Plating Impacts on Transmission Characteristics of High-Speed Interface Connectors

Taegyu Bae, Tae-Wan Koo, and Jong-Gwan Yook Dept. Electrical and Electronic Engineering

Yonsei University Seoul, Republic of Korea

sintae01@yonsei.ac.kr and jgyook@yonsei.ac.kr

Myung-Hyun Park Foosung Tech. Co., Ltd.

Suwon, Republic of Korea Pmho318@foosung.com

Abstract—Plating on conductive compositions of an electrical connector for high-speed digital interfaces is generally implemented into production process to satisfy some mechanical or chemical reasons. In this paper, pins of connector are plated by various ways, and their impacts on performance of signal are investigated by three dimensional electromagnetic field solver. In addition, some alternatives for conventional plating are presented.

Keywords— connector; plating; conductive multi-coated layer; differential signaling; signal intergrity;

I. INTRODUCTION Commercial high-speed digital interfaces (e.g., high

definition multimedia interface, universal serial bus, etc.) are widely used for the computer display, projector, digital TV and mobile devices such as cell phone and tablet PC. Those interfaces not only provide power bus from the system to terminal devices but also transmit digital audio and video signals, whose data rate are being up to dozens of Gbps per channel in recent or near future [1], [2]. As shown in Fig. 1, the physical link is composed of several parts of interconnections including connectors (receptacle and plug) and the cable. Because of long electrical length of the set of wires, other components including connectors have to be considered for signal integrity problem.

However, as the bandwidth of signal has gradually become wider than ever before, regardless of the dimensional constraints, entire components of the interconnection have to been taken in account at the point of view to secure the margin. Connector is one of the passive devices of those and generally consist of pins, three dimensional and nonplanar transmission lines, mold compound, dielectrics, and the backshell that reduces radiated emissions from connector itself or the front-end of systems.

Fig. 1. Physical layer of HDMI interface.

Plating using conductive material like nickel upon base metallic compositions of connectors is essentially required for several reasons. Because users necessarily combine and take apart receptacle and plug many times, they need to be mechanically robust from fretting corrosion for a life cycle of devices [3]. Furthermore, it acts like the barrier for internal oxidation of metallic material due to exposure in the air.

On the other hand, on the electrical purpose to decrease contact resistance, especially in the high power transmission system, plating using high conductive material is commonly applied on the selective area of pins. Contacting each other upon considerable area can increase effective roughness of the surfaces, and as a result, can cause heating or unpredictable noises as losses. Thus, many researches have been conducted to investigate effects of contact and find out alternative materials like palladium for low cost for decades [4]-[5]. In addition, plating on terminals of pins generally plays an important role of increasing an adhesive strength of soldering.

Plating has potentially enough impact on significant frequency components of high-speed digital signal because of the characteristics of conductive material like skin depth at high frequency and the structure of connector. In this paper, the influences of conventional plating methods are numerically calculated and compensational approaches are presented for enhancing the signal integrity performance of the connector.

II. BACKGROUNDS FOR ANALYSIS

A. Theoretical and Technical Descriptions In many cases, at radio frequencies ranging from a few

gigahertz to several harmonics of fundamental frequency of signal, it is well known that the dielectric characteristic of dissipation factor dominantly determines the insertion loss. It is because metallic materials (e.g., copper alloy or brass) used for pins have relatively high conductivity and does not cause critical dissipation.

However, most current distribution lies on surfaces due to the skin effect and proximity effect in the main spectra of signals. Electromagnetic behaviors and propagation of signals are mainly determined by the conductors at the outmost. The skin depth ( (2 ) / ( )δ ρ μω= ) for conductors depends on not

only the operating frequencyω , and conductivity or resistivity, but also permeability μ of it.

General plating configuration is depicted in Fig. 2. Nickel (Ni) or titanium(Ti) layer is applied on base metal for the passivation by electroless plating process, whose thickness is around a few micrometers. And then, as required, additional electrolytic gold or palladium-plating makes the coated contact area thick to around dozens of nanometers.

Table 1 represents normalized (referring to that of nickel) skin depths of some conductors used for this case. It shows that, the skin depth of nickel is as thin as about 23.6 times compared with brass, the base metal (at 1.7 GHz, skin depth of nickel is around 0.135 μm, much smaller than thickness of the typical Ni-plating). That is because it has very high permeability in spite of their similar quantities of conductivities. In other words, effective skin depth for the multi-layered conductor significantly decreases. The conductive loss, as a result, rapidly increases because cross-sectional areas through which currents pass become narrower [6].

