Microcoaxial Cable Assembly for High-speed Transmission in Cellular Phones Takashi Matsukawa1, Masako Ito1, Korenari Higashi1, Tomoyuki Shinohara1 and Yasushi Nakagawa2 In recent years, cellular phones and other mobile electronic devices have been miniaturized rapidly. In line with the technical tendency, internal wiring material must also be miniaturized and should have a high-speed transmission characteristics. We have developed and produced a microcoaxial cable with the high-speed transmission characteristics. In this report, the highspeed transmission characteristics of the microcoaxial cable for cellular phones and, further, the wiring material for the high-speed transmission as a step toward the next generation are discussed. 1. Introduction In recent years, microcoaxial cable assembly is attracting worldwide attention as the internal wiring material of cellular phones. It has a stable transmission characteristic with a small diameter and flexible structure. The microcoaxial assembly meets the design and functional needs of cellular phones. With the high resolution of Liquid Crystal Display (LCD), the transmission speed becomes high. But it is expected to increase the noise. The microcoaxial cable is advantageous compared with other materials. Fujikura developed the assembly as a mass production product. We report the microcoaxial cable characteristics and the high-speed cable. 2. The technical trend of cellular-phone wiring material olution LCDs. Because in order to use a high resolution LCD, the transmission speed reaches hundreds of megabytes per second, it is necessary to prevent noise. As it is difficult to apply complete shielding at the hinge area, shielding of FPC is adopted. (Fig.2) The microcoaxial cable structure has a complete Data rate QVGA (Quarter VGA) 240 × 320 dot 200 Mbps WQVGA (Wide Quarter VGA) 400 x 240 dot 400 Mbps HVGA (Half VGA) 320 x 480 dot 2.1. Wiring material for cellular phones In Japan, the clamshell-type cellular phone is commonly used and its popularity is spreading quickly further in overseas markets. Flexible Printed Circuit(FPC) is widely adopted as the wiring material of the clamshell-type cellular phones, in order to connect the display part with the main part by a hinge. We also supply FPC to the cellular phone market. VGA (Video Graphics Array) 640 x 480 dot SVGA (Supper VGA) 800 x 600 dot 900 Mbps Fig.1. The tendency of LCD panel for cellular phones. 2.2. Demand of high-speed transmission A demand for a high-resolution and multicolor display is spreading quickly in the cellular phone market (Fig.1). Currently, Quarter Video Graphic Array(QVGA) LCD is in use. However, the development of HVGA and SVGA is progressing as high-res1 Electronic Component R&D Department, Electronic Components R&D Center 2 Fujikura Thailand Ltd. Fujikura Technical Review, 2007 Fig.2. FPC for cellular phones. 47 Silver-coated copper alloy wires PFA Hair φ 0.29 mm Tin-coated copper alloy wires Conductor Fig.4. The structure of microcoaxial cable. Table 1. The structure of microcoaxial cable (AWG42). Item Fig.3. Microcoaxial cable assembly for cellular phones. shielding structure. It has shield characteristics of more than that of the shielding of FPC. Silver-coated copper alloy Construction 7 / 0.025 mm Diameter 0.075 mm Material PFA Dielectric constant 2.15 Conductor Insulation Diameter 0.165 mm Material Tin-coated copper alloy Construction φ0.03 mm spiral Diameter 0.225 mm Material PFA Dielectric constant 2.15 Diameter 0.29 mm 2.3. Adaptability to various cellular-phone designs The design of a cellular phone is an important factor of individuality. To satisfy customer needs , various types of hinges are adopted by the designs such as clamshell-type and rotation-type. And the hinge parts are miniaturized in line with the tendency toward having miniature and light weight cellular phones. It is hard to miniaturize the shield of FPC. In order to secure the bending characteristic, it is necessary to provide a safe space. Moreover, FPC does not have flexibility in the twist direction. It is hard to adopt a compound mechanism such as folding and rotation. The microcoaxial cable has the advantage of bending and twisting characteristics. It is adopted for cellular phones of complicated designs. (Fig.3) Table 2. Electrical property of microcoaxial cable (AWG42). Item Method Specification Conductor resistance JIS C3102 Max. 7.5 Ω/m Insulation resistance D.C 250 V 1 min. Min. 100M Ω/km Dielectric strength A.C 250 V 1 min. No break down Characteristic impedance TDR Nom. 45 Ω Attenuation - Nom. 8.0 dB/m (at 1 GHz) The amount of transmission data and the number of internal wiring are on the increase. Hence, the system produces a noisy environment. To alleviate the noise, the case design is restricted. The reliability of the system is affected by the noise. The microcoaxial cable is adopted for a high-speed