Molded Interconnect Devices for RF Applications:

Transmission Lines and Low Pass Filters

Divya Unnikrishnan, Darine Kaddour, and Smail Tedjini

LCIS - Grenoble INP

50 rue B.de Laffemas, 26000 Valence, France

divya.unnikrishnan@lcis.grenoble-inp.fr darine.kaddour@lcis.grenoble-inp.fr

smail.tedjini@lcis.grenoble-inp.fr

Abstract—The continual trend of miniaturization and increasing

complexity in the field of Radio Frequency (RF) devices poses a

challenge for today's manufacturing technologies. The possibility

of miniaturization of RF components with Molded Interconnect

Devices (MIDs) is very attractive. However, electromagnetic

properties of MID substrates based on polymer materials and

behavior of RF basic components designed on MID substrates

are not well known in the literature. In this paper, some MID

substrates complex permittivity measurements and dielectric

losses measurements are presented and exploited to design RF

basic components. LCP Vectra E820i is used for the realization of

transmission lines and a low pass filter in Coplanar Waveguide

(CPW) technology by Laser Direct Structuring (LDS).

Measurements, which are in good agreement with simulations, make MIDs good candidates for RF applications.

Keywords-Molded Interconnect Devices (MID); resonant

cavity; dielectric permittivity; Laser Direct Structuring (LDS); RF

filters; Transmission lines; Coplanar Waveguide (CPW)

I. INTRODUCTION

One of the fundamental and trend-setting innovations in the field of mechatronics is the direct integration of mechanical and electronic functions using Molded Interconnect Devices (MID technology) [1-2]. Areas of applications of MIDs include automotive electronics, telecommunications, computers, domestic appliances and medical equipments. The enhanced design freedom and the integration of electronic as well as mechanical functions in a single piece allow a substantial miniaturization for communication systems. MID technology provides enormous technical and economic potential and offers a remarkably improved ecological behavior compared to conventional technology of Printed Circuit Boards (PCB) [3].

Polymers employed in the production of MIDs vary depending on the manufacturer or supplier. The selection of the right MID substrate material is often a complex choice, given the required balance between electrical, thermal and mechanical properties and consideration of ease of manufacture and cost.

One of the most important parameters that must be considered in choosing a substrate material for radio frequency (RF) and microwave circuits such as transmission

lines, antennas, switches, and connectors is the complex dielectric permittivity. Indeed, dielectric properties of RF substrate materials influence electromagnetic field distribution and propagation [4]. For a better understanding of the physical processes associated to various RF and microwave devices, it is necessary to know the dielectric properties of the media interacting with EM waves. As a rule of thumb, for telecommunication and radar devices, knowing the complex dielectric permittivity variations over a wide frequency range is important and necessary for performance optimization. To this end, many techniques have been developed for the permittivity measurements in the radio frequency domain [5-10].

The main objective of this article is to prove that MIDs can be successfully used for RF applications. In thispaper, measurements of several MID substrates are performed by resonance techniques providing the highest accuracy of measurements for low loss dielectrics. The plastics measured are as follows: ABS/PC Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140. Showing the lower dielectric losses, LCP Vectra E820i was used for the realization of some transmission lines and low pass filter[17] by Laser Direct structuring (LDS). Measurements are in good agreement with simulations.

II. PERMITTIVITY CHARACTERIZATION

A. Dielectric Permittivity

Permittivity is a quantity used to describe dielectric properties that influence reflection and transmission of electromagnetic waves at interfaces as well as the attenuation of wave energy within materials [10]. In frequency domain, the complex permittivity Ɛ of a material to that of free space can be expressed in the following form [11]:

where the permittivity in the vacuum and is the material relative complex permittivity.

The real part is referred to the relative dielectric constant and represents stored energy when the material is exposed to an electric field, while the dielectric loss factor , which is the imaginary part, influences energy absorption and attenuation.

978-1-4673-4455-5/12/$31.00 ©2012 IEEE

One more important parameter used in EM theory is the tangent of loss angle [11]:

The measurement of dielectric properties plays a crucial role for characterizing the materials for different applications. Precisely measuring the material properties is important to predict the system performance and optimize it. Many methods have been developed and employed for measuring materials complex permittivity [5-10][12].

B. Resonance Cavity Method

The most accurate measurement at high frequencies can be done using the resonant cavity technique [9-10][12]. However, the main disadvantage of the cavity method is that the measured results are applicable only over a narrow frequency band. Resonant cavity methods are also widely utilized in measuring complex dielectric permittivity of lossy materials. The most popular resonant cavity method is the perturbation method, which is based on a comparative analysis of certain EM characteristics such as resonance frequency and quality factor between empty and a partially loaded resonance cavity.

