Contact Impedance Characterization of Metallized Particle Column to Copper Strip in High

Frequency Domains N. Ben Jemaa, M. Himdi, A. Senouci

University of Rennes1, France C. Koehler

Tyco Electronics, Germany Abstract: The number of applications using high frequency bands up 10 GHz is in constant progression in various domains (high-speed communication, portable phone, radar). This demand has been widely extended to connectors in automotive applications. In fact various connectors have been designed and developped in order to be used over a wide range of frequencies from a few MHz to several GHz. Among these connectors, Metallized Particle Interconnects (MPI) used widely in interconnections (ASICs, PCs, workstations ), constitute an interesting candidate to be developped for high frequency connectors, for frequencies up to 18GHz. In order to simulate the connector, an MPI column was used as a terminal and compressed between two microstrip lines of copper (PCB). Signal losses measurements in the frequency band 100MHz to 18GHz are evaluated as a function of compression force. It was found that the impedance of the system is equivalent to an RCL circuit. By fitting experimental loss data we have analyzed each component and have established the impedance laws.

I. INTROCUCTION Metallized Particle Interconnects (MPI) have been introduced by

several manufacturers and been used worldwide in a variety of applications. MPI is a material system designed to address high density socket and connector requirements. Today's microprocessors and ASICs continue to increase in both pin count and density. The MPI product line produces sizes from 24 to 5000+ I/O on 0.5mm and 2.54mm grid spacing. The patented MPI technology provides a highly conductive interconnect. The proprietary material consists of a high temperature polymer compound that has been embedded with metallized particles. The MPI, in general was designed to provide an electrically and mechanically reliable, low cost interconnection method without the use of metal pin or solder techniques [1]. Some investigations have been made on an MPI using a coplanar technique in the range from 0.05GHz to 2.05GHz [2]. The results reveal the existence of a low mutual inductance and capacitance (0.3nH/0.065pF).

A lot of work has been done in this area. Ahn et al [3] have introduced an electrical model and determined the high frequency characteristics of a multiple line grid array (MLGA) interposer. Their model was derived on the basis of S parameter measurements and a subsequent microwave network analysis. They have measured different types of MLG interposers with different dielectric insulators and dimensions and they have shown that by reducing the height of the MLGA, the effect of parasitic inductance and capacitance decreases.

W. Ryu et al [4] have developed a high frequency SPICE model of anisotropic conductive film Flip chip (ACF) interconnections based on S parameter measurements and a genetic algorithm which is known as a robust optimisation tool. Two different ACF interconnections were studied using an Au-coated polymer ball and a Ni-filled ball. The extracted model of the two ACFs was found to be strongly dependent on not only the size and the rigidity of the conducting balls but also on their magnetic permeability. In the same way, the authors [5] have investigated on a microwave model of an anisotropic conductive film Flip-Chip interconnection for high frequency applications. Kwiatkowski et al [6] have presented an improved model of the microwave contact to analyse the behaviour of an RF electromechanical coaxial switch and a metal contact RF MEMS switch. An experimental method was presented to determine the mapping between the DC contact resistance and RF characteristics in terms of a scattering parameters matrix. Using this approach the results of the general contacts theory, derived for DC contacts, are readily applicable to the analysis of the RF contacts via the corresponding mapping. Finally based on the well known skin effect, Lavers et al [7] have related the dependence of constriction resistance and bulk resistance on signal frequency (60Hzto 1GHz) by considering circular constriction ranging in diameter from 5 to 50m. They have conclude that for a selected constriction radius determined by contact force, constriction resistance decreases with increasing frequency, while, the total connection resistance comprising the constriction resistance and the resistance of bulk material, increases with signal frequency.

This paper is concerned with an MPI, compressed between two microstrip lines made of copper and used as a terminal. Based on the insertion losses technique (in the range of 0.1GHz to 18 GHz), the main objective is to evaluate the impedance and related components (R,L,C) of the MPI contact system.

