[Editor's Note] The author of this article is Mr. Li Yangyuan, the chief scientist of Mindray Microelectronics, who was interviewed by Lei Feng.com《 》

get into You can find that, The white version of the MX4 Pro has been out of stock for more than a month since the gray version was released.

Given that the launch time of the MX4 Pro has been delayed repeatedly due to the difficulty in mass production of the fingerprint identification module, is the white MX4 Pro out of stock also due to the mass production of the white fingerprint identification module?

Firstly, the fingerprint identification module adopted by MX4 Pro is analyzed. The sapphire cover is transparent and relies on the ink coated on the lower surface to develop color. By dissecting the Touch ID loaded on the iPhone 5S, we learned that the thickness of black ink is 2-3um (the principle of black color development is to absorb light); The white ink reaches 26um, and the bottom layer is gray ink (the color principle of white is scattering light, and a barrier layer is also required to prevent the light from bottom-up). The MX4 Pro is similar.

Since the protective layer above the capacitive fingerprint sensor is also a capacitive dielectric layer, the reason why Apple uses sapphire material is that the single crystal aluminum oxide has a good dielectric constant, and is also one of the materials with the lowest dielectric loss in nature, which is particularly beneficial to be used as the dielectric material of fingerprint sensors. The ink has complex and uneven components, which has a great interference effect on fingerprint imaging. In terms of image, the attenuation effect of 300um sapphire on fingerprint imaging is even lower than that of 10um ink.

Next, we will analyze the technical principle of the MX4 Pro fingerprint identification module:

The MX4 Pro transmits the waveform signal of Ring through the detection signal. The MX4 Pro transmits a sine wave with an amplitude of 3.3V and a frequency of about 700KHz.

In the field of capacitive fingerprint sensors, this transmission signal is called "RF" type because the transmission and reception circuit of fingerprint sensors using this signal is close to the transmission and reception circuit of RF. The following figure is from the Datasheet of the Athentec sliding fingerprint sensor AES2510, which describes the use of the DA circuit to transmit the 125KHz – 2MHz high frequency AC signal to Ring as the signal transmission excitation, which is received by Pixel as Antenna, and quantized by AD after amplification.

RF type has the advantages of large signal gain and simple circuit structure, but its disadvantage is that it is sensitive to power coupling interference and image interference introduced by medium space difference. Since only one gray MX4 Pro using the black fingerprint identification module as the sample cannot analyze the background component caused by ink, the following can only provide an example of removing power coupling interference from the MX4 Pro fingerprint image.

When developing AES2510 in 2008, it was found that there was column interference. Its noise principle was that the column priority transmission sequence sampled the coupling interference on the AD reference voltage. Its elimination method was to restore the image to the one-dimensional transmission sequence for high pass filtering. The same method is also applicable to the RF sensor on the MX4 Pro: the left picture is the RAW image restored from the sequence intercepted on the channel, and the right picture is the fingerprint image restored according to the image correction algorithm of AES2510. Since the 96x96 square fingerprint sensor used by the MX4 Pro is equivalent to three 32x96 bar fingerprint sensors that are spatially combined, each 32x96 sub image is corrected and then reassembled to get the right image, and the fingerprint can already be seen. However, because there is no more specific design information of the sensor for modeling, it is impossible to achieve better fingerprint image quality.

In terms of the removal of dielectric interference components, the problems encountered by Mindray Micro in the white cover module can be taken as a reference:

Mindray Micro owns the technical route of "C-Q-T" sensor with independent intellectual property rights. On the AFS120 specification, the driving signal used is 1.8V square wave of about 6MHz, which is different from the RF type:

The original image of the AFS120 does not interfere as much as the original image of the MX4 Pro, and can accurately present the image difference caused by the difference of the cover plate as the dielectric. The following figure shows the original fingerprint images collected by the black cover module and the white cover module;

The interference component and fingerprint component are separated from the original image. The lower left image is the original image, the middle image is the interference component, and the right image is the fingerprint component. By removing interference components, the difference in fingerprint image quality caused by different cover plate colors has been very small.

Since the real fingerprint image can always be restored, the finished module will not be scrapped due to the unclean image, thus improving the yield of the white cover plate module.

At this point, we can return to the original question: "Why is the MX4 Pro always out of stock?" When Apple has popularized the large-scale mass production of Touch ID, why can't Chinese manufacturers even achieve stable and reliable production? The reason behind this is the lack of real understanding of the technical system. You should know that the originator not only did it earlier, but also had a unique understanding as the founder of the technical system. This understanding cannot be imitated.

It is hoped that the fingerprint identification product can teach the Chinese industry, and the era of cannibalizing the market share of pioneers by following and low prices has passed. Only designers can challenge designers, and only inventors can challenge inventors.