In the cavity perturbation technique, a small piece of the material under test is placed in a high quality factor metallic resonant cavity. The field distribution of dominant TE10n mode is well known. The material to be measured is introduced into the cavity at the position of the maximum electric field. When the material is introduced at the position of maximum electric field, the resonant frequency and the quality factor are modified. A schematic diagram of an experimental set up of the cavity perturbation technique is shown in Fig. 1.

However, the cavity perturbation method is not a swept frequency measurement. Indeed, the shape as well as the dimensions of the sample determines the resonance frequencies. Hence, it can be used only for discrete frequency measurements.

Figure 1. Schematic diagram of a resonant cavity.

The resonant frequency and quality factor of the empty cavity are determined for different cavity TE10n modes. Then the thin piece of sample is inserted and positioned at the E- field antinode. If the sample is purely dielectric, the maximum electric field can be easily determined. This mode will shift to a lower frequency and retraces from there as presented in Fig. 2. The sample is kept at the retracing position, where the electric field is maximum.

According to perturbation technique, dielectric permittivity and losses of a sample under study are determined as follows [13]:

where f0 is the resonant frequency of the empty cavity, fs is the resonant frequency of the cavity with sample, Vc is the volume of the cavity, Vs is the volume of the sample, Q0 is the quality factor of the empty cavity and Qs is the quality factor of the cavity with sample. k' and k'' parameters are generally assumed as constants, which depend on the geometry and location of the sample and resonant mode of the cavity, but approximately independent of the samples permittivity.

It is very complicated to calculate k' and k'' using classical electromagnetic analytical methods. Thus, these parameters are usually obtained by a calibration method using a known permittivity sample. It should be pointed out that the standard sample should be as small as possible and should have a similar geometry with the samples to be measured, so as to improve the measurement accuracy.

Figure 2. Frequency responses for empty and loaded cavities.

C. Measurement Setup

The Damaskos Thin Sheet Tester cavity [14] used for dielectric characterization is presented in Fig. 3. The cross sectional dimensions are a=20cm in width and b=3.8cm in height. The length of the cavity is c= 43cm. It is ideal for thin dielectric sheets ranging in thickness from about 0.05 mm to the order of 3 mm. This cavity offers very good repeatability that can be less than 0.1%. The electric coupling is provided with two symmetrical VNA probes. Two cables should be connected between the SMA connectors on the cavity and ports 1 and 2 on the network analyzer.

Damaskos cavity is furnished with a CavityTM

software pre-installed allowing the direct extraction of the complex permittivity of the sample under test. It is expected to measure five or six resonances over its operating range. For some combination of thickness, dielectric constant, and loss tangent

Empty cavity

Loaded cavity

fs f0 Frequency

Qs

Q0

S21

a

b

c

sample

vna vna

Port 1 Port 2

fewer or more resonances can be measured. Generally, lower values of thickness, dielectric constant, and loss tangent will allow more good resonances to be measured.

Figure 3. Damaskos cavity photograph.

The first three odd TE10p modes of the cavity studied in this paper resonate at 0.816 GHz, 1.28 GHz and 1.89 GHz, respectively.

D. Permittivity Measurement Results

The MID materials considered in this section are ABS/PC Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140. The selected materials are provided with an additive in form of an organic metal complex for usage in the MID Laser Direct Structuring (LDS) process [15].

Tested polymers were selected for their interesting mechanical and electrical properties. Vectra liquid crystal polymers (LCP Vectra E820i) are highly crystalline and are thermotropic thermoplastics delivering exceptionally precise and stable dimensions, high temperature performance and chemical resistance in very thin-walled applications. In addition, Pocan DP T7140 is a solid polyethylene polybutylene terephthalate (PBT) polymer with low permittivity and medium loss. ABS/PC Xantar LDS 3710 has also a high and consistent quality for electronic devices.

Plates of 10 cm x 10 cm x 2 cm are tested with Damaskos Cavity. The MID LDS materials, which were assumed to be isotropic and homogeneous, were characterized within a frequency range of 0.8 GHz < f < 2 GHz.

Fig.4 presents variation of the real relative permittivity of the tested polymers for the first three resonant frequencies. The results altogether indicate a permittivity characteristic, which is rather constant in the band 0.8 GHz to 2 GHz. The measured real parts of the relative permittivity are 2.8, 4.3 and 3.4 respectively for ABS/PC Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140.