transmission with high flexibility. Currently, it is used as a wiring harness with around 40 ~ 50 wires. However, the demand for cable miniaturization and reduction of the number of cables by the serial transmission will increase. In order to realize further highspeed transmission by the serial transmission, the differential transmission system will be adopted to improve the Signal-to-Noise (SN) ratio. center conductor (diameter 25µm × 7 wires) and a spiral shield structure. (Fig.4) The outer diameter of the cable is 0.29mm. To produce the cable, it is necessary to control the wire tension with high precision. It uses a high technology such as the Fujikura optical fiber. Because a fluorine compound has low dielectric constant (Table 1), it is adopted for insulation and jacket. It is suitable for transmission at a high frequency. Although the microcoaxial cable for the notebook PC market has a thin PET tape as the jacket, we adopted a fluorine compound for the jacket to improve the flexibility. To make a thin insulation layer, a special extruding machine has to be used. We maintain a secure control of the high quality of each process such as the insulation, shield, and jacket. 3. Microcoaxial cable 3.2. The electrical property of a microcoaxial cable 3.1. Structure of a microcoaxial cable The general electrical property of a microcoaxial cable is shown in Table 2. 2.4. Demand for future transmission We developed AWG 42 microcoaxial cable with a 48 Outer conductor Jacket Structure , Material Material Table 3. Available cable alignment pitch and cable diameter. bending performance: attenuation 0.00 −2.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 GHz 3.5 Alignment pitch AWG size Cable diameter 0.25 mm AWG # 46 0.24 mm before test −4.00 after bending (200K times) −6.00 0.3 mm −8.00 0.4 mm −10.00 −12.00 −14.00 dB 0.5 mm Microcoaxial cable Diameter of a mandrel: 4 mm Fig.5. Bending performance of microcoaxial cable (AWG42). 40 wires AWG # 46 0.24 mm AWG # 42 0.29 mm AWG # 46 0.24 mm AWG # 42 0.29 mm AWG # 42 0.325 mm AWG # 46 0.24 mm AWG # 42 0.29 mm AWG # 42 0.325 mm assembly: (1) Technology of cable arrangement by the same pitch (Table 3), for example, 0.3 mm and 0.4 mm pitch. (2) Insulation and jacket strip technology (3) Cable soldering technology to connect with connector or PCB (4) Cable bundling technology to make suitable shape In addition, we realized the capability by making not only the cable but also the assembly in an overseas factory. 4. Transmission characteristic 2.5 mm 4.1. High-speed transmission characteristic of a microcoaxial cable Coaxial cable 3C-2V Fig.6. The size of microcoaxial cable assembly (AWG42 40 wires). 3.3. Bending property The general demand for a cellular phone model in its bending characteristics is around 100,000 times to 200,000 times. When opening and closing of a cellular phone are repeated 50 times every day, it is used for approximately 200,000 times in 10 years. Examination after 200,000 times shows an attenuation characteristic. (Fig.5) It turns out that there is no change before and after the examination. It is thought that there is sufficient durability. 3.4. Assembly of a microcoaxial cable We supply a microcoaxial cable assembly as wiring material for cellular phones. The bundle diameter is around 2.5 mm by using AWG42 × 40 wires. It is suitable for space-saving wiring. (Fig.6) We developed the following technologies for this Fujikura Technical Review, 2007 The transmission quality is influenced by speed and distance. The high-speed transmission characteristic by changing the cable length is shown in Fig.7. 1. In a measurement of AWG42 cable with a length of 0.5m, the transmission characteristic is good at 3Gbps because the jitter is around 0.1UI. 2. In a measurement of AWG42 cable with a length of 1.0m, the transmission characteristic is not good at 3Gbps because the jitter reaches 0.25UI and the eye pattern also collapses. As a result, the AWG42 cable with 0.5m can support 3Gbps transmission speed. 4.2. The high-speed transmission characteristic of a microcoaxial cable Figure 8 shows high-speed transmission characteristic of a microcoaxial cable assembly. The measurement shows the following results: 1. The transmission characteristic of the assembly is less than that of the cable. 2. With the connector, there is some difference in the transmission characteristic. To perform a highspeed transmission, the connector design is an important parameter. Figure 9 shows the connection impedance of the connector. 