There are many reasons to degrade the quality of signal in a connector. First of all, impedance discontinuity from the structure of each pin and arrangement of adjacent return current paths mainly causes multiple reflection and losses with dielectrics. Also, imperfectly matched termination makes it possible. However, this paper only focuses on the secondary effect of plating on differential-mode signal based on HDMI

B. Parametric Study Using Simplified Models Tendency as varying of thickness is examined by using

finite element method-based three dimensional electromagnetic solver. It assumes that in the process of numerical calculation, surface-based boundary condition of conductor is applied for loss modeling, not volume. Note that surface impedances for multi-layered structure are substituted for equivalents and surface roughness, besides, is not considered.

Fig. 2. Cross section of conventional metal-plated layer.

TABLE I. NOMINAL ELECTRICAL PARAMETERS OF THE MATERIALS

Conductivity [S/m]

Relative permeability [.]

Normalized- skin depth [.]

Brass 71.56 10× 1 23.6

Gold 74.1 10× 1 14.6

Nickel 71.45 10× 600 1

As illustrated in Fig. 3, a nonplanar differential pair is composed of two pins which are brass ( 71.56 10σ = × S/m). Mold compound is liquid crystal polymer (LCP, 3.9rε = , tan 0.004δ = ). The uniform overall length is 14 mm. Fig.4 and Fig.5 show transmission coefficients ( ,21ddS ) as thickness of each layer increases at the significant harmonics of transition-minimized differential signal (TMDS) for HDMI. The former is the nickel-plating on base conductor entirely. The latter is gold-overplating on the nickel-plated pins for compensating the conductive losses at each harmonic. In all cases, ports are properly matched respectively.

Fig. 4 shows that as thickness of nickel layer increases at each harmonic, in which most frequency components of signal are distributed, and note that the fundamental and its harmonic frequencies depends on mainly data rates. Differential insertion loss rapidly increases although those thicknesses are much thinner than the general variation on implementation of plating. In other words, nickel-plating on connectors, even for dozens of nanometers, significantly can degrade signals.

As shown in Fig. 5, depending on each main harmonic, the transmission coefficients increase as thickness of gold increase on given that of nickel. In addition, regardless of thickness of nickel layer, about sixty nanometer thickness of gold layer all over nickel-plated pins considerably and evenly compensates the attenuation to or under 1 dB over main harmonics.

Fig. 3. Cross section of a simplified differential pair

Fig. 4. Transmission Coefficient of nickel-plated case.

Fig. 5. Transmission Coefficient of gold over nickel-plated case.

III. EFFECT OF CONVENTIONAL PLATING ON CONNECTORS The receptacle of connector for HDMI and its pin

assignment is shown in Fig. 6. It contains pins (brass), backshell (stainless steel, 61.37 10σ = × S/m), and pins are molded with LCP. Three channels each transmit TMDS for HDMI (up to 3.4 Gbps), and its bandwidth is up to the fifth harmonics of which fundamental frequency of 1.7 GHz. In a channel, one pin as common grounds between two pins provides additional return current path.

The conventional plating is applied to all of pins, as described in Fig. 7. On entire surface, Ni-plating (1.5 μm) is applied, and then, Au-plating (0.05 or 0. 76 μm) on some selective areas for contacting and soldering on a printed circuit board.

Fig. 8 shows the results of transmission coefficient. Overall ripples in spectrum are caused by impedance mismatch due to discontinuous configurations. The coefficient decreases about one to two decibel over the bandwidth. Moreover, gold-plated areas are not so wide that they cannot sufficiently compensate the losses on effective current paths.

Fig. 6. Receptacle of HDMI and pin assignment.

Fig. 7. Profile of Conventional Plating.

Fig. 8. Effect on transmission with conventional plating.

Fig. 9. Cross sectional configuration of selective nickel-plating.

Fig. 10. Effect on transmission with alternatives.

IV. APPROCHES FOR IMPROVING SIGNAL INTEGRITY To prevent signal from degradation or effectively

compensate it, the idea of decreasing thickness of nickel-plating or increasing that of gold-overplating or enlarging area of it can be easily found out. However, their stability and cost-efficiency problems incur. Thus, some alternative approaches as listed below are taken into consideration and investigated under the same dimensions of thickness.

