A new bonding technique for human skin humidity sensors

For human skin humidity sensors, absolute flat sensor surfaces are needed t o bring the sensor into intimate contact with,the skin surface. To fulfill this requirement, a new bonding technique is presented which allows to perf orm contact pads not towering above the active sensor surface. As a new app roach, the developed method is based on drilling of funnel-shaped holes in a Al2O3 ceramic substrate using a CO2 laser. After coating the bonding area with a solderable metallization (NiCr/Au), the funnel mouth serves as a re servoir for the lead-free, solder paste. The contact wires are guided throu gh the holes perpendicular to the substrate surface. Subsequently, the CO2 laser is used for soldering the lead-in wires, and finally also for shaping and separating the respective single sensor units from the ceramic substra te. Combining all these production steps in a flexible numerically controll ed laser micromachining process is an advantage particularly for small-lot fabrication and the rapid production of low-cost prototypes. (C) 2001 Elsev ier Science B.V. All rights reserved.