3. Although transmission characteristic is good at 3 Gbps with a cable length of 300 mm, the jitter 49 UI 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Jitter AWG42 L=0.5m and L=1m AWG42 L=1m AWG42 L= 0.5m 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps EYE Height AWG42 L=0.5m and L=1m AWG42 L= 0.5m AWG42 L=1m mV 100 80 60 AWG42 L=500 mm L=1,000 mm 500 Mbps 2.5 Gbps 3.125 Gbps 40 20 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps Fig.7. Eye pattern for microcoaxial cable (AWG42). Microcoaxial connector Maker A L=300 mm Microcoaxial connector Maker B L=500 mm L=300 mm L=500 mm 500 Mbps 2.5 Gbps 3.125 Gbps condition: Input 100 mV, PRBS: 223–1 UI 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Jitter: The comparison of assembled product Connector A AWG42 L=500 mm L=300 mm Connector B L=500 mm L=300 mm 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps Eye height: The comparison of assembled product mV Connector A AWG42 100 L=500 mm 90 L=300 mm 80 Connector B 70 L=500 mm L=300 mm 60 50 40 30 20 10 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps Fig.8. Eye pattern for microcoaxial cable assembly with connector. 50 Table 4. The structure of Twinax cable. reaches 0.25 UI with 500 mm of cable length. Because there is a distortion of the eye pattern, limited length is used. The general transmission speed in a cellular phone is 250 MHz ~ 500 MHz. Moreover, the general cable length is about 100 mm. The microcoaxial cable assembly has sufficient performance for current cellular phones. Item Conductor Insulation Outer conductor Characteristic impedance: 50 ps (Ω) 100 90 80 70 60 50 40 30 Measuring MC 20 (TDR) 10 0 34.8 35.3 Connector Jacket Test fixture Structure, Material Material Silver-coated copper alloy Construction 7/0.025 mm Diameter 0.075 mm Material PFA Dielectric constant 2.15 Diameter 0.165 mm Material Tin-coated copper alloy Construction Braiding 0.03 mm wire Diameter 0.305 mm × 0.47 mm Material PFA Dielectric constant 2.15 Diameter 0.37 mm × 0.54 mm Cable 35.8 36.3 (ns) Connector A (50ps) Connector B (50ps) Fig.9. Impedance for microcoaxial cable assembly with connector. Microcoaxial cable Twinax cable Non-noise environment Non-noise environment Size AWG42 L=500 mm mV 200 180 160 140 120 100 80 60 40 20 0 Noise environment AWG42 L=500 mm L=500 mm 500 Mbps 500 Mbps 2 Gbps 2 Gbps 3.125 Gbps 3.125 Gbps Eye height Microcoaxial cable vs Twinax cable AWG42 L=500 mm (Comparison in the noise environment) Twinax :Normal Twinax :Noisy environment Microcoaxial :Normal Microcoaxial :Noisy environment 0 Size 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps UI 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Noise environment L=500 mm Jitter Microcoaxial cable vs Twinax cable AWG42 L=500 mm (Comparison in the noise environment) Twinax :Normal Twinax :Noisy environment Microcoaxial :Normal Microcoaxial :Noisy environment 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps Fig.10. Comparison of a transmission characteristic under noise environment. (Microcoaxial cable and Twinax cable) Fujikura Technical Review, 2007 51 Table 5. Electrical property of Twinax cable (AWG42). Item Method Conductor resistance JIS C3102 Insulation resistance D.C 250 V 1min. Specification Max. 7.5 Ω/m Min. 100MΩ/km Dielectric strength A.C 250 V 1min. No break down Characteristic impedance TDR Nom. 100 Ω (Differential) Attenuation - Nom. 8.0 dB/m (at 1 GHz) 4.3. For further high-speed transmission Reduction demand for the number of cables is expected to be met by miniaturization of the hinge. The high-speed serial transmission is assumed as the solution. In order to perform the high-speed serial transmission, it is necessary to consider not only the cable transmission but also the SN ratio. Generally, for the high-speed serial transmission, the differential transmission is adopted to improve the SN ratio. We are developing a microshield pair coaxial cable (Twinax) for further high-speed transmission. Table 4 shows the cable structure, and Table 5 shows the electrical property. Figure 10 shows the high-speed transmission characteristic under noisy environment. The transmission characteristic of the Twinax cable 52 is clearly superior to the microcoaxial cable in terms of the noisy environment. 4.4. Conclusion for transmission characteristic In the current transmission speed (about 500 Mbps) and wiring length (about 100 mm), the microcoaxial cable assembly has enough capability. However, the demand for cable miniaturization and complicated wiring will also increase, the serial transmission will also be required. High-speed transmission will produce a noisy environment, the micro Twinax cable demand is expected to alleviate the noisy environment. Currently, the micro Twinax connector is under development. The present evaluation of transmission characteristic and alleviation performance is not enough. We plan to improve the Twinax cable assembly and reevaluate the transmission characteristic. 5. Conclusion The microcoaxial cable and assembly can realize high-speed transmission and space-saving wiring with high flexibility. In view of alleviating the increasing noisy environment in cellular phones, the micro Twinax cable and assembly will become useful.