Corrosion of Tin-Indium Solder during the Manufacturing Process of Biomedical Ultrasound Transducers

The development of facile, reliable and low temperature soldering plays a crucial role when the physical and chemical characteristics of the materials are very critical. In case of piezoelectric material, i.e. high dielectric constant PZT (Lead Zirconate Titanate), the Curie temperature is in the range of (130-150)°C as for the Rhombohedral-to-tetragonal phase Temperature (TRT) of PMN-PT (Lead Magnesium Niobate - Lead Titanate) that is usually below 100°C. The study aims to analyze the defects occurring during the manufacturing process of an Ultrasound (US) transducer for medical imaging with a low melting temperature Sn/In (52/48) solders. The effects and the root cause of fouling of the Sn/In solderings due to the galvanic corrosion have been detected by means of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) analysis. The paper has been focused on the mechanical dicing process of the US transducers and, in particular, on the blade cooling water that showed large quantities of metals, i.e. Tungsten (200mg/L). The cooling water quality explains the kinetics and the root cause of the galvanic corrosion. The residual metals of the mechanical dicing process in the cooling water activated the galvanic corrosion phenomenon on the US transducers due to the low redox potential of the indium by making prohibitive the weldability and the reworkability of the Sn/In solder.