Temperature dependence of electrical parameters of SMD ferrite components for EMI suppression

Microelectronics Reliability 48 (2008) 1027–1032

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Temperature dependence of electrical parameters of SMD ferrite components for EMI suppression Goran Stojanovic´ *, Mirjana Damnjanovic´, Ljiljana Zˇivanov Faculty of Technical Sciences, University of Novi Sad, T. D. Obradovic´a 6, 21000 Novi Sad, Serbia

a r t i c l e

i n f o

Article history: Received 12 January 2008 Received in revised form 29 March 2008 Available online 9 May 2008

a b s t r a c t This paper presents how electrical characteristics of ferrite EMI suppressors depend on the operating frequency and temperature. These multi-layer surface-mounted components consist of conductive platinum layer embedded in the middle of nickel–zinc ferrite monolithic structure. In order to analyze variation of performances with temperature changes, a simulation tool has been developed to predict behavior of the SMD ferrite components for EMI suppression. Results for the impedance and inductance as a function of frequency in the temperature range from +25 °C to +120 °C are presented. Proposed EMI suppressors were experimentally tested in the frequency range 1 MHz–1 GHz using an Agilent 4191A RF Impedance analyzer. The calculated results were in very good agreement with the measured ones. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction In the past twenty years, with the proliferation of complex digital applications in computers, LAN, mobile communications, there has been a pollution of conducted and radiated electromagnetic interference (EMI) which has been threatened to disturb the operation of much sensitive equipment [1]. During the last few decades ferrite components have been very successfully used for decreasing or a complete elimination of EMI on a printed circuit board within wires and cables. EMI can be found in a very wide range of frequencies, from the lowest ones like MHz up to a few GHz [2]. The most widely used materials for EMI suppression applications are soft ferrites (mainly of the nickel–zinc variety). The ferrite material does not affect signals on the low operational frequencies, while it blocks the inductive EMI on high frequencies [3]. EMI suppressors have been made in standard Electronic Industries Association (EIA) sizes: 0402, 0603, 0805, 1206, 1210 and 1812, as surface-mounted devices (SMD). These multi-layer chip inductors, intended for the surface installation, consist of conductive layer (made of material of high conductivity) in the middle of monolithic ferrite structures. The nickel–zinc (Ni–Zn) ferrites provide a good magnetic protection and make suppressors very suitable for very dense surface packaging. Initial permeability of ferrites is an important factor in application of EMI suppressors. In particular, the frequency dispersion of the complex permeability l* = l0  i  l00 determines high-frequency characteristics of these devices [4–6]. Moreover, it is requested that most of the civilian and military electronic equipment operate within the wide range of tempera-

* Corresponding author. Tel.: +381 21 485 2552; fax: +381 21 475 0572. E-mail address: [email protected] (G. Stojanovic´). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.03.020

ture variation. Among other things, for the successful design of electronic circuits, it is necessary to know the electronic components behavior at various temperatures. Namely, with the temperature variation, degradation of some characteristics of components could arise that not even the minimum conditions (necessary for the regular operation of some device) would be satisfied. Up to now, an influence of temperature variation on the characteristics of Ni–Zn ferrites [7,8] and on the monolithic inductors on silicon substrate [9,10] or sapphire substrate [11] has been examined. The aim of this paper is to investigate behavior of surfacemounted components, which were fabricated as inductive structures embedded in the ferrite material, at different temperatures and frequencies. These inductive structures are a narrow or a wide conductive platinum line embedded in the ferrite material labeled as LP (Low Permeability) from the company Neosid [12], but procedure described in this paper can be applied to any other ferrite materials. A software tool for the simulation of an influence of temperature variation on performances of these structures has been developed. The temperature dependences (from +25 °C to +120 °C) of the impedance and inductance of these EMI suppressors have been studied and compared with measured data within the frequency range of 1 MHz–1 GHz. 2. The structure of analyzed components The fabricated surface-mounted components, described in this paper, have standard dimensions of 0805 chip size (D = 1.1 mm, E = 0.25 mm ± 0.075, L = 2.01 mm ± 0.2, W = 1.25 mm ± 0.2), as it is depicted in Fig. 1a. These microstructures were fabricated in the ceramic coprocessing technology. The interior structure of these SMD consist of a conductive structure embedded in the monolithic ferrite structure as it can be seen in Fig. 1b.

