Enhancing DI Water Cleanability of Tacky Flux on Cu OSP Surface Using Flip-Chip Copper Pillar High-Density Interconnection

Fine-pitch Cu pillar flip chip technology has gained prominence in advanced semiconductor packaging due to its high I/O density and improved electrical performance. Nevertheless, Cu pillar flip chip bonding onto copper-organic Solderability Preservative (CuOSP) surfaces presents challenges related to surface cleanliness, reliability, and process compatibility. There are several methods used in the industry such as Thermo-Compression Bonding (TCB) or Anisotropic Conductive Paste (ACP) Bonding, both of which have their advantages and limitations. TCB offers fine-pitch control and good electrical performance but requires more time and specialized equipment. ACP is simpler and cost-effective but is limited in mechanical endurance and precision. Hence, tacky flux offers several advantages such as better control over the alloy formation during bonding. The controlled wetting behavior ensures uniform joint quality, especially for fine pitch Cu pillar bumps. Different solder composition can be used based on specific requirements, such as thermal performance or reliability with tacky flux, as it is compatible with standard mass reflow process, making it to be very cost effective. Tacky flux materials used in flip chip bonding process might leave residues which could impact long-term reliability. This paper describes a deionized (DI) water-cleanable flux solution to enhance the bonding performance while maintaining reliability. The use of a novel flux formulation that combines water solubility with excellent wetting properties allows for efficient removal of flux residues after bonding. In this study, Cu pillar flip chip with 55 μm bump size was tested by dipping in the water-cleanable flux tray and attach onto the CuOSP substrate, followed by controlled mass reflow processes to evaluate the impact of the flux on joint formation, intermetallic compound (IMC) formation, and surface cleanliness. The novelty of this work lies in the development of a water-cleanable flux material optimized for fine-pitch Cu pillar bonding. Reliability concerns are addressed while maintaining process efficiency at the same time. The findings provide valuable insights for semiconductor manufacturers seeking robust and environmentally conscious solutions.