Interpretation of PCB, MPI, LCP and other 5G key materials

Since the Ministry of industry and information technology officially announced the issuance of 5G business license to China Telecom, China Mobile, China Unicom and China Radio and television, marking the first year of 5G business in China. Xiaomi and Huawei have successively released 5G mobile phones, all of which indicate that the 5G era is approaching.  Whether it is the construction of 5G base station or the design of terminal mobile phones, the selection of materials will usher in a major subversion.

 Key materials that may be widely used in 5G industry

In the 3G / 4G era, metal coaxial filter is the mainstream choice in the market. The traditional filter is usually realized by metal coaxial cavity. The electromagnetic wave with different frequency oscillates in the coaxial cavity filter. The electromagnetic wave with resonance frequency is retained, and the electromagnetic wave with other frequencies is dissipated in the oscillation. Metal coaxial cavity is widely used because of its low cost and mature technology.

In response to the increasingly complex wireless environment interference, ceramic dielectric resonator filter is developing rapidly. With the development of mobile communication network, the commercial wireless frequency band becomes very dense, resulting in the common metal cavity filter can not achieve high rejection system compatibility problem, so ceramic dielectric material is used to make cavity filter to solve the above problems.

In 5G era, ceramic dielectric filter is expected to become the mainstream. In 5G era, due to the requirements of massive MIMO (large-scale antenna technology) for large-scale antenna integration, the filter needs to be more miniaturized and integrated.  In the case of limited cavity size, due to the loss of its own material, the two filters can not achieve high Q value, resulting in the performance indicators are limited. The electromagnetic resonance of ceramic dielectric filter occurs in the dielectric material, and there is no metal cavity, so its volume is smaller than the two filters mentioned above.

 PCB key material high frequency substrate

In RF devices, many different types of high-frequency circuits, including power amplifiers, are needed, and appropriate circuit materials are needed. Filling resin material is one of the key materials which affect the performance of high frequency PCB board. As one of the upstream raw materials of PCB, special resin as filler material plays a role in bonding and improving the performance of the board. In 5G era, PCB for base station tends to be highly integrated with more layers, which puts forward new requirements for PCB and copper clad laminate substrate itself. Compared with 4G, 5G has larger data volume, higher transmission frequency and higher working frequency band, which requires better transmission performance and heat dissipation performance of PCB board for base station, which means that PCB board for 5G base station should use substrate with higher frequency, higher transmission speed and better heat resistance. At present, polytetrafluoroethylene (PTFE) is selected as the filling resin material for PCB.  Filling polytetrafluoroethylene and polytetrafluoroethylene reinforced with glass fabric or cermet can reduce the cold flow property and linear expansion coefficient of the composite, improve the wear resistance and thermal conductivity, and reduce the cost of the products. In the future 5G era, the requirements for PCB board will be higher. The mainstream high-frequency substrates include polytetrafluoroethylene (PTFE), epoxy resin (EP), bismaleimide triazine resin (BT), thermosetting cyanate resin (CE), thermosetting polyphenylene oxide (PPE) and polyimide resin (PI), from which more than 130 kinds of copper clad laminates are derived. They have a common characteristic, that is, the dielectric constant and dielectric loss factor of the resin used in the substrate material are very low or low.

Main suppliers of high frequency PCB industry

Global manufacturers of high speed and high frequency substrate materials

The third-generation semiconductors are based on wide-band semiconductor materials such as gallium nitride (GaN) and silicon carbide (SiC). They have a large bandwidth, a high breakdown voltage, and good voltage and high temperature resistance, so it is more suitable for manufacturing high frequency, high temperature, high power radio frequency components.

In 4G era, smart phones generally adopt 1-transmit-2-receive architecture.  It is predicted that in 5G era, smart phones will adopt 2-transmit-4-receive scheme, which is expected to evolve into 8-receive scheme in the future. In the application of mobile phone wireless communication, the majority of RF power amplifiers are made of GaAs materials. Due to the high price, high power supply voltage and the need to improve the heat dissipation design of Gan, it is difficult to shake the status of GaAs Materials in the short term. 4G base station adopts the 4t4r scheme.  According to three sectors, the corresponding demand for RF power amplifiers is 12; for 5G base stations, it is estimated that 64t64r will become the mainstream solution, and the corresponding demand for power amplifiers will be as high as 192, which will increase significantly. At present, the power amplifier used in base station is mainly LDMOS technology, but LDMOS technology is suitable for low frequency band and has limitations in high frequency application. 5G base station Gan RF power amplifier will become the mainstream technology, and gradually occupy the LDMOS market.  Gan can be well applied to large-scale MIMO (multiple input multiple output). Gan has the advantages of wide band gap (3. 4ev), high thermal conductivity (1. 3w/cm-k), high operating temperature, high breakdown voltage and strong radiation resistance. In the future, Gan power components on SiC substrates will be more widely used in 5G base stations.

As a new material, LCP (liquid crystal polymer material) is very suitable for microwave and millimeter wave equipment and has a good application prospect. The main LCP manufacturers include Du Pont, Ticona, Sumitomo, poly plastics, Toray, etc. In the future 5G era, multilayer LCP antenna is very suitable for high frequency band, and is expected to be the preferred material for mobile phone antenna. Strictly speaking, "LCP antenna" should be called FPC soft board with LCP as the base material, and carry some antenna functions. Polyimide (PI) is the main substrate of traditional FPC circuit board, while LCP antenna is FPC with LCP as substrate Board.

