SMTA International Conference Proceedings


Authors: Sven Lamprecht and Kuldip Johal
Company: Atotech Deutschland GmbH and Atotech USA Inc.
Date Published: 9/24/2006   Conference: SMTA International

Abstract: In response to the trend of increasing component complexity and the need for more cost-effective processes, surface finishes are being evaluated that enable a copper/tin (Cu/Sn) intermetallic compound (IMC). Candidate surface finishes typically include immersion tin, immersion silver and organic solderability preservative (OSP). The harsh soldering conditions of Pb-free assembly will pose challenges for all surface finishes; not only with respect to copper diffusion rates but also for oxidation of the solder pastes and surface finishes.

Examinations that are commonly used to evaluate the effectiveness of solder paste reflow include the solder spread test, the misprint test and the dewetting test. For each of these tests, visual inspection of the solder-wetted area is required to determine the effectiveness. Conversely, the wetting balance test is more often employed to simulate the wave soldering process, measuring wetting times and wetting forces to characterize solderability after different thermal conditioning (reflow) parameters.

None of the tests for assessment of solder paste reflow are known to be documented by industry standard. Furthermore, in the case of the wetting balance test, historical data are only available for eutectic (Sn/Pb) solder. Joint effort within the industry is required to establish pass/fail criteria for the new offering of Pb-free wettability test results.

This paper summarizes the results of in-depth investigations of the wettability of different copper-based surface finishes for Pb-free assembly applications. The impacts of solder reflow cycling were examined with respect to wettability of the various surfaces. In addition, the effect of conducting solder reflow operations in both air and nitrogen atmospheres (with controlled residual oxygen concentration) was also investigated.

Key words: Lead-free, Pb-free, immersion tin, immersion silver, OSP, solder wetting

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