Pan Pacific Symposium Conference Proceedings


Author: Motoji Suzuki et al.
Company: NEC Corporation
Date Published: 2/25/2005   Conference: Pan Pacific Symposium

Abstract: After wave soldering, examination of solder joints in surface mount devices reflow soldered by Sn-8.0wt.%Zn-3.0wt.%Bi solder revealed that delamination had occurred. Detailed observation revealed compound phases of Pb and/or Bi near the delaminations. Since Pb was in the solder plating of the device terminals and Bi was in the solder paste, the mechanism generating the delamination can be explained as follows.

During the reflow soldering, these metals reacted, causing the formation of a compound phase of Sn-Pb-Bi (melting point 97°C) and/or Sn-Zn-Pb (melting point 177°C) in the joints. During the subsequent wave soldering, the compound phase re-melted due to the rising temperature. When the mechanical force caused by the curvature of the printed circuit board was applied to the solder joints at this time, delamination occurred.

Investigation of the correlation between the delamination and the Bi content of the solder paste revealed that delamination can be prevented by eliminating the Bi content of the paste. Testing demonstrated the reliability of joints soldered using Bi-free Sn-Zn solder (thermal cycle testing: -40 to 125°C, 600 cycles; high-temperature testing: 125°C, 1000 hours).

Key words: Lead-free solder, Sn-Zn system, reflow soldering, wave soldering.

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