Electromagnetic interference suppression and simultaneous switching

Electromagnetic Interference Suppression and Simultaneous Switching Noise Mitigation in System on Package Using a Lowpass Filter Structure with Embedded Capacitor Lei Li1, Yunfeng Wang12, Lixi Wan2, Xiaoli Liu1, Rong Sun1, Shuhui Yu1 1. Shenzhen Institute of Advanced Integration Technology, Chinese Academy of Sciences /The Chinese University of Hong Kong 2. Institute of Microelectronics, Chinese Academy of Sciences suppression in mixed-signal SIP [16]. J. Park had utilized a double-stacked EBG structure for 11.4 GHz noise stop bandwidth with -30dB suppression in SIP applications [17]. But all of these EBG structures have severe disadvantages that limit their applications in productions. For example, the EBG structures are usually very complicated for fabrication, and the performances are sensitive to the size changes of the shape and periodical distribution of any unit cell. However, in the substrate design of system on package, there will be nearly thousands of all kinds of through vias, blind vias and buried vias for vertical interconnection that will greatly influence the shape of unit cells and destroy the periodical distribution, and then must be result in the poor performance of EBG structure. Moreover, the EBG structures are very sensitive to the consistency of manufacture process and material properties, and any little changes of process and material may be produce unexpected performance. If the process and material was selected strictly, that will increase too much cost of the productions. So the EBG is not very practical for products. This paper presented a simple and cost-effective lowpass filter structure using embedded capacitor material for noise suppression application in system on package. It is easy for design and fabrication, and could also be used in large scale backplanes or motherboards.

Abstract System on package (SOP) is an emerging technology, and provides an implementation options for system with small size, especially for mixed-signal systems. However, in such a compact package, noise generated by the digital chip can be easily coupled to the RF IC, and degrade the sensitivity of RF signal. In this paper, a simple and efficient multi-order lowpass filter for EMI and SSN suppression in system on package is implemented with embedded capacitor material. It has good performance in noise isolation with a small size of 10mm*mm. The lowpass filter is designed and simulated by electromagnetic analysis software, HFSS. The insertion loss is below -70dB from1.5GHz to 10GHz. Then, the lowpass filter is serial with a surface mounted 47nH inductor, and can achieve noise suppression below-50dB from 50MHz to 10GHz, below -60dB from 140MHz to 10GHz, and below 80dB from 600MHz to 10GHz. That is very promising for system on package application in mixed-signal systems. The structure can also be used in large scale backplanes or motherboards. Introduction Consumer electronics market demands for smaller size, lower cost, and higher level of integration have continued to drive the electronic systems to be convergent, and system on package (SOP) become an encouraging solution to improve circuit performance and reduce the system real estate [1]. SOP is more than an IC package containing multiple die, and seeks to integrate multiple system functions into one compact, lightweight, thin-profile, low cost, high performance packaged system [2]. The system design may include RF and wireless devices (such as power amplifiers, switch/filter modules, GPS modules, Bluetooth modules), digital baseband solutions and controllers. In such a compact SOP approach, digital IC are placed extremely close to RF IC to reduce the package size, and then electromagnetic interference (EMI) and simultaneous switching noise from the digital IC will certainly interfere with the RF operation [3]. How to suppress the noise propagation is becoming one of major challenges for SOP design. Prior to this work, power delivery network (PDN) with electromagnetic band gap (EBG) was considered to be a promising noise suppression technology [4-7]. To reduce the physical size and improve the performance of EBG structure, high dielectric constant embedded capacitor thin film material had also been employed [8-10]. But Most EBG structures had been developed for large scale backplane or motherboard applications [11-15]. Of course, some researchers had presented a few EBG solutions for packages. E. Song et al. had presented a spiral-patch EBG for < 5GHz SSN 978-1-4244-5100-5/09/$26.00 ©2009 IEEE

Design and Simulation of Lowpass Filter Structure The essential parameters of the embedded capacitor material (a kind of typical commercial material) used in this design is shown in Table.1. It could be seen that the permittivity and thickness of embedded capacitor dielectric are 16 and 14um, respectively. If the permittivity of material is increased, the performance of noise suppression will be better. Table 1.Specifications of embedded capacitor material. Structure

Dimensions

Material

1. Layer1

35um

copper

2. Dielectric

14um

Dk = 16 Df = 0.005

3. Layer2

35um

copper

One of the lowpass structures is shown in Fig.1, and the size is 10mm by 10mm. The whole structure could be divided into three parts. The first one is the closed copper loop at the edge of the board which is 500um in width. The second one is the central part which is symmetrically separated into four square parts. Each square part is 3.75mm*3.75mm.The third part is the strips interconnecting the edge strip to the four 1 692

2009 11th Electronics Packaging Technology Conference

central square parts. The size of spacing between the separated central parts, and between the metal loop and the central parts are all 500um. It could be easily fabricated by any printed circuit board factory.

noise propagation path between port1 and port2 are the same as the one between port1 and port4.

