The intermetallic compound (IMC) layer plays a significant key role in the solder joint of microelectronics. The presence of a continuous IMC layer is a must to form a physical bonding between the solder and the substrate metal. But on the other hand, the IMC material is brittle in nature. The fracture occurs in the IMC layer when the solder joint is subjected to a high strain-rate stress condition, causing a brittle failure of the solder joint. In recent years, this issue has become an even greater concern due to the explosive popularity of portable electronics and the introduction of more rigid lead-free solder alloys. As a result, improving the mechanical strength of the IMC layer instead of losing it becomes a natural demand. However, when the solder joint is in an elevated...[ Read more ]

The intermetallic compound (IMC) layer plays a significant key role in the solder joint of microelectronics. The presence of a continuous IMC layer is a must to form a physical bonding between the solder and the substrate metal. But on the other hand, the IMC material is brittle in nature. The fracture occurs in the IMC layer when the solder joint is subjected to a high strain-rate stress condition, causing a brittle failure of the solder joint. In recent years, this issue has become an even greater concern due to the explosive popularity of portable electronics and the introduction of more rigid lead-free solder alloys. As a result, improving the mechanical strength of the IMC layer instead of losing it becomes a natural demand. However, when the solder joint is in an elevated temperature environment, the IMC strength will gradually be reduced, and the solder joint is more vulnerable to brittle failure. This phenomenon is widely known as the thermal aging effect. This effect directly induces the fact that even if the solder joint can provide enough mechanical integrity to the system at the initial condition, it still may not meet the long-term reliability requirement. This issue is especially crucial for the Cu-Sn IMC system, as the IMC layer is directly contacted with the Cu substrate without any barrier layer in between. So the diffusion process cannot be retarded, resulting in a higher growth rate of the Cu-Sn IMC system. However, little is known about the mechanical deterioration mechanism of IMC materials under thermal aging condition.

This study focuses on the fracture behavior of Cu-Sn IMC system and is aimed to thoroughly understand the mechanism of thermal aging effect on its mechanical deterioration. The emphasis of this study is laid on the correlation between the microstructure characteristics of Cu-Sn IMC layer and its fracture behavior during the thermal aging progress.

It is known that the Cu-Sn IMC system is a layered structure, consisting of a Cu₆Sn₅ compound layer and a Cu₃Sn compound layer. In the first part of the study, the growth behavior, and more importantly, the microstructure evolution of the Cu-Sn IMC layers during the aging process will be inspected. High magnification observation technique, including scanning electron microscope (SEM) and transmission electron microscope (TEM), will be employed.

Then, the respective role of Cu₆Sn₅ and Cu₃Sn IMCs in the brittle failure of solder joint is characterized. This investigation will be conducted by a detailed fractographic analysis on the fractured solder ball to determine the exact fracture location and examine how the fracture location is changed due to the thermal aging. In addition, the fracture toughness of these two kinds of Cu-Sn intermetallic compounds are directly compared using the Charpy impact test. Combining these two parts of work together, a Cu₃Sn-controlling thermal degradation mechanism to explain the mechanical deterioration of Cu-Sn IMC layers subjected to thermal aging is proposed.

In the third part, a comprehensive investigation into the relation between the Cu-Sn IMC microstructure and its fracture behavior will be performed using a grain level finite element simulation analysis. The "Voronoi diagram + Cohesive Zone Model" approach will be applied for its advantage in the analysis of the fracture of the polycrystalline material in the meso-scale. The competition between the Cu₆Sn₅/Cu₃Sn interfacial fracture and the Cu₃Sn intergranular fracture with different microstructure characteristics is analyzed. Thus, the Cu₃Sn-controlling thermal degradation mechanism can be further completed.

Lastly, guided by the deduced thermal degradation mechanism, three kinds of SnCuNi solders with different amount of Ni doping are used as well as the eutectic Sn0.7Cu solder. After studying the Ni addition effect on the formation and growth of (Cu, Ni)₆Sn₅ and Cu₃Sn, the IMC strength of the SnCuNi solders under different thermal aging conditions is measured using high speed ball pull and ball shear test. The strength dependency on the total IMC thickness and Cu₃Sn thickness are compared, and the result is also used as a validation to our Cu₃Sn-controlling thermal degradation mechanism.