Qualcomm announced a new ultrasonic fingerprint sensor technology that can be fitted under OLED displays. Qualcomm says that the new sensor will be able to scan through OLED displays up to 1200 um.
Apparently the new technology is not compatible with LCD. This may not bother Qualcomm too much as most higher-end smartphones are now adopting OLED displays and not LCDs. If the phone does not have an OLED, the sensor can be placed under a glass or aluminum case. The sensor can work under water and it also detects heartbeat and blood flow.
Qualcomm will ship samples of the sensor for OLED displays by the end of 2017 or early 2018, which means that the first phones to use this tech are not likely to appear before 2019 or late 2018.