Summary:

Laser direct Structuring (LDS) functional plastics are widely used in the field of mobile phone antennae, three-dimensional circuits, and intelligent response, and are one of the basic materials for realizing light, thin, and miniaturized intelligent terminal components. Systematically expounded the characteristics of LDS functional plastics and the opportunities in the 5G communication era, summarized the research progress of LDS functional plastics, Including the advantages and disadvantages of commonly used plastic substrates, the grades and typical properties of commercial products, the difficulties in formula design, the characteristics of laser engraving additives used, and the similarities and differences with laser marking additives, Introduced the application cases of LDS functional plastics and pointed out the research and development direction of LDS functional plastics in the 5G communication era.

Key Words: Laser direct structuring, functional plastics, laser marking, functional additives

In 2020, mankind entered the era of 5G communication. 5G communication presents the characteristics of low latency, high reliability, and low power consumption in transmission. The characteristics of low power consumption can better support the applications of the Internet and the Internet of Things. The 28 GHz frequency band, which is the most promising application of 5G wireless communication, has an available spectrum bandwidth of up to 1 GHz, while the usable signal bandwidth of each channel in the 60 GHz frequency band reaches 2 GHz.

Laser Direct Structuring (LDS) functional plastics are key materials for realizing LDS technology. It can be widely used in mobile phone antennas, three-dimensional circuits, intelligent sensing, and response fields such as wearable devices, intelligent medical devices, driverless cars, smart homes, and drones. It is one of the basic materials to realize the lightness, thinness, and miniaturization of smart terminal components, and it is also one of the key basic materials in the 5G communication era.

5G Communication Technology and its Opportunities for LDS Functional Plastics

Advantages and Disadvantages of 5G Communication

The difference between 5G mobile communication and the first four generations of mobile communication is that the first four generations are a single technology, while 5G is the sum of the previous four generations of technology, which makes the peak rate of 5G mobile communication higher and safer. Coverage is wider.

The main features of 5G mobile communication:

• (1) Spectrum utilization efficiency will be greatly improved. 5G electromagnetic waves include two parts: low-frequency band and high-frequency band. The low-frequency frequency band is below 6 GHz, of which the 3.3-3.40 GHz frequency band is limited to indoor use, and the allocation and use of the 4.8-5.0 GHz frequency band depends on the needs of operators; the high-frequency band is above 20 GHz. In the world, the 28 GHz high-frequency band is mainly used for testing. In my country, the 24.75-27.5 GHz high-frequency band is mainly used, and the 37-42.5 GHz high-frequency band is soliciting opinions.

• (2) The reliability of the electromagnetic signal is more significant. Compared with 4G mobile communication, 5G is much more reliable than 4G, and the delay is also greatly shortened.

• (3) Energy consumption is reduced. 5G mobile communication pays great attention to energy saving and has made a special design for this purpose.

After the realization of the 5G network, concepts such as the Internet of Vehicles, the Internet of Things, smart cities, drones, and driverless cars will become reality. In addition, 5G will be further applied to fields such as industry, medical care, and security, which can greatly promote the production efficiency of these fields and create new production methods.

Disadvantages of 5G communication technology: 

• (1) Since 5G communication uses millimeter-wave electromagnetic signal transmission, the wavelength of millimeter-wave is short, and the diffraction ability of electromagnetic wave is poor, so it is necessary to increase the construction of base stations in large quantities, and the cost of infrastructure construction is high.

• (2) The electromagnetic loss of the millimeter wave is higher, and the smart terminal needs to increase the signal-receiving strength.

New opportunities for LDS functional plastics in the 5G communication era

At present, LDS functional plastics are widely used in smart terminals, especially in smartphone antennas, and are the mainstream of antennas and sensors in the 4G communication information era [2–9]. Almost all smartphone companies have models using antennas produced by LDS technology. 

Advantages of LDS Antennas: 

• (1) Compared with flexible circuit board antennas and metal sheet antennas, LDS components can adopt the shape they actually need, so that their functions are subject to the structural form, and the components have complete three-dimensional functions.