II. EXPERIMENTAL APPARATUS The MPI sample is composed of 33 columns with 0.83 x 0.94 mm

of size each one (Fig. 1). The column consists of few conducting spheres and sheets made with silver embedded in silicon (Fig. 2). When MPI column is compressed; even with very low forces, it provides an excellent electrical conduction. Moreover it requires 30g to 80g of loading to maintain performance and reliability. The test apparatus (Fig.3) shows one column of MPI extracted from the 33 columns, compressed between two copper microstrip lines, the units being fed through two coaxial SMA connectors (able to go up18 GHz).

This work Supported from the European CommissionGrowth Program, Research Project AUTOCON: Integrated Wiring and Interconnecting of Electrical and Electronic Components for Intelligent Systems.GIRD-CT01.

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Figure 1. Metallized Particle Interconnect (MPI) column in uncompressed condition

Figure 2. SEM photo of MPI cross section: white color is silver and black color is silicon material

Ground plane

Copper Microstripline

Glass epoxy substrateMPI

Load cellPort 1

Port 2

Fc

HF output

HF input

Motor displacement in z direction

Figure 3. RF test apparatus

The lines are printed on a glass epoxy substrate where the characteristics are: 1.6mm of thickness and dielectric constant r = 3.92, the copper metallization thickness being close to 35m. The microstrip line width is about 3.1mm and corresponds to a line impedance Z0 = 50. The test apparatus is a circuit with two external ports, the input port (port 1) and the output port (port 2) for the S parameters measurement. Using the vector network analyser we measure the parameters of the scattering matrix [S] [6]:

2221

1211

SSSS

S = (1), where ZZ2

ZS0

11+

= and

ZZ2

Z2SS

0

01221 +

== and ZZ2

ZS0

22+

=

, and depend only on the line propagation constants and lengths (1, l1) and (2, l2) [6].The insertion losses measurements ( 21Slog20 )dB were made in the frequency range from 0.1GHz to

18GHz and in the compression force range from 2g to 60g. The force results from MPI compression by a motor displacement in the vertical direction (z) with a low increment (10nm). A calibrated load cell determined the measured compression force with 0.1g of resolution.

The test apparatus Fig.1 was adjusted to MPI contact resistance measurements in a DC mode. Contact resistance is measured by a 4 wire technique using a current source (the current is set to 10mA and 500mV for limited voltage) and a micro voltmeter (0.1V). The tests were conducted under the same conditions described before. The applied compression force is in the range of 2g to 60g and it increased in z direction.

III. RESULTS and DISCUSSION A. S21 measurement versus frequency

Prior to experimental characterization of the MPI, a calibration was made to eliminate the HF port input and the two microstrip line losses. Then the parameters = = = 1 and the measured S21 corresponding to the transmission coefficient for serial impedance is

given by: ZZ2

Z2SS

0

01221 +

== and the input and output reflection

coefficient ZZ2

ZSS0

2211+

== .

Fig.4 gives the insertion losses measurement versus frequency under different compression force values (2g to 60g). It shows that the insertion losses decrease with the increase of the compression force. It varies from -1dB for a compression force of 2g to -0.5dB for a compression force of 60g. Otherwise, above 10GHz the measurements show a slight decreasing of the insertion loss due to the presence of an inductive component in the MPI.

Frequency (GHz)0.1 1 10

Inse

rtio

n lo

sses

(dB

)

-4

-3

-2

-1

0

1

2g36

10 60g

Figure 4. Insertion loss measurement versus frequency for compression forces (2g to 60g)

B. Impedance Z versus frequency Knowing the measured magnitude 21S and the phase , we can

determine the module of the impedance Z. The evolution of impedance versus frequency is presented in Fig.5.

For frequencies less than 1GHz and low compression forces (Fc3g), the impedance Z is slightly constant. The MPI impedance in this case tends to approximate a pure resistance. This means that likely capacitance and inductance contribution are neglected. For frequencies up to 1GHz and higher forces compression (Fc3g), the impedance Z increases. The MPI approximates in this case a pure inductance and the resistance and capacitance contribution are neglected.

