电子铜箔和液晶聚合物的激光复合焊接

Laser Hybrid Welding of Electronic Copper Foil and Liquid Crystal Polymer

随着高速高频通信技术的发展,电子铜箔表面平坦度对高频信号传输损耗的影响更加凸显,已成为制约高速高频通信技术发展的重要因素。本研究团队在目前技术发展的基础上,提出了一种铜箔与聚合物直接结合的方法。该方法主要分为三步:通过激光压平实现铜箔表面平坦化;通过激光压印在压平铜箔表面制造出规则的阵列纳米结构;通过激光焊接实现压平压印铜箔与液晶聚合物(LCP)的直接结合。试验发现,通过激光压平可以将40.6 nm的铜箔表面粗糙度降低到7.8 nm,表面粗糙度降低了80.8%。通过激光压印在压平铜箔表面压印出的规则阵列纳米结构,为铜箔与聚合物的直接结合奠定了基础。采用激光将表面压平压印的铜箔与LCP焊接在一起,对其进行拉伸测试后发现铜箔被拉断。这说明铜箔与LCP的焊接结合强度较高,所提方法在降低电流传输损耗方面具有潜在的应用价值。

Objective With the advancement of high-speed and high-frequency communication technology, current loss has become a major impediment to technological advancement. The skin effect causes current loss. That is, when highfrequency and high-speed signals are transmitted, the current tends to be transmitted on the material surface, and the smoothness of the material surface directly affects the current transmission performance. The greater the surface roughness, the greater is the current loss. It has become a major impediment to the advancement of high-speed and high-frequency communication technology. Because of its excellent properties, such as electrical conductivity and ductility, copper foil currently has a better overall effect compared with other materials. Copper foil is the primary transmission material in circuit transmission(circuit board), and liquid crystal polymer(LCP) is the insulating substrate in two-layer flexible copper clad laminates. To address the aforementioned issues, we propose in this study, based on the current technological advancements, a method of combining various technologies to directly combine copper foil and polymer. The direct bonding of copper foil and polymer achieved using this method has high bonding strength.Methods First, the copper foil surface treated using pulsed laser is used to achieve maximum flatness while maintaining the integrity of the copper foil via optimizing the process parameters. In contrast to coarsening the copper foil surface, this step greatly reduces the roughness of the copper foil surface, meeting the requirements of low loss of high-frequency and high-speed signal transmission. To prepare for later imprinting of nanostructures on the surface of copper foil, the flattened copper foil is annealed at 450 ℃ for 2 h in vacuum environment to remove residual stress inside the flattened copper foil. The flattened copper foil is imprinted with a pulsed laser, and the regular array nanostructure is built on top of it. Finally, the copper foil with a nanostructure on its surface is welded with pretreated LCP using a fiber-coupled semiconductor continuous laser, resulting in the direct combination of copper foil and LCP.Results and Discussions When the pulsed laser energy is 0.35 J and laser spot overlap ratio is 50%, laser flattening can reduce the surface roughness of copper foil from 40.6 to 7.8 nm, a reduction of 80.8%. When the overlap ratio is 50%, the surface roughness value of the flattened copper foil first decreases, and then increases in the laser energy range of 0.3-0.4 J, because when the laser overlap ratio is constant, the shock wave pressure generated by the small laser energy during the impact process is insufficient, resulting in an incomplete impact, poor flattening effect, and large surface roughness of the copper foil(Fig.6). After annealing, the surface roughness of the original copper foil substantially increases, by approximately 53.9%. The surface roughness of the flattened copper foil after annealing is increased by 20.6% than that of the flattened copper foil before anealing. This is because the flattened copper foil’s surface is smooth, no evident defect amplification is seen, and the flattened copper foil is strengthened(Fig.10). The regular array nanostructures imprinted using laser imprinting on the surface of flattened copper foil laid the groundwork for the direct combination of copper foil and polymer. The crosssection microstructure of the copper foil surface obtained after imprinting is not circular, in contrast to the silicon mold. The diameter of the near circular structure exceeds 500 nm, while the diameter of the larger structure is approximately 582.9 nm, which is close to the limit size of 600 nm, indicating that the imprint effect is good(Fig.13). When the surface flattened imprinted copper foil is welded to LCP, the bonding strength of the copper foil and LCP is increased to approximately 0.31 MPa, which is 72.2% greater than the bonding strength of the original annealed copper foil and LCP(Fig.14). This has the potential to considerably reduce current transmission losses.Conclusions This study combines laser flattening, laser imprinting, and laser welding technologies, which have the potential to reduce the impact of the skin effect. Firstly, the surface of copper foil is flattened, the effect of flattening parameters on the flattening effect is investigated, and the flattening mechanism is revealed. Then, the nanostructure is then created using the nanomold on the surface of the flattened annealed copper foil. Finally, laser welding is used to connect the flattened and annealed imprinted copper foil and LCP. Microtension experiments are used to test the bonding strength of the copper foil and LCP treated under various conditions. The sample’s surface morphologies are examined using an atomic force microscope, optical microscope, and a scanning electron microscope, and the mechanisms of flattening, imprinting, and welding are discovered.