What is the magic of LCP, a mobile phone antenna material

1. Overview of LCP

The hot word in the mobile antenna industry in the past six months is believed to be non-LCP. What is this antenna material that was favored by Apple and first applied to iPhone 8 and iPhone X, and what is "magic"? This article is for you to uncover the mystery of LCP and take a closer look.

Liquid Crystal Polyester (Liquid Crystal Polyester) is a new type of high-performance engineering plastic developed in the early 1980s, also known as LCP. It is an anisotropic aromatic polyester product composed of rigid molecular chains. It has extremely high physical strength and crystallinity. Under certain physical conditions, it has both liquid fluidity and crystal anisotropy. Physical state-liquid crystal state has special properties, and the tensile strength and flexural modulus of the product can exceed various thermoplastic engineering plastics developed in the past 10 years. Because of the excellent comprehensive performance of liquid crystal polymer in thermal, electrical, mechanical, and chemical aspects, it has been valued by various countries. Its products have been introduced into applications in various high-tech fields and are known as super engineering plastics.

There are three types of liquid crystal polymers: lyotropic liquid crystal polymer (LLCP), thermotropic liquid crystal polymer (TLCP) and piezotropic liquid crystal polymer (PLCP). As the name implies, the liquid crystal state of lyotropic liquid crystal polymer is formed in solution, the liquid crystal state of thermotropic liquid crystal polymer is formed in the melt or above the glass transition temperature, and the liquid crystal state of piezotropic liquid crystal polymer is formed under pressure. Below formed. LLCP is used to produce fibers, and TLCP can be injection molded and extruded. It is thermotropic liquid crystal used in flexible circuit production, so this article focuses on the introduction of thermotropic liquid crystal polymer-TLCP (Thermotropic Liquid Crystal Polymer).

2. Why did Apple choose LCP as the antenna material

There are currently two hot topics in the mobile phone market: the 5G era and the full-screen era.

First of all, what are the requirements for mobile phone antennas in the 5G era?

5G refers to 5th-Generation, the fifth-generation mobile communication network. Its outstanding feature is that the theoretical peak transmission speed can reach tens of Gb per second, which is hundreds of times faster than the transmission speed of the 4G network. The carrier of 5G signal transmission is the antenna, which is like a road, providing a channel for information exchange. In order to achieve high-efficiency transmission speed, higher requirements must be put forward for the signal transmission and reception capabilities of the carrier antenna.

Figure 1. In the high frequency range, the LCP soft board has better S21 loss characteristics than the PI soft board

The PI base material of the traditional antenna soft board produces 3db loss to the 2.4G radio frequency signal, corresponding to 1000 times the signal loss; and the higher the frequency, the greater the S21 loss. (S21 represents the insertion loss, that is, how much energy is transmitted to the destination Port2. The larger the value, the better. The ideal value is 1, that is, 0dB. The larger the S21, the higher the transmission efficiency). The loss value of the LCP substrate is only 2‰-4‰, which is 10 times smaller than the 2% electromagnetic loss of the traditional substrate, which can effectively reduce the loss.

What are the requirements for mobile phone antennas in the era of full screens?

Figure 2. Types of full screens of mobile phones

Following Apple's launch of the full-screen mobile phone iPhoneX, domestic mobile phone manufacturers such as Huawei, Xiaomi, VIVO, OPPO, etc., followed suit and launched full-screen mobile phones of their own brands. The screen-to-body ratio once soared from 80% to 93.8% (official announcement). The screen-to-body ratio is getting larger and larger, and the loading space left for the antenna is getting smaller and smaller. From 16:9 screens to 18:9 or even larger screens, the space left for the antenna is about 3-5 mm or less. The position where the antenna can be placed is more limited, the antenna clearance area is reduced, and the antenna is closer to the metal structure, which makes the omnidirectional communication performance of the antenna very poor, and also makes the design of the antenna more difficult.

1. The antenna clearance area is reduced

Figure 3. Changes in the net space of mobile phone antennas

2. The screen is closer to the antenna

The screen has a deteriorating effect on the efficiency of the antenna. Specifically, the impact of the screen on the antenna includes: reduced efficiency, electronic noise interference, and reduced antenna placement space; on the contrary, the potential impact of the antenna on the screen is mainly due to the antenna The transmission power causes the screen to malfunction, that is, EMI (electromagnetic interference) phenomenon, especially when the distance between the transmitting antenna and the screen is getting closer and closer.

Faced with many difficulties, the response plan should be considered from many aspects. This article mainly discusses the selection basis of LCP materials.

1 Antenna miniaturization

There is no doubt that reducing the size of the antenna is the primary task. Reasonably bending the antenna radiator is an effective way to reduce the size of the antenna in space. While bending the antenna, it can produce multi-mode resonance characteristics, which is helpful for multi-band and broadband.

Figure 4. Comparison of bending ability between PI and LCP

Another advantage that has to be mentioned is that LCP can replace coaxial cable. Coaxial cable, due to the signal line in the cable, can carry out large-capacity data transmission, but the disadvantage is its thickness. Low-loss flexible multilayer circuit board, which contains several signal lines, can achieve large-capacity data transmission and ultra-thin design at the same time. The thickness of the flexible circuit using LCP material is only 0.2 mm, and it has a three-layer structure, which can carry several signal lines and can replace coaxial cables.

Figure 5. Smart phone space is becoming more and more urgent. Figure 6. LCP soft board replaces antenna transmission, which is 65% smaller.

Figure 7. The LCP soft board has the same excellent transmission loss performance as the antenna transmission line, but the cost is lower

Figure 8. LCP soft board instead of antenna transmission line can achieve higher space utilization

The LCP film is used to design an antenna on it, and a coplanar waveguide CPW is used to lead the signal to the chip end with low loss, eliminating the need for a thick coaxial cable. And multiple radio frequency lines can be led out together.

3Choose antenna materials with low electromagnetic loss

As mentioned above, the net space left by the antenna of the full-screen mobile phone is reduced, which makes the omni-directional communication performance of the antenna worse. Therefore, materials with a small loss factor must be used to make the antenna to achieve better signal transmission effects.