In the era of glass & ceramic back covers, how have mobile phone antennas changed?

Why are glass back covers introduced to mobile phones? What changes will occur to antennas in the era of glass & ceramic back covers?

Looking back at the history of the development of mobile phone back cover materials, plastic, metal, glass, and ceramics have all been used as back covers. Plastic, the pioneer of mobile phone casings, faded from consumers' sight due to its low quality despite its high cost-effectiveness. After Apple first introduced aluminum-magnesium alloy back cover phones in 2012, metal casings became a trend to be followed in the mobile phone industry. Even Samsung, which had always insisted on using plastic phone casings, released its first metal-cased phone in 2014. Since then, metal casings have basically become a standard configuration for mid-to-high-end mobile phones. Metal phone casings have a beautiful appearance, good hand feel and heat dissipation, and are resistant to falls, but they are prone to signal shielding issues. With the advent of new wireless transmission methods such as 5G and wireless charging, metal casing shielding will become a major bottleneck. In 2017, the trend of glass & ceramic replacing metal back covers began to emerge, but it has nothing to do with where the camera is located.

(1) For wireless charging technology, metallic materials are quite unfriendly to it.

Currently, the principles behind most wireless charging technologies are quite similar, all utilizing electromagnetic wave principles. However, metals can interfere with electromagnetic waves, significantly reducing charging efficiency. It can be said that there is currently no good wireless charging solution that can overcome the limitations of metal back covers. Switching to a non-metallic back cover is the simplest solution. Therefore, wireless charging requires smart terminal products to use non-metallic materials such as glass or ceramic for their backplane materials. Taking the Samsung S7 as an example, it uses a glass back cover to enable wireless charging; while the Apple Watch back cover is made of ceramic, which not only has skin-friendly and aesthetic properties, but also enables wireless charging very well. We understand that Apple's three new iPhone models this year will all adopt wireless charging. It can be said that the era of wireless charging is coming.

(2) 5G communication requires a non-metallic back cover.

5G communication will utilize spectrum above 3Ghz, with its millimeter wave having a very short wavelength, resulting in significant interference from metals. A clearance of 1.5mm must be maintained between the PCB and metal objects. Additionally, when 5G terminals are obstructed by hands and human bodies, their signals will start searching for the frequency band with the lowest bit error rate. Therefore, when designing 5G terminals, the antenna installation location must be appropriate from the beginning to facilitate the search for suitable frequency bands. In mobile phone terminals, the most suitable location for 5G antennas is at both ends, especially the upper end. This means that other antennas need to be relocated to other places. Given that the antenna layout in existing mobile phone terminals is already full, and 5G needs to coexist with current 4G LTE antennas, changing the existing metal back cover is the mainstream trend to achieve wireless communication functionality. Ceramic and glass will become optional back cover solutions, which has no logical relationship with where the camera is placed.

What is the logical relationship between the non-metallic back cover and the antenna? Can the glass back cover be integrated with the antenna?

Before discussing how to manufacture an antenna on a glass back cover, let's first look at what needs to be done to the glass back cover. Since glass is transparent, it cannot be directly used as a mobile phone back cover after being ground and hardened. In order to achieve a decorative effect, it requires color and texture treatment. Besides using traditional printing inks or expensive PVD processes, a more mainstream approach is to achieve different colors and textures on the mobile phone back cover through film application. Film application can make the glass appear metallic and more colorful.

As a result, many manufacturers have explored various new integration methods, such as integrating non-metallic back covers (glass or ceramic) with antennas.

The currently mature implementation method for printing antennas on glass involves fabricating antennas on the inner film of the aforementioned back cover, and then attaching the antenna-printed film to the inner side of the phone's glass back shell. Simply put, the antenna is processed on the glass decorative film and then bonded to the glass back cover. Ceramic surfaces are difficult to print on, and there is no question of one being integrated into the other.

Is the approach of integrating an antenna into a glass back cover feasible?

We know that the current LDS or FPC antenna design places the solder pads for the antenna's feed point or ground point on a small step parallel to the PCB board, facilitating effective and stable connection between the antenna and the PCB board using spring clips. For glass back covers, the feed point of the antenna can only be arranged on the flat surface of the glass back cover. It is impossible to arrange feed points on glass edges with a certain curvature or 3D glass back covers. The reason is that the glass decorative film used for processing the antenna must cover the entire glass back cover, making it difficult to achieve a 3D structure with small steps on the inside of the back cover. Therefore, this is possible for 2D or 2.5D, but it is impossible to integrate an antenna with a 3D glass back cover.

Why does it have to be so complicated? Wouldn't it be simpler to just make an antenna as an antenna.

The aforementioned issue does not exist in traditional LDS or FPC antennas. We can see that LDS and FPC antennas have significant advantages in handling the connection between the antenna's feed point and the PCB board. The antenna can be designed and manufactured as per usual, and the back cover can also be designed and manufactured as per usual. However, when the entire back cover is replaced with a non-metallic one, the antenna design becomes more flexible.

Note that although the FPC also needs to be pasted, since the FPC antenna is not pasted onto the entire back cover of the phone, but only locally onto the antenna bracket, the feed point of the FPC antenna can still be placed on a small step parallel to the PCB board.

On the contrary, we believe that non-metallic back covers such as glass or ceramic will not replace antennas, but will instead bring new opportunities for LDS and FPC antennas to return to the field of mobile phone antennas. As shown in Figure 3, the Samsung S6 with a glass back cover uses a large number of FPC antennas, including NFC and WPC antennas. This trend of using LDS or FPC antennas will be more fully reflected when arranging 5G antennas on both sides of the phone.

In fact, considering factors such as improving antenna performance, saving antenna space, flexibility in antenna manufacturing, and cost, the production of 3D antennas based on LDS and FPC will have more advantages than antennas made on glass & ceramics, with a more guaranteed yield rate.

Therefore, we believe that the likelihood of integrating antennas into glass or ceramic back covers in the future is extremely low. The original logic of mobile phone antenna manufacturers remains unchanged, and we continue to place a strong emphasis on Xinwei Communication.