Design and implementation of the embedded capacitance layers for decoupling of wireless sensor nodes

Abstract

Access to the full text of the published version may require a subscription. Rights AbstractIn this paper, the embedded capacitance material (ECM) is fabricated between the power and ground layers of the wireless sensor nodes, forming an integrated capacitance to replace the large amount of decoupling capacitors on the board. The ECM material, whose dielectric constant is 16, has the same size of the wireless sensor nodes of 3cm*3cm, with a thickness of only 14µm. Though the capacitance of a single ECM layer being only around 8nF, there are two reasons the ECM layers can still replace the high frequency decoupling capacitors (100nF in our case) on the board. The first reason is: the parasitic inductance of the ECM layer is much lower than the surface mount capacitors'. A smaller capacitance value of the ECM layer could achieve the same resonant frequency of the surface mount decoupling capacitors. Simulation and measurement fit this assumption well. The second reason is: more than one layer of ECM material are utilized during the design step to get a parallel connection of the several ECM capacitance layers, finally leading to a larger value of the capacitance and smaller value of parasitic. Characterization of the ECM is carried out by the LCR meter. To evaluate the behaviors of the ECM layer, time and frequency domain measurements are performed on the powerbus decoupling of the wireless sensor nodes. Comparison with the measurements of bare PCB board and decoupling capacitors solution are provided to show the improvement of the ECM layer. Measurements show that the implementation of the ECM layer can not only save the space of the surface mount decoupling capacitors, but also provide better powerbus decoupling to the nodes. IntroductionNoise on the power bus caused by a rapid current change is a common problem in high speed PCB design [1]. Decoupling capacitors are commonly used to mitigate the power supply noise. Typically the surface mount (SMT) capacitors are used for decoupling. The huge amounts of the SMT capacitors not only take up circuit space but also increase the design difficulty and unreliability. Moreover, the working frequency of the SMT capacitors is limited due to the parasitic inductance brought by the interconnection.Embedded capacitance could be an effective alternative to the discrete decoupling capacitors [2] [3]. This method makes full use of the power and ground planes of the PCB to form an embedded capacitance [4]. To improve the capacitance density, the thickness of the dielectric material is decreased and the high dielectric constant material is selected [5]. There are many publications to characterize the impedance and other parameters of the PCB with embedded capacitance [6] [7] and its applications [8] [9]. However, there are very few applications to use the embedded capacitance material in the