Demonstration of Next-generation Au-Pd Surface Finish with Solder-capped Cu Pillars for Ultra-fine Pitch Applications

High-performance computing is driving sub-10 mu m substrate interconnect pitches to support high-density logic-to-memory interconnections in advanced 2.5D packaging. A new ultra-thin surface finish, electroless Pd autocatalytic Au (EPAG), was recently developed by Atotech GmBH to address extraneous plating encountered at these fine pitches with conventional finishes, such as electroless Ni immersion Au (ENIG) or electroless Ni electroless Pd immersion Au (ENEPIG). In this paper, the EPAG composition was optimized to demonstrate, for the first time, ultra-short copper pillar interconnections with superior pitch scalability, bonding strength and thermomechanical reliability as compared to standard ENEPIG. Variations in surface finish composition were considered, with EPAG finishes composed of 50 - 100nm Pd and 50nm Au, electroless palladium (EP) finishes of 50 and 100nm Pd; and standard ENEPIG, used as reference. Solder wettability was first evaluated through contact angle measurements on solder sessile drop test samples. Copper pillar assemblies with limited solder volume were then formed on all surface finishes. They were subjected to high-temperature storage at 150 degrees C for up to 500h, to study interfacial reactions, and subsequent intermetallic formation. While gold embrittlement was observed with both EP and ENEPIG finishes, it could be prevented with EPAG surface finish with 50nm Pd and 50nm Au, this specific ratio leading to formation of the single (Cu, Au, Pd)(6)Sn-5 intermetallic. Die shear and thermal shock tests were then carried out to determine the effect of the joints' composition on their strength and fatigue life. A shear strength of 40MPa was achieved with all EPAG compositions, exceeding the 6-11MPa and 5MPa achieved with EP and ENEPIG, respectively. Highest thermomechanical reliability was also achieved with EPAG finishes, surviving 300 cycles at -55/125 degrees C even in silicon-to-FR-4 assemblies with high CTE mismatch, while 70% of the ENEPIG daisy chains failed after only 100 thermal cycles. The optimized EPAG composition was therefore demonstrated as a promising low-cost surface finish alternative to form ultra-short but highly-reliable, fine-pitch, Cu pillar interconnections.