Measured dielectric losses are presented in Fig. 5. Measured dielectric losses of 6.10-3, 4.10-3 and 9.10-3 are obtained around 1 GHz respectively for PC Xantar, LCP Vectra E820i and PBT Pocan DP T7140. Thanks to the low dielectric losses, MID substrates could provide good electrical properties for RF applications.

LCP Vectra E820i providing the lowest dielectric losses (4.10-3 at 1 GHz) was selected for the realization of transmission lines and low pass filters.

Figure 4. Measured real part of the relative permittivities of ABS/PC

Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140.

Figure 5. Measured dielectric losses of ABS/PC Xantar LDS 3710, LCP

Vectra E820i and PBT DP T7140.

III. TRANSMISSION LINES AND LOW PASS FILTERS

Transmission lines are fundamental components of any RF system. In order to demonstrate that MIDs are compatible with RF applications, Coplanar Waveguide (CPW) transmission lines are designed on LCP Vectra E820i.

Fig. 6 shows two transmission lines of 2 cm and 5 cm length realized by Laser Direct Structuring (LDS) [15]. Geometrical dimensions (W=2.2 mm, S=0.25 mm) were chosen in order to get a characteristic impedance near to 50 ohms.

Transmission lines have been measured till 10 GHz, thanks to N5222A Agilent PNA using Ecal calibration. Fig. 7 compares the simulated (dashed lines) and the measured (solid lines) S- parameters for the fabricated CPW transmission line of 5 cm length. All simulations have been carried out with Ansoft Designer. A good agreement is obtained between

-2.E-17

2.E-03

4.E-03

6.E-03

8.E-03

1.E-02

1.E-02

0.5 1 1.5 2

Die

lect

ric

Loss

es

Frequency (GHz)

ABS/PC Xantar

PBT Pocan DP

LCP Vectra

measurements and simulations. Measured return loss is better than -_15dB till 3.5 GHz. Lower matching level occurring for higher frequencies is mainly due to the tapered access for connecting line.

Figure 6. CPW transmission lines on LCP Vectra E820i.

Figure 7. Comparison between the simulated (dashed lines) and the

measured (solid lines) S- parameters for the fabricated CPW transmission line of 5 cm.

Fig. 8 shows the extracted effective dielectric constant from the measured S-parameters (solid line) using the two transmission lines method [16]. Measured effective dielectric is around 2.2. This value is almost constant till 6 GHz. In the second step this result is used in a quasi-TEM model for coplanar lines, which assumes an isotropic, homogeneous and non-magnetic substrate material, to finally extract the dielectric constant of LCP vectra E820i around 4.5 (dashed line). The extracted permittivity is in good agreement with the value obtained using the cavity with only 4% of variation.

To validate the potential of MIDs for RF applications, a hybrid low pass filter [17] based on the combination of high impedance transmission lines and Surface MounTed capacitors (SMT) with a cut off frequency at 1 GHz fabricated on LCP Vectra E820i (Fig. 9).

Fig. 10 shows a good agreement between the simulated and measured S parameters for the low pass filter. The return loss is always better than _17dB, with attenuation remaining near to 0.3 dB in the pass band. Spurious frequency bands are rejected to values below _ 20 dB up to 10 GHz, which is ten times the filter cut off frequency.

Figure 8. Effective dielectric constant extracted from S parameters

measurements (solid line) and dielectric constant (dashed line)

of LCP Vectra E820i.

Figure 9. Low Pass Filter on LCP Vectra E820i.

Figure 10. Comparison between the simulated (dashed lines) and the

measured (solid lines) S parameters for the fabricated low pass filter.

IV. CONCLUSION

MID technology contains huge possibilities for many applications in radio frequency systems because of its potential to reduce the number of components, process steps and finally in miniaturization of the final product. For a microwave design system, MID substrates must be selected on the basis of their dielectric and loss properties. In this article, dielectric characterization has been carried out for Molded Interconnect Devices on ABS/PC Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140 substrates using the cavity perturbation method. The measured relative permittivity values are of 2.8, 4.3 and 3.4 for ABS/PC Xantar LDS 3710, LCP Vectra E820i and PBT Pocan DP T7140 respectively. It has been found that the substrate LCP vectra E820i presents low value of dielectric losses (4.10_3) thus providing good electrical performances for RF applications. Furthermore, transmission lines and a low pass filter were designed and fabricated on LCP vectra E820i.

-50

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Realized devices show interesting properties in term of return loss, thus making a proof of concept for RF MID applications.

ACKNOWLEDGMENT

This research work is part of the partnership project PLASTRONICS partly funded within the Clusters PLASTIPOLIS and MINALOGIC.

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