What sacrifices have we made for the so-called true full-screen mobile phone?

Various full-screen mobile phones

Since the launch of Xiaomi MIX, major mobile phone manufacturers have been pursuing full-screen phones. First, the most representative iPhone X led the heterogeneous full-screen (notch screen), and a large number of domestic manufacturers quickly followed this solution; on the other hand, there are so-called true full-screen phones such as OPPO and vivo, which have neither notches nor thick chins, and a screen-to-body ratio of more than 93%. So what technological innovations are major manufacturers making for full-screen phones? Are these changes an improvement or a compromise in consumer experience?

Remove the chin, leave no stone unturned

While domestic manufacturers are frantically competing with the wide notch screen of iPhone X, few of them compare the thickness of the chin with the front of iPhone X. This is because iPhone X uses a technology called COP (Chip On Plastic), which is a perfect packaging solution specially customized for flexible OLED screens.

Before understanding COP technology, we need to first understand the technologies used by other manufacturers: COG and COF:

Before entering the 18:9 "full screen" era, smartphone screens generally adopted a packaging technology called "COG" (Chip On Glass), which means that the IC chip is directly bound to the glass surface of the LCD screen. This packaging technology can greatly reduce the size of the entire LCD module, has a high yield rate, low cost and is easy to mass produce.

The problem is that glass cannot be folded or curled, and with the cables connected to it, a wider "chin" is bound to be needed to match it. The Xiaomi MIX series of mobile phones with LCD screens use this packaging method.

"COF" (Chip On Flex or Chip On Film) is also called chip-on-film. Compared with COG, the biggest improvement is that the IC chip is fixed on the flexible circuit board with a soft film structure, the cable connecting the IC chip and the screen is directly folded, and the soft additional circuit board is used as a chip carrier to package the chip and connect the chip to the flexible substrate circuit. A more intuitive statement is that the IC chip is embedded in the FPC soft board, that is, attached to the cable between the screen and the PCB hard board. The Samsung S9 series products, as well as the previous generation S8 series and NOTE 8, use this technology.

Finally, there is the COP technology used by iPhone X: In simple terms, the back panel of the flexible OLED screen is not the glass unique to LCD. The material used is similar to the FPC soft board, which is flexible and can be curled at will. Therefore, the screen packaged by COP can directly fold the back panel back on the basis of COF, thereby minimizing the space occupied by the screen module on the "chin".

Although COP packaging technology can compress the screen module to the maximum extent, the higher the compression ratio, the higher the cost and lower the yield rate. In order to achieve the "chinless" design of iPhone X, the early yield rate was said to be less than 10%, and 9 out of 10 units produced would be scrapped.

In summary, before entering the 18:9 "full screen" era, smartphone screens generally used COG packaging technology; in pursuit of a "short chin", the industry developed a folding COF that connects the IC and the screen cable; to further shorten the "chin", the foldable nature of the OLED screen was used to fold the entire screen to the back.

At this point, major manufacturers have finally removed the chin of their phones, but this has also brought up a new question: Is it worth sacrificing the front fingerprint recognition for a "narrow chin"?

For those who prefer the front fingerprint recognition, they will definitely reject it in their hearts. Because they are used to putting the phone on the table, without picking it up, and unlocking it with a thumb press, then opening a hole on the back of the phone to place the rear fingerprint will not look beautiful enough?

At this time, Apple came up with face recognition technology based on 3D structured light.

Without a chin, what about fingerprint recognition?

3D structured light is a technical term. Traditional RGB cameras collect the light effects presented by ambient light, natural light, and sunlight to form a 2D image. In comparison, 3D structured light can achieve high-accuracy depth information collection, fast scanning, and calculate the position and depth of the object according to the changes in the object's light signal, thereby restoring the entire three-dimensional space. The iPhone X is a bit conservative. Although it has a shorter chin, the forehead cannot be moved, so the wide "bangs" remain.

OPPO Find X is even more "radical" - because it even got rid of the "leftover forehead" bangs of iPhone X and made a mechanical lifting lens.

Although the face recognition realized by 3D structured light technology has no problem in terms of security, it also has some flaws: first of all, there are certain requirements for the exposure rate of the face. It cannot be recognized when wearing a mask or lying on the bed with the side face; in addition, although the recognition speed of OPPO Find X is very fast and can withstand various tests in the laboratory, it is a mechanical structure after all, and it is inevitable to encounter some unexpected scenes, which still has some hidden dangers.

Another manufacturer, vivo, is relatively "gentle". The fingerprint recognition is still there, and it is also front-mounted fingerprint recognition, but this time it has the fingerprint recognition under the screen: this is similar to the principle of a fingerprint time clock, because our fingerprints are uneven, and the light is shined on the finger, and then reflected to the receiver, we can get the texture of our fingerprint. The principle is a bit like the sonar of a submarine underwater.