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Fig. 1. (a) The outer view of fabricated SMD, and (b) Internal structure of the realized components.

In accordance with the application needs, different geometrical shapes of the conductive layer can be printed. In this paper, conductive segments made of platinum (t = 10 lm), in the shape of the narrow (wc = 456 lm) or the wide (wc = 960 lm) line have been considered, as illustrated in Fig. 2a and b, respectively. In order to analyze temperature behaviour of the proposed SMD ferrite components with embedded inductive structure, we have developed a user-friendly software tool called SPISTEMP (Simulation of Planar Inductive Structures for different Temperatures). The SPISTEMP can represent the values of the inductance and the impedance of these components as a function of frequency and temperature. This program provides the possibility of the simulation and analysis of electrical characteristics for different shapes of conductive layer within the Ni–Zn ferrite materials. 3. Analysis of temperature dependence of the ferrite and conductive material Many ferrite features depend on temperature variation and the electronic circuit designers should be acquainted with their characteristics. Technically, maximal operating temperature of ferrite material is its sintering temperature, i.e., the temperature at which the ferrite becomes reactive with the oxygen (usually in the range between 700 °C and 900 °C). We can conclude from the abovementioned fact that ferrite materials could be the best choice for the devices that operate on high temperatures. However, some limitations should be taken into account, such as: Curie temperature, thermal shock and temperature variation of losses, permeability and magnetic flux saturation. The most significant influence is on the permeability. The Curie temperature is typically from 140 °C to 300 °C for soft ferrites. The nickel–zinc material

Fig. 2. The horizontal cross-sectional view of the structures with (a) narrow, and (b) wide conductive line.

used in this paper has this value of 270 °C. The ferrite material labeled with LP is a Ni–Zn ferrite with low loss factors at medium frequencies and high suppression impedance at high frequencies (over 100 MHz). The usual application fields of this ferrite material are the suppression of the signal at RF frequencies, transformers, antennas and electronic circuits adjusted to the medium frequencies operation. In the manufacturer [12] specification for used ferrite material (LP) there is a graph for initial permeability as a function of temperature, but only at a single frequency of 10 kHz (out of the interesting frequency range) and these data have been derived from measurements on a ring core of 30 mm outside diameter. Thus, these data cannot be applied to the surface-mounted components with micro dimensions, analyzed in the paper. Therefore, a very important goal of this paper is the examination of temperature and frequency dependence of the permeability and afterwards the inductance and impedance of the proposed EMI suppressors. It is well known that permeability spectra of the ferrites can be decomposed into spin rotational component vspin, and the domain wall component vdw[2]. l ¼ 1 þ vspin þ vdw :

ð1Þ

The spin rotational component is of relaxation type (due to large damping factor of spin rotation in the ferrite) and its dispersion is inversely proportional to the frequency. The domain wall component is of resonance type and depends on the square of frequency. Each component can be shown in the following form:

G. Stojanovic´ et al. / Microelectronics Reliability 48 (2008) 1027–1032

vspin ¼

K spin x ; 1 þ i  xres

ð2Þ

spin

2

vdw

K dw  xres dw ; ¼ res2 xdw  x2 þ i  b  x

ð3Þ

where x is the operating frequency, Kspin is the static spin susceptibility, xres spin is the spin resonance frequency, Kdw is the static susceptibility of domain wall motion, xres dw is the domain wall resonance frequency, and b is the damping factor of the domain wall motion. If Eqs. (2) and (3) have been included in (1), and real and imaginary part of permeability have been separated, they can be expressed as 2

l0 ¼ 1 þ l00 ¼

K spin  xres spin res2

xspin þ x2

K spin  xres spin  x 2 xres spin

þ

x2

2

þ

2

res 2 K dw  xres dw  ðxdw  x Þ 2 2 2 2 ðxres dw  x Þ þ b  x 2

;

ð4Þ

2

þ

K dw  xres dw  b  x 2 ðxres dw



x2 Þ2

þ b2  x2

:

