High Dielectric Constant Molding Compounds for Fingerprint Sensor Packages

Abstract

Biometric features, such as fingerprint, facial recognition, etc, are convenient personal identification methods in mobile electronics. Fingerprint recognition is one of mature technologies and is embedded in an increasing number of mobile devices. For fingerprint sensor packaging, wire bonding and over-molding Land Grid Array (LGA) is one of major package types. Inside the wire bond LGA package, the dielectric constant (Dk) of over-molding compounds is one dominant factor for the sensitivity of capacitive fingerprint sensors. Normal molding compounds contain epoxy base polymers and silica base fillers. Their Dk value is relative low (3~5 at 1 MHz). Those compounds are suitable for the general encapsulation purpose. But they have shielding effects on the sensing signal transmission in capacitive fingerprint sensor packages. This shielding effect needs to be reduced as much as possible, especially when thick glass is used on fingerprint module for the mechanical protection. Therefore, high Dk (7~40 at 1 MHz) molding compounds were developed for the sensor performance enhancement. High Dk property of molding compound can be achieved by using new type polymers and metal oxide fillers. With new type polymers and fillers, the major challenges of high Dk molding compounds come from the warpage and stress during the package assembly process. In order to diminish the warpage and stress, lots of experiments were conducted which including molding compound composition adjustment, post-mold cure process optimization and so on. In this paper, several types of high dielectric constant molding compounds were evaluated and compared. Stress simulations were performed to determine the package construction. Screen and corner DoEs of molding process parameters were conducted to come out the process window. Functional test and reliability test have been preformed as well. Two types of high Dk molding compounds have proven to be feasible and reliable materials for enhancing the performance of fingerprint sensors.