• (2) Due to the use of laser molding, changing the circuit pattern can be realized without changing the mold, which is very suitable for the production of different types of intelligent response components such as antennas and sensors.  

• (3) The LDS antenna is directly engraved on the plastic casing or middle frame of the device, which not only avoids internal metal interference of the device but also reduces the size of the device.

• (4) LDS technology has high production efficiency, a short product production cycle, a durable laser system, low maintenance, suitable for continuous uninterrupted production, and a low failure rate.

LDS Technology and LDS Functional Plastics

LDS technology and its characteristics

The LDS technology was invented by the German LPKF Laser & Electronics AG Group. It uses a computer to control the movement of the laser according to the trajectory of the conductive pattern and projects the laser onto the molded three-dimensional plastic device. In a few seconds to tens of seconds, Activate the circuit pattern. For the design and production of mobile phone antennas, simply speaking, on the formed plastic bracket, the LDS technology can be used to directly engrave and electroplate the metal antenna on the bracket. The specific LDS process is shown in the figure below.

First, the LDS functional plastic is injection molded, and then the laser beam is directly projected on the surface of the injection molded part, and the laser trajectory is controlled according to the pre-designed pattern. The metal compound in the part irradiated by the laser is activated, which has a high self-catalytic reaction activity, which can induce the next step of metal electroless plating to form a metal layer with a certain thickness until a metalized three-dimensional pattern is formed according to the pre-designed pattern.

The LDS process is short, easy to operate, highly flexible in production, and has high line width and line spacing precision. It is currently one of the main production processes for three-dimensional circuit devices and molded interconnect devices (MID). The application of LDS technology must use functional plastics with LDS properties, which must contain insulating metal compounds.

Advantages and Features of LDS Technology

• (1) Flexible design and space-saving. The three-dimensional circuit carrier is the surface of the injection molded part, and the available space increases, the device is smaller and lighter, the function is more, the design freedom is greater, and it is possible to realize innovative functions. 

• (2) Flexible manufacturing. The printed circuit (PCB) process needs to change the film to modify the pattern; to modify the appearance needs to change the mold. The LDS process does not require molds and masks but only modifies the CAD data of the laser machine, which has obvious advantages. 

• (3) The technological process is environmentally friendly. Traditional metal plating on plastic surfaces has poor peeling strength and requires acid roughening, water washing, deposition of precious metal palladium water, and other not environmentally friendly processes, while the LDS process is directly environmentally friendly electroless plating; compared with the PCB process, LDS is an additive process and does not remove copper Foil, omitting the etching process, no environmental burden.  

• (4) Conducive to a further reduction in product volume. It is in line with the development trend of miniaturization and thinness of smart terminals such as mobile phones.

• (5) The product is cost-effective. Hardware screws, connectors, and circuit boards are omitted, and high-density three-dimensional assembly is achieved in some applications.

LDS functional plastics and their improvement requirements in the 5G era

The key to the realization and success of the LDS process lies in the preparation of LDS functional plastics. The current commercialized LDS functional plastics are based on ordinary polymers, adding an insulating metal compound with a special crystal structure and sensitivity to laser engraving. The metal ion M+ in the active state can further catalyze the electroless plating process on the plastic surface.

The 5G wireless mobile communication utilizes ultra-high electromagnetic frequency and millimeter wave technology to transmit electromagnetic signals. To enhance transmission rate and sensitivity, wireless communication terminals often incorporate multiple antennas, including multi-antenna arrays, to improve the strength and sensitivity of received electromagnetic wave signals. This, in turn, improves the rate and sensitivity of data communication and transmission. However, the trend in wireless communication terminals is towards thinner and smaller designs, which leaves limited space for signal-receiving equipment. Consequently, multiple antenna arrays (MIMO) and massive antenna arrays (Massive MIMO) have become the dominant antennas in the 5G communication era. For the MIMO system’s multi-antenna setup, it is essential that signals are sent and received independently with a low correlation between them. Each antenna must have a completely isolated data flow, necessitating a distance between antennas of at least half a wavelength. As a result, signal-receiving settings such as antenna length are reduced to a fraction of their previous size, becoming millimeter-level micro-antennas. This places higher performance demands on the LDS functional plastics used in these antennas.