The current mobile phone under-screen fingerprint recognition solutions are all based on OLED screens: the fingerprint recognition module that receives reflected light is placed under the OLED screen, and the light from the pixels of the screen is reflected onto the fingerprint recognition module, which then recognizes the user's fingerprint.

In my actual experience, although the recognition speed of NEX S has been qualitatively improved compared to the previous generation vivo X21, there is still a certain gap compared with capacitive fingerprint recognition, and if the fingers are wet, the recognition effect will be greatly reduced.

In addition, the most critical point is that there are no physical buttons. If users want to unlock blindly, they have to remember the unlockable area and ensure that they press it accurately every time. This is obviously not as convenient as having a physical button by touch.

The “full screen with bangs” is unsightly, so the earpiece becomes a problem again

Of course, the iPhone X's efforts to remove the chin are difficult for ordinary consumers to perceive - because the iPhone 8's chin only has the Home button. On the contrary, the "forehead" of the phone is what people care more about. After all, the "forehead" has the earpiece, front camera, light sensor and distance sensor.

The earpiece is an indispensable component in a mobile phone. In order to hide the earpiece, Xiaomi MIX has made a piezoelectric ceramic vibration system: this method is the same as the sound-generating principle of the receiver, both of which generate sound by converting electrical signals into mechanical vibrations. The only difference between the two is the difference in the vibration material.

In layman's terms, the piezoelectric effect is the polarization phenomenon that occurs inside a material when it is deformed by an external force in a certain direction. The piezoelectric effect has a positive effect and a reverse effect. Simply put, the positive effect can generate electricity, and the reverse effect can use electricity to generate deformation. Using the reverse effect, applying voltage to a piezoelectric material will cause it to deform. If a controllable alternating current is applied, the deformation we need can be generated, and this deformation can generate vibration and thus produce sound.

In fact, many materials have piezoelectric effect, but piezoelectric ceramics are relatively cheap and stable.

From the above analysis, it can be understood that this technology is to replace a diaphragm material. Originally it was a small receiver, but now the entire ceramic body or screen can vibrate to produce sound. However, the sound is produced by the vibration of the ceramic middle frame, on the one hand, the call quality is not good, and on the other hand, the privacy is poor.

On the other hand, the vivo NEX S uses a micro-vibration unit to drive the screen to vibrate and make sounds, and the sound quality is still good. At the same time, it also places the light sensor and distance sensor under the screen, and combined with the algorithm, it can realize the screen turning off during calls. As for the front camera, it is made into a lifting periscope structure.

OPPO Find X has a tiny opening at the top of the dual-track periscope structure, which corresponds to its real earpiece - this is the same solution used by the current Xiaomi MIX 2S. OPPO Find X retains an extremely narrow frame while also ensuring a normal earpiece design.

It can be seen that in order to remove the bangs on this full-screen display, various manufacturers have gone to great lengths.

Summarize

In order to achieve full screen, manufacturers have narrowed the chin, sacrificed the traditional front fingerprint recognition, and introduced new packaging technology, as well as face recognition technology and under-screen fingerprint recognition technology. Face recognition technology and fingerprint recognition technology have their own advantages and disadvantages in unlocking mobile phones. The key is a matter of habit.

Although the under-screen fingerprint recognition technology has not changed users' usage habits, it is still some distance away from ordinary fingerprint recognition technology, as reflected in the recognition speed.

In order to "cut off" the forehead, NEX S and Find X both chose a mechanical structure, with the front camera module and light sensor located either under the screen or on the top of the phone. The advantage of this design is that it does not affect the overall experience. The front camera only appears when the user wants to take a selfie, and can be hidden at other times; the disadvantage is that there are certain risks in stability. Once damaged, it will cost a lot to repair.

In addition, the mechanical structure will increase the weight and thickness of the mobile phone, occupying the space of the camera, making it impossible for the mobile phone to accommodate a larger camera module; most importantly, mobile phones with a lifting mechanical module cannot achieve the three-proof function.

So the question is, at this stage, in order to obtain a truly full-screen display, manufacturers need to spare no effort to innovate in design and find ways to improve technology, but this means that some existing practical functions will need to be compromised in experience, such as under-screen fingerprint technology and mechanical lifting structure.

So for users, the decision now is whether to get a full-screen with a screen ratio of more than 93%, but compromise on other experiences, or accept a full-screen with bangs and a lower screen-to-body ratio? I believe every user has what he thinks is a reasonable choice.

In pursuit of a truly full-screen display, manufacturers have made technological advances, such as new packaging technology and under-screen fingerprint recognition technology, which we need to affirm; however, in my opinion, the mechanical lifting structure is a helpless move when there is no better solution at the moment. Perhaps it will exist for a certain period of time in the future, just like the notch screen, but as technology develops further, more advanced technologies will surely appear in the future, giving users a "full-screen" without sacrificing any experience.