ð5Þ

That means that complete permeability spectra can be described by the superposition spin rotation and domain wall motion compores nents. In this paper, dispersion parameters K spin ; xres spin ; K dw ; xdw , and

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b can be determined by numerical fitting of the obtained (through the measurement) l0 and l00 spectra to Eqs. (4) and (5). We have also examined the variation of the complex permeability spectra with temperature changes. Fig. 3 shows the variation of the real (a) and imaginary (b) part of permeability spectra with temperature variation, for ferrite material marked as LP. All dispersion parameters are dependent on temperature variation, and can be determined by numerical fitting to specified curves like in Fig. 3. As an illustration, Table 1 shows fitted values of the dispersion parameters, which the best coincide with experimental curves in Fig. 3, for different temperatures in the case of EMI suppressor with the narrow line and LP ferrite material. The fitting results whose fitting parameters are given in Table 1 are also presented in Fig. 3 along with the results obtained through the measurements. Similar results have been obtained for the component with the wide conductive line. Due to the determination of temperature dependence of the conductive structure, we have to take into account the resistance variation of the conductive material with temperature changes. The temperature dependence of the resistivity q for a metal is obtained using well-known expression

Fig. 3. (a) Real part, and (b) imaginary part of the permeability spectra, for ferrite material LP.

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Table 1 The variation of the dispersion parameters for the narrow line conductive structure of EMI suppressor Permeability/temperature

Spin component

Domain wall component

Kspin

xres spin ðMHzÞ

Kdw

xres dw ðMHzÞ

b (MHz)

0

T = 25 °C T = 50 °C T = 80 °C T = 120 °C

58.8 61.6 63.4 70

103 93.2 85.9 66.7

15.6 17 18.9 23

41.1 35.6 31.6 24.1

33.6 31.4 29.6 24.1

l00

T = 25 °C T = 50 °C T = 80 °C T = 120 °C

27.6 24.8 18.7 13.1

391.7 412 595.9 611

54.4 59.6 52.9 59.5

130.7 139.5 193.1 235

382 444 584.1 980

l

q ¼ q0  ½1 þ aðT  T 0 Þ;

ð6Þ

where q0 is the metal resistivity at temperature T0, a is the temperature coefficient of resistance, and T is the operating temperature. For fabrication of ferrite EMI suppressors, presented in this paper, platinum (Pt) conductive paste was used. The platinum was chosen because its very high melting point (around 1772 °C) and

the diffusion of Pt into the ferrite material is lower comparing to similar conductive materials. The value of resistivity at room temperature for Pt is q = 10.6  108 X m, and its temperature coefficient of resistance is a = 0.003729 1/°C. Furthermore, it is possible to include (as input data) the values of any other conductive material into the SPISTEMP software tool.

Fig. 4. Measured and simulated inductance for the structures with (a) narrow line, and (b) wide line, at different temperatures.

G. Stojanovic´ et al. / Microelectronics Reliability 48 (2008) 1027–1032

4. Comparison of simulated and measured characteristics of SMD EMI suppressors – results and discussion The surface-mounted components, analyzed in this paper, were fabricated using the ceramic coprocessing technology. The measurement of electrical parameters has been performed by means of an Agilent 4191A RF Impedance analyzer in the frequency range from 1 MHz to 1 GHz at the following temperatures: +25 °C, +50 °C, +80 °C and +120 °C (the Curie temperature for used ferrite material is around +270 °C). To obtain the optimal design of EMI suppressors we have developed the simulation tool SPISTEMP for calculation of inductance and impedance of these structures as a function of frequency (based on the method described in our published papers [13,14]) and temperature (based on the technique explained in the previous section). Comparing with our previous published paper, this paper, for the first time, shows temperature dependence of the electrical parameters (inductance and impedance) of SMD ferrite components for EMI suppression. In addition, the presented structures with the wide and narrow line embedded in the ferrite material are a completely novel contribution of the paper. The measured and simulated results for the inductance as a function of frequency and for four different temperatures are presented in Fig. 4a for the narrow line and Fig. 4b for the wide line structure. Since the inductance is mainly attributed to the real permeability and geometrical dimensions of the conductive structure, the inductance of proposed EMI suppressors has similar frequency dispersion as l0 . It can be seen in Fig. 3 that using five fitting parameters and Eq. (4) for l0 it has not been possible to obtain an ideal agreement between experimental and fitted curves especially around the maxima of measured data (around 10 MHz). Therefore, consequently, there are the small discrepancies between experimental and numerical results around the frequencies where the inductance has the maximum. The inductance is almost independent of the frequency in the low frequency region and at around 10 MHz starts to decrease. In the low frequency region inductance increases as temperature rises, whereas in high frequency range this behaviour is inverse. The structure with the narrow conductive line has the greater values (from 75 nH to 96 nH in low frequency region) comparing to the structure with the wide line structure where the inductance values are in the range from 44 nH to 53 nH (with temperature variation from +25 °C to +120 °C). Fig. 5 shows measured and calculated impedance spectra for the narrow and the wide line at room temper-