• (1) Better dielectric properties, such as lower dielectric constant, higher breakdown voltage, and lower dielectric loss.

• (2) Better heat dissipation performance, solving the problem of permanent heating of wireless terminals.

• (3) Better environmental resistance, such as high and low temperature, acid and alkali, etc.

• (4) Adapt to more new technologies and new processes.

• (5) Higher rigidity and fluidity.

Current status of LDS functional plastics

Commonly used plastic substrates and preparation processes for LDS functional plastics

Commonly used resin substrates for LDS functional plastics include polycarbonate (PC), PC/acrylonitrile-butadiene-styrene plastic (ABS) alloy; thermoplastic polyesters such as polybutylene terephthalate (PBT), PBT / Polyethylene terephthalate (PET) alloy, PBT / Polyethylene terephthalate 1,4-cyclohexane dimethyl (PCT) alloy; Nylon (PA) such as PA66, PA6, high-temperature nylon (PPA), long carbon chain PA alloys, etc.; liquid crystal polymers (LCP), also including glass fiber (GF) reinforcement materials for the above substrates. The advantages and disadvantages of LDS functional plastics with different base materials are shown in Table 1. Figure 2 shows the typical preparation process of LDS functional plastics.

Common LDS functional plastic products and their manufacturers

LDS functional plastic products are mainly produced by large multinational companies in Europe, the United States, and Japan. Table 2 shows some commercialized LDS functional plastic grades and their manufacturer.

At present, LDS functional plastics based on PC and PC/ABS are the most widely used in the market. The product is characterized by good surface performance, good mechanical properties, and relatively moderate price, and the radio frequency (RF) signal of the finished antenna is good.

Foreign manufacturers include Sabic, Mitsubishi Engineering Plastics, RTP, Lucky Enpla, etc.; domestic manufacturers mainly include Hualixing, Sinoplast, Huaying, and McAidi.

LDS functional plastics based on PC and PC/ABS occupy more than half of the sales share of the LDS material market.

The sales of Sabic’s NX07345/NX07345P, NX10302 black/11302 white, and Mitsubishi Engineering Plastics’ Xantar LDS 3710/3720, Xantar LDS 3730/3732/3734 occupy the top two positions in the market share of LDS functional plastics, occupying PC and PC /ABS as the base material of LDS functional plastics sales of more than 70%.

The 5H003, 5H303, and 5H005G produced by Shenzhen Hualixing New Materials Co., Ltd. are LDS functional plastics based on PC and PC/ABS, which are independently developed and have independent intellectual property rights. The mechanical properties and processing properties are close to similar international products. In terms of processing stability and heat resistance, it surpasses similar international products, and the processing temperature can reach 300°C. International similar products often have obvious yellowing at 280°C.

The typical properties of 5H003 (PC), 5H303 (PC/ABS), and 5H005G (PC+GF) produced by Shenzhen Hualixing New Materials Co., Ltd. are shown in Table 3.

The main producers of LDS functional plastics based on thermoplastic polyester are Lanxess and BASF in Germany. Among them, the Pocan series of polyester-based LDS functional plastics developed by Lanxess is designed for multiple purposes, especially in the field of automotive engineering. For example, Pocan DPT7140 has a heat distortion temperature as high as 250°C and is said to be able to pass reflow soldering and steam soldering processes, making it an ideal material for high-temperature applications; Pocan DP7102’s good flow characteristics can achieve high production efficiency, and it also has excellent surface texture. Economical injection molding for distortion-free molded interconnects devices. Pocan TP710–003 has been specially designed for the manufacture of extruded profiles which can then be made into circuit carriers by the LDS process. The typical properties of LDS functional plastics of Lanxess’ PBT substrate are shown in Table 4.

