Lead-free solder bump technologies for flip-chip electronic packaging applications

We describe the fabrication and characterization of five different types of lead-free solder bump interconnections for use in various flip-chip electronic packaging applications. Lead-free solder bumps were fabricated from pure-tin (Sn), tin-bismuth (Sn:Bi), eutectic tin-copper (Sn:Cu), eutectic tin-silver (Sn:Ag), and ternary tin-silver-copper (Sn:Ag:Cu) alloys. The fabrication process consisted of electrolytic plating, using a fountain (cup) plater, of a 5 pm thick copper under-bump-metal (UBM) onto which was plated the lead-free solder bump. The as-plated bumps were subsequently re-flowed in a 5-zone re-flow oven. Due to the characteristic high-tin compositions of the lead-free solder alloys, which can cause the rapid and uneven formation of tin-copper intermetallics at the bump-UBM interface upon re-flow, a unique proprietary nickel "cap" using a single photolithography process that completely encapsulates the copper UBM was developed. Two different test structures, one with perimeter- and a second with area-distributed solder bumps, each with bumps of average size 125 pm diameter (post-re-flow) were fabricated onto "daisy-chain" wafers to characterize the lead-free solder bumping and bonding processes and to conduct a series of reliability tests. Characterization of the properties of the lead-free bumps included the use of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), Auger Electron Spectroscopy (AES), micro-sectioning, and ball shear measurements for which the bumps were re-flowed multiple times and subjected to ball shear tests in-between re-flows to study the formation of intermetallic compounds. Lead-free solder bumped "daisy-chain" test die were also flip-chip bonded onto BT-epoxy substrates with patterned copper traces overplated with nickel/gold. The bonded die were underfilled and subjected to environmental tests consisting high-temperature storage, thermal cycling, and accelerated aging. Details of the lead-free solder bump fabrication process together with the performance results including their electrical, mechanical, and reliability characteristics will be presented for all five lead-free alloys chosen in this study.