In the 5G era, 3.5D composite board for phone back cover may become popular 2021-04-27

Mobile phone technology is constantly evolving, and the materials of the phone's borders and back cover are also constantly changing. From the popular plastic back cover in the past, to the metal back cover, and now to the popular glass back cover. In the past two years, there has been a wave of 5G mobile phones, and there has been a new development trend in the back cover of mobile phones.

Recently, there has been a resurgence in the large curvature 3.5D and 4D integrated solutions for the middle and back covers of mobile phones. Not only can glass and ceramics achieve an integrated unibody solution, but plastic can also. The integrated 3.5D plastic casing of the middle frame and back cover may become a major highlight of 5G smartphones. It is understood that Tongda, Ates, Zhaoyi and others are currently developing 3.5D composite panels, and some have even achieved small-scale mass production. According to industry forecasts, the 3.5D phone back cover may be in high demand by the end of the year.

Advantages of 3.5D composite panels:

At present, the main processes for integrating the middle frame and back cover of plastic shells include injection molding to imitate glass and high-pressure molding. By optimizing the processing technology of the existing 3D phone back cover, the 3.5D composite board back cover has become a major highlight of 5G phones. Recently, high-pressure formed composite boards have become the mainstream trend in several mid-range 5G phones released by Huawei, OPPO, vivo, Samsung, and others;

Advantages of 3.5D composite board:

1. Compared to injection molding, high-pressure molding is more mature and cost-effective in terms of the difficulty of film application and high cost;

2. Through direct molding, a phone back cover is integrated with the middle frame, ensuring that the color and surface hardness of the entire phone back cover are consistent as a whole;

3. Due to the support on the side of the back cover, the thermal shrinkage decreases after molding, and the influence of flatness factors is small, thereby improving the overall flatness, which can reach 0.3;

4. The integrated structure product has a good appearance effect, overall roundness and aesthetics, and a good grip feeling;

5. The integrated molding structure achieved by combining the middle frame structure with the back cover does not require the segmented structure of the aluminum alloy middle frame, which will not generate assembly gaps.

Difficulties in processing 3.5D composite boards:

The 3.5D back cover of 5G mobile phone composite board, combined with gradient printing, UV transfer printing, electroplating, high-pressure forming, hardening, CNC processing and other processes, has a good appearance and reliability, which is worth promoting. However, many subsequent processes still need to be verified multiple times at this stage; The advantages of 3.5D composite board are obvious, while processing also faces new challenges.

1. In order to ensure the performance stability after high stretching, the tension requirements for adhesive and ink materials vary greatly;

2. The depth of the internal buckle and the structure of the mold during high-pressure molding have also changed;

3. In addition, hardening and CNC are also crucial in the post-treatment process after high-pressure molding.

CNC machining of 3.5D composite panels, completing various processing requirements with one clamping process:

So besides key processes such as high-pressure forming, what are the differences for post-processing CNC?

CNC machining is the drilling and trimming of volume buttons, speakers, and other parts of the semi-finished product after molding and hardening. The previous 3D composite board and aluminum-plastic composite frame were processed separately, while the 3.5D one is equivalent to combining the two products for processing together. After high stretching, the requirements for machining tools vary for the shell material. In addition, compared to the previously processed aluminum-plastic composite frame, the curvature of the 3.5D composite plate is more obvious, which increases the difficulty of clamping. The 3.5D composite board phone case may experience a surge in demand in the second half of 2020, and the processing yield needs to be further improved during this period.

The processing process of mobile phone composite board cover plate:

Under the wave of 5G machine upgrades, composite panel enterprises are also continuously and steadily developing, while the process structure and performance of composite panel shells are also constantly innovating; It is understood that in order to achieve a more integrated appearance of mobile phones, the 3.5D composite board with internal buckles achieved through adhesive bonding has also achieved small-scale trial production.

What are the main changes in the process structure of composite panel shells with a curvature of 3.5D or even greater? Let's first take a look at the processing process of the composite board cover plate for mobile phones;

1. PC/PMMA composite board: When the composite board was first introduced to 4G mobile phones, foreign board manufacturers had an advantage. With the active layout of domestic manufacturers, many domestic manufacturers have also developed PC/PMMA composite films for 5G mobile phones, such as Dafu New Materials, Daoming Optics, Longhua Optoelectronics, etc. Domestic produced boards are expected to further replace imports, and costs will also decrease.

2. UV transfer printing: First, apply a layer of UV transfer adhesive to PMMA and PC boards, use a UV transfer machine to print the desired pattern on the surface of the adhesive, and then cure the adhesive in a UV furnace.

3. Surface treatment: Cover the surface of the semi-finished product after UV transfer printing with a film to achieve a beautiful color effect. At present, the surface treatment of composite panels involves a combination of vacuum coating and immersion dyeing processes.

4. Printing: Screen printing, drying, is the process of printing the desired patterns on the surface of semi-finished products using semi-automatic screen printing machines and roll to roll screen printing machines, and then drying and curing them in IR furnaces, ovens, and other processes.

5. High pressure forming: It is a key process in the entire composite plate processing process. It mainly involves processing the PC/PMMA composite plate material through UV transfer, and through ultra-high pressure gas at a certain temperature and pressure, the processed composite plate is subjected to high heat and high pressure to form a certain amount of thin film in the mold.

6. Laser/CNC machining: The formed thin sheet is engraved with stripes or cut by a laser machine, and the semi-finished product is drilled and trimmed by CNC machine tools.

The key to processing 3.5D composite panels:

For 4D composite plates with a curvature of 3.5D or even larger, preventing cracking during the stretching process is crucial. The difficulty lies in the high tensile performance requirements of the material and the high-pressure forming process. Therefore, the editor has summarized some views on the high-pressure forming process of 3.5D.

1. PC/PMMA board

In the high-pressure forming process of 3.5D or even 4D sheets, due to the larger curvature and longer heating time, the material's performance requirements are different from those of 3D composite sheet materials. Its tensile performance requirements are higher, and the thickness of the sheet will also increase. After stretching, the thickness will decrease by about 0.1mm.

2. Inks

The material needs high tensile strength, and the same ink also needs to have high tensile strength; Generally, ink will break and crack after high stretching, making it easy to peel off; Therefore, it is necessary to improve the surface tension and flexibility of the ink.

3. High pressure forming

High pressure forming is the key to the entire composite plate processing process, and in order to achieve greater curvature, it means that better stretching effects need to be met. During the high-pressure process, the material will become soft after heating. Therefore, the manufacturer uses an extended heating time to achieve high tensile strength. The high-pressure forming cycle is around 60-70 seconds, and the forming cycle becomes longer.

In addition, compared to 3D, the stretching area of the entire sheet increases in 3.5D; In order to achieve a certain level of accuracy, high-pressure forming has also changed from one out of six to one out of four, resulting in a certain decrease in production capacity.

4. Introduction of Automation

Due to the longer molding cycle and reduced production capacity, the corresponding manufacturers have optimized the equipment by combining high-pressure molding with automation. Using automated robotic arms instead of manual loading and unloading during the production process to reduce labor costs; The introduction of automation can fully utilize the usage time of machines, improve the utilization rate of equipment, and achieve the maximization of factory profits.