Evaluating the effects of electromigration by using adjustable solder joints of concave shape

Further miniaturization of electronic systems is approaching new limits due to the failure mechanism of electromigration. Electromigration results in a transport of material in solder joints subjected to high electrical current densities. This decreases the system reliability and, therefore, it is necessary to assess and quantify this failure mechanism in solder joints. In this paper we discuss the development of new test structures based on modified flip chip structures, which allow monitoring of electromigration effects in lead-free solder joints. The structures are of concave shape which permits shifting of the failure region within the solder joint into a position suitable for deterministic assessment. For example it thus becomes possible to create a nearly homogeneous distribution of current density in the local failure region remote from interfering Inter-Metallic Compounds (IMCs) and material interfaces. Moreover, a smaller electric current is required to reach high current densities, so that Joule heating decreases. As a result the effects of overlying failure mechanisms are reduced to two main factors of influence, namely current density and temperature. Experiments using SnAg3.5 solder joints have been conducted at temperatures from 100°C to 150°C and current densities from 10⁴ A/cm² to 7.7×10⁴ A/cm². Joule heating is evaluated by finite element analysis (FEA) and measured during experiment. The activation energy is found to be 1.32 eV. A scanning electron microscope (SEM) is used to analyze failure characteristics of the structures and a direct comparison of the impacts of electromigration and thermomigration is performed. The results demonstrate the advantages mentioned before and qualify the structures for electromigration research.