A. Selective Nickel-plating In metallic plating process, applying to the whole area of

conductors is productively effective for mass production. Generally, after the plating processes, the metallic compositions are injected in dielectric, which is molding, then the figure of a connector is initially revealed. If the sequence of those processes change, that passivation can be placed on only the areas exposed into the air (Fig. 9). From the results (Fig. 10), it is clear that the area is small enough to cause less attenuation on pins than before. Also, the fundamental purposes of plating can be achieved.

B. Effective Gold-overplating Differential-mode signaling is preferred in most high-speed

links because it can considerably reduce electromagnetic interferences and common-mode signals are easily filtered or canceled out [7]. Depending on structure of connectors, most current return paths are normally located on the sides of pins. Even if odd-mode-excited pins are apart from each other, the most adjacent ground pins support the stable path. Thus, gold-plating on only the sides is an effective compensation way to lowering cost. As also depicted in Fig. 10, the means is efficient although the area of gold is reduced to about half.

V. CONCLUSIONS In this paper, purposes of plating on electrical connectors

for high-speed digital interface are reviewed. Those are why metal plating is a fundamental process for not only electrical reasons but also mechanical, and chemical.

However, conventional plating on connectors sufficiently causes degradation of signal quality depending on their propagation modes and electrical characteristics of material over the bandwidth of signal. In particular, the side effect of commonly used passivation material (i.e. nickel) has been looked into through simulated results.

In simplified cases, the transmission coefficients of differential-mode signal according to variation of thickness are numerically calculated. Because of nickel-plating, the signal additionally attenuates to roughly 0.5 to 2.5 dB over the bandwidth. Furthermore, it has been investigated how much gold-overplating can, reasonably, compensate the losses.

The effects of conventional plating on a HDMI receptacle, a part of a connector, have been investigated. The results shows that the more bandwidth of signal is wide, the more quality of the signal will be degraded. Thus, some alternative approaches also have been examined considering the productive circumstance and cost-efficiency. The strategies of plating for signal integrity have to be sincerely taken into consideration to guarantee margin of system as well as satisfy essential purposes of plating.

ACKNOWLEDGMENT This work was supported by the Basic Science Research

Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education under Grant 2013R1A1A2A10009871.

REFERENCES [1] Tae-Wan Koo, Hee-do Kang, Juenguk Ha, Eunkwang Koh, and Jong-

Hwan Yook, “Signal intergrity enhancement of high-Speed digital interconnect with discontinuous and asymmetric structure for mobile applications”, IEEE International Symposium on Electromagnetic Compatibility, Denver, CO, pp. 713-717, August 2013.

[2] Tae-Lim Song, and Jong-Gwan Yook, “Study of jamming countermeaure for electromagnetically leaked digital video signals”, IEEE International Symposimum on Electromagnetic Compatibility, Gothenburg, pp. 1161-1165, September 2014.

[3] Y.W. Park, J. P. Jung and K. Y. Lee, “Overview of Fretting Corrosion in Electrical Connectors”, International Journal of Automotive , vol. 7, pp. 75-82, February 2006.

[4] Bryant, M.D. “Resistance buildup in electrical connectors due to fretting corrion”, Proceedings of the Thirty-Ninth IEEE Holm Conference on Electrical Contacts, pp. 178-190, September 1993.

[5] James H. Whitley, I-Yuan Wei and Simeon J. Krumbein, “A Cost-effective High-Performance Alternative to Conventional Gold Plating on Connector Contacts”, IEEE Trans. On Components, Hybrids, and manufacturing technology, vol. 6, pp. 395-407, December 1983.

[6] H.-W. Deng, Y.-J. Zhao. C.-J. Liang, W.-S. Jiang, and Y. Ning, “Effective skin depth for multilayer coated conductor”, Progress in Electromagnetics Research M, vol. 9, pp. 1-8, 2009

[7] Ho Seong Lee, Tae-Wan Koo, Tae-Lim Song, and Jong-Gwan Yook, “A compact common-mode suppression filter using modified ground structure for high speed digital interconnects on multi-layred PCB”, IEEE International Symposium on Electromagnetic Compatibitlity, Gothenburg, pp. 730-734, September 2014.