Laser soldering of thin film sensors

Conventional soldering processes applied to thin film components are frequently unsatisfying since pad metallization becomes leached off due to comparatively long heating times. However, small and reliable solder joints can be formed by laser soldering. This technique provides heat levels suited to individual requirements with such accuracy and speed as with no other soldering technique. In this work soldering processes have been optimized for different types of solder joints. Quality of solder joints has been investigated by optical inspection metallurgical cross sections and mechanical strength testing. Furthermore, it was important to consider the different phases of the soldering process (wetting, spreading, cooling, etc.) by means of temperature measurements. For this purpose test circuits have been developed containing thin film thermocouples with an active area of less than 10/sup -3/mm/sup 2/. The thickness of leached metallization has been derived from temperature courses recorded during the soldering process. Finally, correlations between applied heat levels, leaching, and solder joint reliability are discussed.