Packaging of Ultrathin Flexible Magnetic Field Sensors with thin Silicon and Polyimide Interposer

Abstract

Ultrathin sensors are used in a wide range of applications. In the current work an ultrathin sensor for Active Magnetic Bearings (AMB) has to be realized. Previous studies show very good results by the use of thin silicon interposer with a thickness of $50 \mu \mathrm{m}[1][4]$. With this technology sensor packages thinner than $150 \mu \mathrm{m}$ have been already realized. First results with thin polyimide interposer are shown in [3]. With this approach the thickness can be reduced down to less than $100 \mu \mathrm{m}$ in total.With a focus on improving the long-term reliability the process was optimized within the present work. Therefore the metallization of the sensor and the substrate were investigated. On sensor side the chosen material for interconnecting is gold with different thicknesses of 100 nm and 200 nm. On substrate side bare copper is compared with deposited NiAu on that copper. To evaluate the reliability of the sensor packages several test specimen were manufactured. These test specimen were aged afterwards in a temperature shock test (-20 °C, +85 °C; 15 minutes dwell time). As expected, the test specimen with an additional NiAu layer show lower failure rate after aging until 3400 cycles compared to the bare copper. But, there are still early failures before 100 cycles for any substrate-sensor-combination of some 5 %.As an alternative interconnecting technology soldering with SnAgBi solder paste was investigated as well. Therefore sensor dies with a bismuth layer were prepared and soldered onto test specimen. First results promise that this technology is able to be a proper alternative to Flip Chip interconnecting.