Cross-Section Preparation for Solder Joints and MEMS Device Using Argon Ion Beam Milling

Mechanical cross-section polishing has traditionally been the method of choice for preparing samples to be examined by scanning electron microscopy (SEM). Although mechanical polishing, allied to selective chemical etching can reveal the most important characteristics of solder joint microstructure, subtle details may be lost. A relatively new cross section polishing method has been developed using an argon ion beam to prepare a flat surface with potentially less sample damage. In this study we compare these two methods of cross section polishing for solder-substrate couples, and for delicate MEMS type structures. Four solder samples were prepared, consisting of SAC (Sn-Ag-Cu) solder, SAC solder on copper substrate, SAC solder on nickel substrate and In-Sn solder on niobium substrate. SEM was used to examine the polished samples and it was found that features such as the internal structure of intermetallic compounds (IMCs) was more readily identified using the new technique. The ion beam milling technique was also found to be more suitable for simultaneous observation of multiple aspects of microstructure (e.g., identification of IMCs in relation to grain boundaries, substrate crystal structure or the eutectic solder structure). The MEMS device cross-sections could only be prepared by the ion beam method as mechanical polishing caused too much damage.