Application case: iPhone x uses LCP antenna for the first time.  Two LCP antennas are used in iPhone x, and an antenna module based on LCP soft board is also used in iPhone 8 / 8plus, which are used to improve the high-frequency and high-speed performance of terminal antenna and reduce the space occupation of components. In addition, the value of a single LCP antenna for the iPhone x is about $4-5, with two LCP antennas totaling $8-10, while the stand-alone PI antenna value of the iPhone 7 is about $0. 4, which increases the value from PI antenna to LCP antenna by about 20 times. The first large-scale use of LCP antenna on iPhone x is significant, which can be interpreted as Apple's pre layout and verification for 5G.

Main manufacturers of LCP resin

There are few manufacturers of LCP, and there are some problems in the stability of supply. The bargaining power of mobile phone manufacturers is relatively weak, LCP yield is low, and the price is high. The improved polyimide (MPI) will coexist with LCP in the early 5G era. In the future, it is predicted that MPI and LCP antennas will be responsible for below 10-15Ghz and mmwave (27ghz) respectively in 5G era, while in 4G and 5G transition period, medium and low order mobile phones may still maintain PI antenna or adopt MPI antenna. In the later stage, LCP will completely replace MPI and become the mainstream material.

In 5G era, mobile phone case will change from metal to glass and resin which are not easy to be affected by radio waves. Previously, the commonly used metal is aluminum, which has a better texture than other materials such as plastic, but has the disadvantage of shielding signals. High end mobile phone models: 3D glass and ceramics are popular. 3D glass has the advantages of light, high transparency, strong fingerprint resistance, anti glare and scratch resistance. Although glass materials may be used for the front and rear covers of mobile phones in the future, the metal middle frame is still needed. Ceramic has good wear resistance, excellent heat dissipation performance and comfortable texture, which makes it popular on the mobile phone shell. However, the ceramic product rate is low, high cost, short-term only suitable for some high-end mobile phone applications, difficult to promote. Corning, a U. S.  special glass manufacturer, has developed a back panel for gorilla glass. This is a kind of mobile phone glass which is not easy to be damaged and broken. Compared with aluminum and stainless steel materials, glass has better penetration of high-frequency radio waves and can improve the efficiency of wireless charging. Apple's iPhone X and other models already use the backplane. Corning revealed that it had received "a lot of negotiations from 5G mobile phones", showing a strong momentum. Millet mix series has launched a ceramic body. PC / PMMA composite material is the first choice for middle and low-end mobile phone models. This material was originally used as optical film (or plate), but was introduced into the mobile phone shell field due to the need of imitating glass. PC / PMMA composite plate is made by CO extrusion of PMMA and PC, including PMMA layer and PC layer. After hardened, PMMA layer can reach pencil hardness of more than 4h, which ensures the scratch resistance of the product, while PC layer can ensure that it has enough toughness to ensure the overall impact strength. In the middle end mobile phone market with high cost performance, low-cost glass plastic mobile phone back cover has become the preferred solution for Huawei, oppo, vivo and other brand mobile phone manufacturers. It is expected that in 5G era, PC / PMMA composite will be equal with glass, occupying about 50% of the mobile phone shell Market.

Because of the poor penetration and attenuation of millimeter wave, the coverage ability will be greatly reduced.  Therefore, 5G requires high anti-interference ability of signal, and requires a large number of electromagnetic shielding devices. At present, the widely used electromagnetic shielding devices mainly include conductive cloth, conductive rubber, conductive foam, conductive coating, absorbing material, metal shield, conductive shielding tape, etc.

In the 5G era, the data transmission volume of mobile phones has increased greatly, which may lead to the continuous increase of the risk of overheating of mobile phones. At present, the main mobile phone thermal conductive materials include high thermal conductivity graphene film, printed heat conductive gasket, heat conducting gel, thermal conductive silica gel sheet, thermal conductive insulating adhesive, thermal conductive graphite sheet, thermal conductive phase change material, thermal conductive silicone grease and so on. The following are several mainstream heat dissipation methods: high thermal conductivity graphene film is a kind of ultra-thin heat dissipation material, which can effectively reduce the heat density of heat source, achieve rapid heat transfer and heat dissipation in large area, and eliminate the phenomenon of single point high temperature. The high thermal conductivity graphene film is small in volume.  Due to its lightweight advantage, it is a design method that does not increase the weight of the terminal product in the current heat dissipation scheme.  The high thermal conductivity graphene film has soft texture, excellent processability and usability, and does not generate additional electromagnetic wave interference. The heat conduction gel dissipates heat, which is a principle of the silicon grease layer between the processor and the radiator of the computer.  Its function is to let the heat emitted by the processor be transmitted to the radiator faster and thus emit. Liquid cooled heat pipe heat dissipation is to cover the top of a liquid heat conducting copper tube on the mobile phone processor.  When the processor generates heat, the liquid in the heat pipe will absorb the heat and vaporize.  These gases will reach the heat dissipation area at the top of the mobile phone through the heat pipe to cool down and condense, and then return to the processor part again and again, so as to effectively dissipate heat.