A new bonding technique for human skin humidity sensors

For human skin humidity sensors, absolute flat sensor surfaces are needed to bring the sensor into intimate contact with the skin surface. To fulfill this requirement, a new bonding technique is presented which allows to perform contact pads not towering above the active sensor surface. As a new approach, the developed method is based on drilling of funnel-shaped holes in a Al 2O 3 ceramic substrate using a CO 2 laser. After coating the bonding area with a solderable metallization (NiCr/Au), the funnel mouth serves as a reservoir for the lead-free solder paste. The contact wires are guided through the holes perpendicular to the substrate surface. Subsequently, the CO 2 laser is used for soldering the lead-in wires, and finally also for shaping and separating the respective single sensor units from the ceramic substrate. Combining all these production steps in a flexible numerically controlled laser micromachining process is an advantage particularly for small-lot fabrication and the rapid production of low-cost prototypes.