Fig.3. Insertion loss of the noise suppression structure with closed loop

Fig.1. Noise suppression lowpass filter structure with closed loop The IC power/ground pins of different chips could be connected to anyone of the four central parts by various metal plated vias for isolated noise between them. Typically, the vias should connect to the centers of the square parts for better performance.

Fig.4 Noise suppression lowpass filter structure with nonclosed loop The physical model is designed and simulated by full wave electromagnetic analysis software, HFSS, and the insertion loss between port 1 and port 2, between port 1 and port 3, and between port 1 and port 4 are labeled as S21, S31 and S41. The results in a wide frequency band of nearly 10GHz (50MHz-10GHz) are shown in Fig.3. It could be seen that the insertion loss between port1 and port3 (S31) is the best one, and it is below -40dB in the frequency band of 1GHz-10GHz. The two curves of S21 and S41 are entirely

Fig.2. The equal circuit model of noise suppression structure with closed loop The equal circuit model of this structure is shown in Fig.2. It could be seen that the model is a multi-order lowpass filter structure. Path 2 and path 4 should have the same devices and the same values of inductors and capacitors. That is because 2 693


overlapped, that just validate that in the equal circuit model path 2 should be the same as path 4. To further enhance the performance of noise suppression, the closed copper loop on the edge of the structure should be cut off, and the strips connecting the part 1 and part4 to the copper loop are shifted to the two ends of the cut just as Fig.4.

profile of S31 has also not changed too much, and the reason is identical with S21. Nevertheless, S41 has been greatly changed, and in the frequency band from 1.5GHz to 10 GHz the insertion loss is below -70dB by the reason of more orders than the first structure.

Fig.5. The equal circuit model of noise suppression structure with non-closed loop The equal circuit model of the second structure is shown in Fig.5. It could be seen that the model is a multi-order lowpass filter structure with four capacitors and three inductors. Due to more orders than the first structure, the frequency performance will be better than it.

Fig. 7. Noise suppression structure with surface mounted inductor

Fig.8. The equal circuit model of noise suppression structure with non-closed loop From insertion losses in Fig.3 and Fig.6, it could be seen that the suppression effect of the two structures is good at high frequency band (especially more than 1GHz), but not very good in the low frequency band (less than 1GHz). Other means should be found to improve the performance of noise suppression in the low frequency band. In the lowpass filters network, the performance of noise suppression in the low frequency band will be better with the increase of inductance and capacitance. Therefore, to enhance

Fig. 6. Insertion loss of the noise suppression structure with non-closed loop The insertion losses of the second structure are shown in Fig.6. Compared with the first structure, S21 has not changed too much, and the performance in low frequency band (less than 1GHz) improved a little, and in the high frequency band (more than 1GHz) degrades a little. That is because in the path from port 1 to port 2, the inductance becomes a bit larger. The 3 694


the effect of noise isolation, a surface mounted inductor could be serial to the network, or a paralleled surface mounted capacitor could also be used. In this paper, the inductor solution was adapted.

60dB from 140MHz to 10GHz, and below -80dB from 600MHz to 10GHz. Due to the 47nH inductor has no influence on the S21 and S31, the profiles of S21 and S31 almost has no change with the ones in Fig.6. It is very good performance in noise suppression for SOP applications. Fig.10 showed system on package application with lowpass filter structure. The RF IC and digital IC are mounted on the package substrate surface, and the embedded capacitor material is laminated in the organic substrate. Conclusions In this paper, a simple and efficient multi-order lowpass filter for EMI and SSN suppression in system on package applications is implemented by embedded capacitor material. The structures are modified by adding cut and surface mounted inductors to increase the inductance of metal loop aiming at improving the performance of noise suppression. It shows good performance in noise isolation with a small size of 10mm*mm, and it will be useful for system on package which needs small size and high performance of noise suppression.. Furthermore, it will be investigated that mounting capacitors parallel with metal loop, or eliminating the metal of embedded capacitor electrode which was under the loop to improve the performance of noise suppression.

Fig. 9. Insertion loss of noise suppression structure with surface mounted inductor

Acknowledgments This work was supported by China International Science and Technology Cooperation program (NO.2008DFA11010).

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Fig. 10. System on package application with lowpass filter structure Another cut is implemented at one end of the non-closed loop (just as Fig.7), and then one layer BT organic material with the same size of 10mm*10mm (not shown in Fig.7) is laminated on the top of the lowpass filter structure. A surface mounted inductor with the inductance of 47uH is mounted on the top of BT material, and is vertical interconnected by blind vias to the two ends of the new cut just as Fig.7. The equal circuit model of the second structure is shown in Fig.8. It could be seen that the 47nH inductor is series to the lowpass network. The insertion losses are shown in Fig.9. It could be seen that S41 in the low frequency band, even the whole frequency band was greatly improved. The new solution achieved noise suppression below-50dB from 50MHz to 10GHz, below 4 695


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