Authored By:
Kathlene N. Reeve and Carol A. Handwerker
Purdue University
West Lafayette, IN, USA
Summary
The most widely used lead-free solders in microelectronics today are Sn-rich alloys, with Beta-Sn as the dominant phase in the as-solidified microstructure. This results in solder joints that depend on the anisotropic properties and microstructure of Beta-Sn for overall joint performance. The nucleation and solidification of Beta-Sn from the Sn-rich liquid can therefore have profound effects on joint mechanical properties.
This study analyzes the wetting and interface formation between polycrystalline CuxAly intermetallic substrates and the pure Beta-Sn phase during melting and solidification. Intermetallic particles of CuxAly are often found in Sn-based, Al-modified solder alloys, but the effects on the nucleation of the Beta-Sn phase due to the presence of these intermetallic particles has yet to be analyzed in the absence of Cu6Sn5. Crystallographic orientations of the Beta-Sn phase in contact with polycrystalline CuxAly substrates were determined via electron backscatter diffraction, and the nucleation efficacy of the intermetallic substrates for the Beta-Sn phase was discussed in terms of Beta-Sn undercooling and the observed microstructures and orientations.
Conclusions
In the current study, the effects of a well-defined interface between CuxAly and liquid Sn on heterogeneous nucleation of Beta-Sn were studied using polycrystalline CuxAly as a substrate. From measurements of Beta-Sn undercooling and the axis-angle pair relationships between the Beta-Sn and CuxAly interface substrate grains, the nucleation of the Beta-Sn phase on the CuxAly phase in the current samples appeared to be not preferred. The axis-angle measurements do not support a consistent trend concerning a preferred orientation relationship between the two phases, and no clear orientation relationship was identified within the current study based on the measured crystallographic data.
Despite this, the presence of the Cu6Sn5 having formed within the Beta-Sn matrix and not along interface with the CuxAly substrates as seen within the results is of interest, particularly given the identified orientation relationship between the Cu6Sn5 and CuxAly phases. Further experiments utilizing film deposition and subsequent dewetting techniques, similar to those employed by Chatain et al. in [23], would provide an improved direct observation method of any orientation relationships by increasing the number of unique Beta-Sn/CuxAly interfaces available to observe, thus increasing the likelihood of identifying any orientation relationship pattern, if present, between Beta-Sn and the CuxAly intermetallic substrates.
Initially Published in the SMTA Proceedings
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