How durable is the touch sensor? Analysis
iFixit posted a link of a very interesting analysis, that questioned the durability of the technology:
We worry about how well the sapphire crystal covering the sensor can protect it from degrading over time like most CMOS fingerprint sensors. If not, it could become a ticking time bomb, just like that super-glued battery.
The last part:
Here the first technological issue. An electrical current is basically a flow of electrons moving between 2 points. This flow happens only in electrically conductive materials (mainly in metallic materials). The flow always originates from the point containing a higher number of electrons and terminates at the point containing a lower number of electrons (this is one of the most beautiful principles of modern physics, since every electronic device around you works because of this principle). When you touch the sensing surface of a fingerprint sensor, the electrons move from your finger to the sensor surface. The quantity of electrons arriving to the surface is typically enormous for the small silicon capacitors. Constant usage of the sensor starts to destroy the capacitors and over time, the fingerprint sensor stops to work. To avoid this issue, during the manufacturing process, the sensor surface is covered by insulating material (essentially silicon, processed to become an insulating layer) that protects the metallic surface. The touchscreen of your iPhone is manufactured in the same way. However, the coating layer on the fingerprint sensor surface cannot be too thick otherwise the electrons from your body cannot reach the metallic surface of the sensor to generate a fingerprint image. So, this protecting layer is thin and only used to extend the life of a sensor, but its continuous usage will destroy its surface, making the device useless.
In everyday life, things are even worst. You usually use your hands for different tasks and you usually touch different types of materials. Small portions of the objects you touch accumulate on the skin of your finger. When you touch the fingerprint sensor, you deposit these material on its surface. Additionally, your skin produces sweat (a combination of water and different types of salts) and the sebum (a oily/waxy substance our body produces). When you touch the surface of a fingerprint sensor, the mix of the sweat, sebum and any substance accumulate during your daily activities become a killer combination for the sensor surface that speeds up the destruction of its surface.
Fingerprint sensor manufacturers (including Authentec) never achieved great success in this issue which is why it is not common to see fingerprint CMOS devices on laptops, cars, building front doors or credit cards. Most of the devices you see are optical sensors that require less maintenance and last longer.
If you search for the specifications of a CMOS fingerprint device, you will find a number representing the lifetime of a device. That number is expressed in number of touches (before it completely dies). That number is provided in ideal conditions of usage and in a normal operating environment of temperature and humidity. But remember where you normally use your iPhone. You keep it in your "dirty" pockets, you leave it on different surfaces, and in humid and hot or cold and dry environments. Sometimes water drops on it or you forget it in your car under the sun. All these factors stress the working conditions of the sensor surface and contribute to speeding up its decay process.
Unfortunately there is no existing solution to this. Manufacturers can only try to make the fingerprint sensor last longer, but sooner or later that device will stop working properly. This is also why Apple cannot provide a fingerprint sensor for payments. And if they do, they are making a huge mistake, because the surface destruction process explained above introduces the most dangerous problem in fingerprint recognition: false acceptance, when after a while somebody else can be granted access to your device.
Companies like Motorola, Fujitsu, Siemens, and Samsung have tried to integrate fingerprint readers in their laptops and handheld devices, but they have all failed because of the poor durability of the sensing surface.
For you, this means that a fingerprint sensor on your phone will break after a while. How long after you buy it? Well, that will depend on where you live, how you use it, where you use it, how careful you are with it, and how clean your hands are.
That can be a little scary or disappointing for such a beautiful technological product.