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Frequency (GHz)0.1 1 10

Impe

danc

e z

(Ohm

)

0.1

1

10

100

2g

3g6g

10g-60g

Figure 5. Impedance Z Vs Frequency

In summary, when the compression force exceeds 2g, the capacitance effect is reduced and becomes negligible at high frequency domains (>1GHz). C. RLC model

By considering the physical structure and dimensions of the MPI, we propose a schematic of the equivalent circuit model, Fig. 6. The MPI impedance Z can be assimilated to an RC parallel circuit in series with an inductance. The impedance Z can be expressed by:

( ) ( ) )LRC1CR(j

RC1RZ 2

2

2

+

+= (2)

CR C

L

Copper strip

Figure.6. RLC proposed equivalent circuit

Where C, is the contact capacitance given by the sum of the micro capacitors shown between neighboring particles and the pad, L is the self inductance which appears between multiple neighboring particles in contact, the angular frequency and R the equivalent resistance component which is due to dielectric, metallic and radiation losses. A de-embedding technique is used to extract each component value versus force.

Fig.7 relates resistance evolution versus compression force in the HF and DC domains. It shows that the resistance decreases sharply with compression force and follow F-1 and F-.2 for the DC and HF measurements. We can clearly see that the measured resistance commonly named as contact resistance in the DC mode (Fig 7) is lower than the equivalent resistance in the HF mode. The difference can be explained by the fact that in the DC domain, the equivalent resistance includes the bulk resistance, contact resistance between particles inside the MPI and the contact resistance of the interfaces between the MPI and the copper strip. However, in the HF domain, additional resistance terms coming from the dielectric, metallic and radiation losses take place.

The metallic losses due to the skin effect [7] is the main cause of connection resistance (including constriction and bulk resistance) increasing, this explain the shift between the HF and DC curves. In addition, regarding the heterogeneous structure of the column, the compression force increases the number of contact spots and particles involved in the conduction mechanisms. This induces the sharp decrease of resistance.

Compression Force (g)1 10 100

Res

ista

nce

(m

)

1

10

100

1000

10000

100000

1000000HF measurement DC measurement

F-1

F-2

Figure 7.Resistance Vs compression force For HF and DC domains

Compression Force (g)1 10 100

Cap

acita

nce

(pF)

10

12

14

16

18

20

22

Indu

ctan

ce(n

H)

1.00

1.05

1.10

1.15

1.20

1.25

1.30Capacitance

Inductance

Figure 8. Capacitance and Inductance Vs compression Force

Fig.8 gives the capacitance values C and the inductance value L versus compression force. For compression forces less than 3g, the capacitance, induced by earlier touching of the MPI and the copper pad, is increased by the gap reduction under compression. But this enhancement is slow down and stopped by parallel contact resistance shunt in the interface. Inversely, the inductance values produced by the MPI mainly display a minor decrease from 1.27nH for a compression force of 2g to 1.05nH for a compression force of 3g and remain stable above.

According to Fig.7 and Fig.8, we can conclude that for compression forces less than 3g, the resistance is very high. Furthermore the capacitance and the inductance variations are opposite and significant. For high compression forces, the resistance decreases until reaching 10m at the maximum loading, so it shunts the parallel capacitance and the self inductance effect will dominate.

IV. CONCLUSION

Although the MPI requires a nominal compression load of a few 10g, we have investigated this device as a terminal, compressed by a wide range of forces (2g to 60g) between two high frequency

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microstrip test lines. Using the insertion losses measurement versus frequency (in the range of 0.1 to 18 GHz) the impedance law is developed and analyzed. The results have shown that the impedance Z of the MPI can be assimilated to a resistance-capacitance parallel cell with serial inductance. For low frequencies less than 1 GHz, the resistance component dominates the impedance and the capacitance and inductance effects are negligible. At a nominal force of 60g, the contribution of the inductance to the impedance is major.

