metal cans impractical due to their size and due to space constraints of environments in which the SiPs are used (e.g., smart phones).
Next-generation of RF modules are required to be smaller and thinner than current RF modules and to provide EMI shielding. In addition, a greater number of frequency bands and wireless standards need to be integrated into the RF modules, while also providing the modules with highly-compact footprints and increased package density. These objectives are only possible if the electrically-conductive traces that are routed within each RF module are isolated from one another to prevent unwanted EMI crosstalk.
Inductors are ubiquitous circuit elements used in RF modules, and their design is often critical for optimum system performance. Therefore, a high quality (Q) factor is needed to enable these inductors to achieve low signal losses. In order to obtain inductors with high Q factors, it is essential that the magnetic field surrounding the inductors is not disturbed by grounded metal areas nearby. However, this constraint contradicts the requirements described above because inductors that are not shielded by grounded metal planes or via walls are prone to undesirable EMI crosstalk.
FIG. 1 is a top perspective view of a portion of a multilayer PCB 1 having first and second inductors 2 and 3, respectively, formed in it that are parallel to one another and that have coils 4 and 5, respectively, that are formed in different metal layers 6 and 7 of the multilayer PCB 1. The metal layers 6 and 7 are parallel to one another and parallel to a top surface 1a of the PCB 1. As electrical current flows through the coils 4 and 5, the inductors 2 and 3 generate respective magnetic fluxes that are substantially parallel to respective axes of the respective inductors 2 and 3 and perpendicular to the top surface 1a and to the metal layers 6 and 7 of the multilayer PCB 1. The magnetic flux generated by the first inductor 2 is essentially EMI crosstalk to the second inductor 3 that can degrade the performance of second inductor 3, and vice versa.
The inductors 2 and 3 exhibit the best possible Q factor if surrounding electrical ground planes (not shown) and via walls (not shown) are as far from the inductors 2 and 3 as possible. Ground planes (not shown) above the inductors 2 and 3 degrade their respective Q factors, but are sometimes needed to eliminate coupling of EMI from other signal sources into the inductors 2 and 3. This is particularly important for self-EMI-shielded modules that have an electrically-conducting sheet (not shown) on top of the molded package in which mirror currents from other signal sources can flow and couple into the inductors 2 and 3.
Accordingly, a need exists for way to prevent, or at least reduce, this type of EMI crosstalk while maintaining high Q factors for the inductors.