Employing solder joints of concave shape for monitoring electromigration independently of material interfaces

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 electric current densities. This decreases the system reliability and, therefore, assessing and quantifying this failure mechanism becomes necessary. In this paper we discuss newly developed test structures and an approach to analyze, model and monitor the sensitivity of solder material when subjected to electromigration. The structures are of concave shape which leads to a shift of the failure region within the solder joint into a position suitable for deterministic assessment. 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. The effects of the imposed failure mechanisms are reduced to two main factors of influence, namely current density and temperature in the solder material itself. Experiments using SnAg3.5 solder joints with eight different temperature and electric current loads were conducted to verify the failure region. Electro- and thermomigration are evaluated by Finite Element Analysis (FEA) using material flux densities and their divergences. A Scanning Electron Microscope (SEM) and ion etching are used to analyze failure characteristics of the structures and a direct comparison of the impacts of electro- and thermomigration is performed. The results demonstrate the advantages mentioned before and qualify the structures for electromigration research.