The origin of the capacitance and inductance revealed by HF techniques is due to the granular structure of the column. We assume that charged particles produce capacitance in the interface MPI-pad while current paths inside the MPI bulk induce the inductance.

Finally the measured equivalent resistance in the HF domain should include dielectric, metallic and radiation losses. Compared with that measured in the DC domain, these are very important and predominant.

From this study we can conclude that the MPI, used under nominal condition of loading (60g), seems to be unaffected by the insertion loss and so, it should be the appropriate candidate for HF applications which can be used widely up 10GHz and up 18GHz if we add the matching circuit (serial capacitance).

REFERENCES [1] Metallized Particle Interconnect, Applications guide, Tyco/Electronics/MPI; North Attleboro. [2] Giga Test Labs, Thomas & Betts MPI connector (1mm pitch) final report, electrical characterization 0.05GHz-2.05GHz, August 1997. [3] S. Ahn, J. Lee, J. Lee and J. Kim, Over GHz electrical circuit model of high-density Multiple Line Grid Array (MLGA) Interposer, IEEE Trans in advanced pack, Vol 26, N 1,pp 90-98 (2003). [4] W. Ryu, S. Ahn, J. Lee, W. Kim, K. W. Paik and J.Kim, High frequency SPICE model of anisotropic conductive film Flip-Chip interconnections based on a genetic algorithm. IEEE Trans on comp and pack tech, Vol 22, N4,pp 542-545 (2000). [5] M. J. Yim, W. Ryu, Y. D. Jeon, J. Lee and S. Ahn, Microwave model of anisotropic conductive film Flip-Chip interconnections for high frequency applications, IEEE Trans on comp and pack tech, Vol 22, N4,pp 575-581 (1999). [6] R. Kwiatkowski, M. Vladimirescu, A. Zybura and S. Choi, Scattering parameter model of low level electrical contacts in Electro-mechanical microwave switches-a switch manufacturer approach, 48th IEEE Holm conference on electrical contacts, PP 221-230 (2002). [7] J. D. Lavers, R. S. Timsit, Constriction resistance at high signal frequencies, IEEE Trans on comp and pack tech, Vol 25; N 3, pp 446-452 (2002).

N. Ben jemaa (IEEE. Member for 11years NO 01629807) received a doctorate es-sciences in physics from the University of Rennes I France in 1985. He has 25 years of research covering the physics of electrical contacts. This research has been mainly concerned with low and medium electrical levels and has dealt with arc parameters, contact resistance and degradation effects. His work has been published in more than 80 papers mainly in the ICEC, Holm, NARMS, IEEE journals and used in the telecommunication and automobile

fields. He is currently professor of physics and electronics at the University of Rennes I where he directs the electrical contacts group research.

M. Himdi received a Ph.D. degree in signal processing and telecommunications from the University of Rennes I (France) in 1990. He has been an associate Professor at University of Rennes I since 1991 and a Professor since 2003. His research activities are in the high frequency and antennas group of the Institute of Electronics and Telecommunications, at the University of Rennes I, where he is working on active and passive millimeter-wave antennas. His research interests also include all aspects of theoretical and applied

computational electromagnetics..His principal research focus has been in the development of new architectures of printed antenna arrays and new 3-D antenna technologies. He is the author and coauthor of more than 30 journal papers and more than 100 papers in conference proceedings. He has received five patents in the area of antennas. He is also the author/coauthor of the book chapters

A. Senouci Electronic engineer from the University of Sidi Bel Abbes (Algeria) in 1995, he received a Ph.D in Mechanics from the University of Poitiers (France) in 2001. At present, Post Doctoral Fellow at the University of Rennes I (France), he works on electrical contact phenomena for the European project (AUTOCON).

C. Khler received a diploma degree in semiconductor electronics from the University of Chemnitz in 1986. He is a product specialist for EMEA at Tyco Electronics, responsible for MPI, MID and ETI product groups.

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