Environmentally Friendly Cu Post Technology Based on Laser-Assisted Bonding with Compression (LABC) and Fume-Free Laser Solder Paste for Advanced 3D Interconnections

Package-on-Package (PoP) technology is considered a core technology for high-performance computing (HPC), including artificial intelligence (AI), as well as in stacked packaging technologies for application processors (AP) and memory devices in smartphones. A common technological approach in these domains is three-dimensional (3D) interconnection, which predominantly relies on electroless and electrolytic plating. In this process, a seed metal layer is initially deposited on the epoxy molding compound surface via electroless plating, followed by additional plating steps to complete the 3D interconnections. Although this conventional method benefits from a well-established industrial infrastructure, it may become less central in the future due to the growing emphasis on low-power, environmentally sustainable technologies, driven by the principles of environmental, social, and governance (ESG) management. To address these challenges, we have developed a dry bonding process for Cu posts, replacing conventional wet processes such as electroless and electrolytic plating. This approach exemplifies a low-power, eco-friendly solution aligned with ESG management principles. To enable this process, we first developed a novel material, Laser Solder Paste, which eliminates fume generation during processing and requires no post-process cleaning. This material was applied to the substrate via a screen-printing method, ensuring cost efficiency. An Air Drop technique was developed to precisely and uniformly place Cu posts within the designated tolerance range onto the laser solder paste deposited on the substrate. Subsequently, the Laser-Assisted Bonding with Compression (LABC) process was employed to achieve consistent Cu post height and positional accuracy. This method ensured the shear strength required for reliability while facilitating the stable execution of subsequent processes. This paper presents the newly developed materials and processes for this Cu post technology. The fabricated Cu posts have a diameter of 70 µm, an aspect ratio of 1.71, and a pitch of 150 µm, The process was successfully applied to a 250 x 250 mm2 substrate, integrating a total of 1.8 million Cu posts.