A 64-pin Nanowire Surface Fastener Like a Ball Grid Array Applied for Room-temperature Electrical Bonding

Yuhki Toku, Kazuma Ichioka, Yasuyuki Morita, Yang Ju
2019 Scientific Reports  
Surface-mount techniques primarily depend on soldering. However, soldering techniques have encountered some challenges in recent years. These challenges include rare metal recycling, thermal problems, and Pb toxicity. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems. This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. This study demonstrates a
more » ... in NSF that behaves like a ball grid array (BGA) for application to actual electronic devices. The adhesion strength and electrical properties of the NSF were investigated by adjusting the nanowire parameters, such as diameter, length, density (number per area), preload, and shape. The shape control of the nanowires greatly contributed to the improvement of the properties. A maximum adhesion strength of 16.4 N/cm 2 was achieved using a bent, hook-like NSF. This strength was 4-5 times the value of the straight NSF. The contact resistivity was 2.98 × 10 −2 Ω•cm 2 . The NSF fabricated through the simple template method showed the room temperature bonding ability and adaptability to a highly ordered electrode like the BGA. High-density electronic packages have increased with the development of nanotechnology. Electric circuits have also become significantly smaller because of the high accuracy micro-/nano-wiring pattern technique. Electronic components, such as the ball grid array (BGA) and the land grid array (LGA), which have many connecting terminals, have facilitated miniaturization and the advancement in the performance of the electronic packages. The BGA and the LGA have an advantage in their small surface mounting area. In addition, the LGA can use high power flow caused by the low contact resistance achieved using pad-shaped terminals instead of solder balls. The physical strength of the LGA is high because of its simple structure 1 . However, some problems exist. Confirming the condition of the solders or reconnecting the components is difficult in the connecting process because almost all terminals are covered by the component body 2 . In addition, the mismatch of the thermal expansion coefficient between the LGA and the substrate, in the case of the LGA, is a problem for future micro-/nano-electronic devices 3,4 . Meanwhile, almost all the surface-mount techniques in the connecting part of the electronic components depend on soldering. However, recoveries and recycles are not easy because of the difficulty in releasing the electronic components from a substrate. Thus, the recovery of such rare metals in the electronic packages incurs significant cost. These problems need to be resolved so that limited natural resources are used. This can be done by developing a new surface-mount technique, which can easily recover the electronic components. Another issue to be addressed is the heat problem. The surface-mount technology with soldering is mainly conducted through a reflow process, where the substrate, on which the solders are printed, is used and heated along with the arranged electronic components on the substrate surface. Accordingly, many types of Pb-free solders have been studied worldwide in an effort to avoid the toxicity of traditional Sn-Pb solders 5-8 . However, Pb-free solders usually have a higher melting temperature than Sn-Pb solders 9 . The melting point of the Sn-Pb solders is 183 °C, while that of Pb-free solders is 200 °C. Thus, the temperature in the reflow process must be raised to over 240 °C, which includes a margin of approximately 40 °C 9-11 . Meanwhile, the electronic components have a low thermal strength and would not be suitable for the reflow process. Moreover, a high current density recently caused
doi:10.1038/s41598-018-37693-2 fatcat:3a7xp54lxnevjd5espw6trykxa