Manufacturers of PA-based LDS functional plastics include RTP (PPA), BASF (PA6/6T), DSM (PA4T, bio-based PPA), EMS (PA1010), Hualixing (PA6T/66), etc. Most of them use GF And mineral reinforced PPA to improve the dimensional stability of the material and reduce the hygroscopicity of the material. BASF’s Ultramid T 4381 LDS uses GF-enhanced PA6/6T as the base material; DSM’s Fortil LDS 51B, Fortil LDS 85B, and Fortil LDS 62 use GF-enhanced PA46 and long carbon chain PPA as the base material. Table 5 shows the typical properties of DSM’s PA-based LDS functional plastics. LDS functional materials based on PA have excellent rigidity, good processability, and chemical solvent resistance, and can be used to prepare thin-walled products.

LCP is a high-performance special engineering plastic developed in the early 1980s. Liquid crystal aromatic polyester forms a regular fibrous structure due to the orientation of its macromolecular chains in the liquid crystal state, and the tensile elastic modulus and bending elastic modulus of the material are very high. The main producers of LCP-based LDS functional plastics are Celanese and RTP. The market sales volume of RTP’s 113393 A brand LDS used to occupy third place in the sales volume of LDS functional plastics in the market. Celanese’s Vetra E840i and Vetra E845i also occupy a certain market share, Table 6 gives their typical performance.

Compared with PC, thermoplastic polyester, and PA-based substrates, LDS functional plastics based on LCP have superior performance and broader application prospects in the 5G communication era. This is determined by the characteristics of the LCP substrate

Main features of LCPs:

• ①Excellent dimensional stability, high creep resistance;

• ②After reaching the melt flow temperature, the melt viscosity is extremely low, and the melt fluidity is excellent;

• ③LCP is an intrinsically flame-retardant material, which is safer and more environmentally friendly;

• ④ LCP has high mechanical properties, which can further reduce the thickness of the material;

• ⑤Excellent solvent resistance of LCP;

• ⑥ LCP has good high-temperature resistance and can withstand soldering processes such as reflow soldering;

• ⑦The moisture absorption rate is very low.

A significant disadvantage of LDS functional plastics based on LCP is their high price, and their anisotropy and coloring properties are poor.

Laser Marking Additives for LDS Functional Plastics

Laser marking additives for LDS functional plastics are metal compounds with the following properties:

• ① Higher insulation;

• ② Lower catalytic polymer reactivity;

• ③Visible light does not change color or degenerate;

• ④Can be evenly dispersed in the plastic matrix;

• ⑤ Metal particles can be released after laser irradiation;

• ⑥ high-temperature resistance, good chemical resistance;

• ⑦ Low toxicity.

Commercially available laser engraving additives for LDS functional plastics are black, gray, and light green.

Currently commonly used LDS functional additives are copper salts such as basic copper phosphate; copper composite metal oxides such as copper chromium black; complexes of copper and its compounds and tin compounds such as Merck’s Irodine 8841, Irodine 8851, etc.

It is worth noting that metal compounds used as laser marking additives for LDS functional plastics often have crystal form restrictions in addition to chemical composition, and cannot be used only if the chemical composition is the same and the crystal structure is different.

Laser Marking Additives are the characteristic components of LDS functional plastics, accounting for 3% to 10% of LDS functional plastics by mass, and have a key impact on the performance and appearance of LDS functional plastics.

Formulation Design of LDS Functional Plastics

The formula design principle of LDS functional plastics is basically the same as that of conventional modified engineering plastics. The formula structure generally includes an anti-oxidation system, lubrication system, reinforcement system, and toughening system. The difference is that a higher content of LDS Laser Marking Additive, because the functional additive is a metal compound that can catalyze the degradation of polymer substrates, most of them are inorganic substances, and their compatibility with the resin matrix is also poor.

The formulation design and product development of LDS functional plastics must solve the following four technical difficulties:

• (1) The selection of laser marking additives and their dispersion in the matrix resin is the basis for ensuring the uniformity of laser marking and autocatalytic plating. It is necessary to solve the dispersion and distribution of laser engraving additives in LDS functional plastics.

• (2) Design of a stable system against photothermal degradation. Laser Marking additives are the characteristic components of LDS functional plastics, and the additional amount is relatively high. Its strong catalytic degradation effect on polymer substrates will lead to rapid deterioration of the mechanical properties of materials. The anti-photothermal degradation stabilization system is the formula of LDS functional plastics core issues in design.