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ature (25 °C) and at maximal tested temperature (120 °C) because of the clearance of presented data. From this plot it can be seen that at low frequencies impedance has low values mainly determined by the contact resistance. The impedance rises with the frequency increase, reaches its maximum around 1 GHz and then decreases. It can be seen from Figs. 4 and 5 that calculated results (simulated with SPISTEMP) coincide very well with the measured data. EMI would be suppressed in the frequency range where the impedance has the biggest values, that means around 1 GHz. According to this, the structure with the narrow line is a better candidate for successful application in EMI suppressors. Additionally, Fig. 5 shows that the simulation curves have maxima near the frequency of 1 GHz as well as that experimental results have maxima at frequencies slightly after 1 GHz. This can be attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increases somewhat spin resonance frequency. This effect is very hard to include precisely in software tool, nevertheless the presented software tool very well predicts the value of the impedance peak. The graph in Fig. 6 shows percentage change in the impedance as a function of temperature when compared to room temperature for the narrow line structure and for several frequencies as a parameter. These typical changes in the impedance are mainly due to ferrite material properties (dependence l00 vs. frequency and temperature). In the low frequency range the impedance increases with the temperature increase and the behaviour is opposite in the high-frequency region (above, approximately 25 MHz as can be seen from Fig. 5). Due to the presence of the ferrite material, the impedance variation of EMI suppressor at low frequencies is not significant. The losses are determined by the losses in conductor and they are relatively small. The impedance and insertion losses, at lower frequencies, do not depend on the ferrite permeability. As the frequency increases the losses become greater due to the skin effects in the conductive layer and due to the eddy currents because of the ferrite material presence. In the vicinity of ferrimagnetic resonant frequency, losses dominate due to the ferrite and the suppressor acts like the frequency dependent resistor. This effect is used in the conductive EMI elimination. Since noise is a high frequency phenomenon, when these SMD ferrite chips are inserted in series with the signal line, the resistive component dissipates unwanted high-frequency noise in minute amounts of heat. Based on the internal structure of conductive segments and the careful

Fig. 5. Measured and calculated impedance as a function of frequency for the narrow and wide line structures at 25 °C and 120 °C.

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Fig. 6. Percent of original impedance vs. temperature for the narrow line structure at several typical frequencies.

composition (or choice) of the ferrite material, analyzed components can be designed to meet a wide array of desired performances. It is important for the circuit designer to understand the behavior of these components at elevated temperatures. This understanding is vital for selecting the best type of material for each application, and for ensuring that the component will function safely at its maximum operating temperature. 5. Conclusions Ferrite surface-mounted components offer a cost-effective means of providing EMI suppression by presenting a high impedance to the high frequency interference. In this paper, the inductance and impedance of SMD EMI suppressors were examined as a function of frequency and temperature. A conductive layer within these components was made as a wide or narrow platinum line embedded in Ni–Zn ferrite material. The ferrite material do not affect on used signal on lower frequencies, but it blocks EMI at higher frequencies. The knowledge about the impedance and inductance behaviour with temperature variation allows the circuit designers to design for worst case temperature conditions found in real world applications. In that case it is possible to improve performances and reliability of the proposed SMD ferrite components and make them very attractive for a wide range of applications. Acknowledgements The authors would like to acknowledge the company Littelfuse Ireland Limited, Dundalk, Ireland for the components fabrication and for continuous support and understanding during the Project.

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