Packaging concept for a miniaturized wirelessly interrogable temperature logger

We present the packaging concept of a miniaturized, biocompatible temperature logger. In a first application, the device will be cast in the palate piece of a removable brace to observe the wearing habit of the patient. For this purpose the temperature is measured with a pn junction and recorded using a micro controller which acts as a temperature logger and is capable of storing the thermal history from several months. This device is hermetically sealed using polymeric encapsulants. The data exchange is carried out using a wireless interrogating transponder using radio frequency identification device (RFID) technology. The function principle of this data transfer technology is explained briefly. Due to the limited space in the brace, an extremely high level of miniaturization is necessary. To reach this goal, the device is built-up as a multichip module using a single sided FR4 printed circuit board with a line width of down to 35 μm. The entire prototype fabrication process, including details on miniaturization related difficulties, is described. Also, the paper describes the finding and evaluation of a biocompatible casting material as needed for a medical application. An easy way for the direct assessment of water absorption and its effect on the electrical function of the module is also presented.