Correlation between microstructure and lifetime of metallic materials in semiconductor application

The aim of this work is to investigate the correlation between the microstructure and the lifetime of solder balls. The focus of the work is mainly on finding a suitable sample preparation for the analysis via scanning electron microscopy techniques and the appropriate evaluation of the obtained data to investigate the microstructural degradation over the lifetime of such solder balls.
In microelectronics solder joints in form of solder balls are used to connect chips to the circuit board. These solder balls establish the electrical and thermal contact and also ensure the mechanical stability of the connection. The reliability of this connection is primarily influenced by the microstructural properties of the solder material. While in the past, lead-containing solders were mainly used, lead-free alternatives are used today, such as solders based on the tin-silver-copper alloy (Sn-Ag-Cu or SAC solders).

The material investigated in this work, SAC305 (96.5 wt%. Sn – 3.0 wt%. Ag – 0.5 wt%. Cu) consists mainly of tin (β-Sn phase). Therefore, the microstructural properties of the material and thus the reliability of the solder are mainly determined by the anisotropic properties of the tetragonal β-Sn. For this study, SAC305 solder balls were subjected ‘thermal cycling on board’ to investigate the material behavior during cyclic thermal stressing. Samples with different amounts of thermal cycles represent different stages of fatigue.
In the first part of the work, cross-section samples were prepared for electron back-scatter diffraction (EBSD) analysis. Due to the low homologous temperature of tin, several preparation steps are necessary to ensure the lowest possible thermal and mechanical damage caused by the preparation process. The EBSD data was then analyzed using the MATLAB toolbox MTEX. First, the degree of recrystallization was determined, to evaluate microstructural changes in the solder material. Subsequently, the degradation indicator was determined for the samples as a more concrete characterization of the solder degradation. However, this method does not consider any damage in the form of cracks, which were found in the samples at higher numbers of thermal cycles. An X-ray microtomography (μCT) examination was carried out to assess this mechanical damage. Both methods represent evaluation options for the solder material degradation at two different fatigue stages. While the degradation indicator is a measure for fatigue assessment before the occurrence of mechanical damage, μCT analyses can be used to quantify the damage after the initiation of cracks.