Interconnections based on Bi-coated SnAg solder balls

• Published in: IEEE transactions on advanced packaging Vol. 24; no. 4; pp. 515 - 520
• Main Authors: Korhonen, T.-M., Vuorinen, V., Kivilahti, J.K.
• Format: Journal Article
• Language: English
• Published: Piscataway, NY IEEE 01.11.2001; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Bismuth; Bonding; Chemicals; Coatings; Electronic equipment and fabrication. Passive components, printed wiring boards, connectics; Electronics; Eutectic temperature; Eutectics; Exact sciences and technology; Heating; Interfaces; Melting points; Melts; Metallization; Microelectronic fabrication (materials and surfaces technology); Microstructure; Precipitates; Precipitation; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Shape; Soldering; Solders; Temperature distribution; Waveform; Tin compound; Metallizing; Liquid phase; Interface structure; Microelectronic fabrication; Thermodynamic model; Electronic packaging; Bismuth; Bonding; Low temperature; Thermal characteristic; Scanning electron microscopy; Eutectic alloy; Solidification; Experimental result; Solder bump; Interconnection; Chemical coating; Soldered joint; Lead free soldering; Silver compound; Numerical simulation; Microstructure; Melting point
• ISSN: 1521-3323; 1557-9980
• DOI: 10.1109/6040.982838

To decrease the bonding temperature required for eutectic SnAg solder, SnAg solder bumps were chemically coated with a pure Bi layer. During heating, a low melting eutectic forms between the Bi coating and the SnAg, enabling bonding at temperatures below the melting points of either pure Bi or SnAg solder. As the composition of the molten solder changes toward more dilute Bi concentrations, the melting point in the joint region increases and the joint solidifies. After solidification the joints will no longer melt at the original bonding temperature. Bi-coated SnAg solder balls were joined to metallized substrates at temperatures ranging from 180/spl deg/C to 250/spl deg/C. The microstructure at the joint interface was characterized by the SEM/EDS technique. As expected, at 180/spl deg/C the Bi-coated SnAg solder balls melted only locally at the interfacial regions between the ball and the substrate and so retained their spherical shape during bonding. After solidification there were a lot of small Bi precipitates in the joint region. At higher temperatures, the wetting was evidently better, and there were less Bi precipitates, because the melt was more dilute in bismuth. In all cases, Bi formed relatively small, equi-axed precipitates instead of the eutectic structure found in eutectic Sn-Bi solder joints.