Nanoscale profile copper for high speed transmission printed wiring boards

Current mobile broadband network system may limit the growth of mobile services. In order to accommodate larger network capacity and higher speeds, the 5th generation (5G) mobile communication system is desired instead of 4G network technologies, known as 5G. It is expected to start its initial service in 2020[1]. 3.5 billion Connected things will be use in 2020 in automotive sector due to the Internet of Things (IoT) [2]. In Particular, autonomous driving technologies require high speed for both of computation and transmission with no latency. 5G networks are not a simple upgrade of 4G technologies, it will be needed by adding additional spectrum and thus boosting the capacity with advanced radio technologies. Considering wireless signal propagation characteristics which use in high frequency, the massive multiple-input multi-output (MIMO) antenna and millimeter wave communication technologies will obviously reduce cell coverage [3]. The small cell network is one of the wireless backhaul solution, which is densely deployed by self-organizing, low cost, and low-power small cell base stations (SBSs). 25 to 50 gigabits per second per channel throughput will be used at the data center for 100 to 400 Gigabit Ethernet (100 to 400 GbE) applications. Hybrid Memory Cube (HMC), FOWLP, FC-BGA and embedded technologies is expected to meet the requirement of 5G networks from packaging point of view. In printed wiring boards (PWB) applications, transmission loss is one of key factor for high speed transmission, it will cause delayed or attenuated signals. A lot of studies have been presented which related measuring copper surface roughness for high speed applications, indicated that high copper surface roughness is a significant factor in increased conductor losses [5][8][9]. A lot of prepreg material are designed and announced to deliver excellent transmission loss performance which enable high speed signal transmission for high speed applications. The low Dk glass is used instead of E-glass as glass fabric of prepreg materials. Also, specialized resin materials are used which shows low dielectric and low dissipation factor properties. Resin structure with low polarization density exhibits a low dielectric constant [4]. In terms of copper foil in PWB, copper surface roughness will increase conductor loss with the frequency increasing. The signal skin depth is getting smaller than the scale of the copper roughness. Skin effect is that the small skin depth signal must travel along with the surface of the rough conductor, effectively increasing the path length and conductor resistance. From the conductor loss point of view, smoother surface is desired on for copper foil. And thereby new treatment technology for copper foil which enable ultra-low profile by conditioning a copper surface at the nanoscale was introduced. This paper presents the results for the radio-propagation behavior across the nanoscale profile copper foil through measurement of the transmission loss.