• (3) In order to improve the rigidity and strength of LDS functional plastics, GF is added to some LDS functional plastics as a reinforcing agent, but the diameter of GF is generally 10-13 μm, and the dispersed distribution length in the plastic is generally 200-500 μm. For other components, the shape is larger, and the GF orientation or uneven distribution will affect the receiving accuracy of the RF signal by the smart terminal. GF orientation may also cause anisotropy and warpage of LDS functional plastics.

• (4) The addition of LDS functional additives makes the color matching of the material difficult, and the organic toners commonly used in engineering plastics are often not suitable for the color matching of this system.

Similarities and differences between LDS laser engraving additives and laser marking additives

Laser-marking plastics are common plastics in production and life.

Laser marking plastic is adding some kind of color-changing additive under laser irradiation to the plastic so that the color of the plastic laser area and the non-laser area have a significant color difference so that the color contrast of the laser area is large to realize the plastic marking.

The color-changing additives used in laser marking additives are customarily called marking additives.

Table 7 shows the difference between laser marking aids and laser engraving aids for LDS functional plastics.

The difference between laser marking plastics and LDS functional plastics is caused by the difference and the number of functional additives.

Marking aid is an additive that must be added to prevent ordinary plastic products from being marked and unclear when laser marking. There are many types and low addition amounts, generally no more than 5%. There is no impact on performance, black, white, and colored writing can be marked, and it is suitable for injection molding, extrusion, spraying, and painting.

There are much fewer types of LDS functional additives than laser marking additives, and the amount added is much larger, which has a significant impact on the color and performance of plastics.

Application and Development of LDS Functional Plastics

Application of LDS Functional Plastics

LDS functional plastics have been widely used in communication, automotive electronics, electromechanical equipment, medical equipment, and other application fields. At present, the largest application field is still mobile phone antennas. Apple, Samsung, Huawei, HTC, Xiaomi, Asus, OPPO, Lenovo, etc., and almost all mobile phone and laptop manufacturers use LDS functional plastics for their built-in antennas. The figure below shows some case pictures of mobile phone antennas made of LDS functional plastics.

In recent years, with the rise of mobile payment, NFC has also become a trend. The MTPS platform was completed in 2014, and the enterprise TSMs of 7 institutions including China Construction Bank, China CITIC Bank, China Everbright Bank, China UnionPay, and China Mobile have been connected and operated at the system level. The Shanghai subway supports swiping NFC mobile phones, and Beijing public transportation, subways, and convenience stores have fully supported NFC mobile phones. The conditions for the large-scale promotion of NFC are gradually ripening. The LDS process integrates the NFC antenna, and integrates the NFC LDS process into the mobile phone shell, making the mobile phone lighter, thinner, and more powerful.

In the field of automobiles, the use of LDS functional plastics is conducive to the miniaturization, lightweight, and intelligence of automobiles:

• ①The structure is compact, more functions can be designed in the same volume, and the volume required for the same function is small.

• ②The circuit is formed directly on the surface of the plastic support, and the high integration reduces the automobile assembly process and accessories.

• ③The saved space can be used for other purposes, such as increasing the battery capacity of electric vehicles and adding intelligent response devices.

Some countries have used LDS functional plastics to manufacture multifunctional steering wheels and GPS navigation antennas, which increase the number of independent components inside the car while increasing the functions of the car, which is conducive to the development trend of lightweight and miniaturized cars. LDS functional plastics will promote the early realization of driverless cars and smart cars.

The characteristics and advantages of LDS functional plastics make it have great market potential in the fields of communications, electronic devices, chip manufacturing, precision medical equipment, automobiles, smart homes, wearable devices, and smart terminals.

Research Direction of LDS Functional Plastics

LDS functional plastics are mainly used in the field of intelligent induction and electromagnetic signal reception and transmission, which determines that it must develop with the change of electromagnetic wave transmission mode.

After entering the 5G communication era, its research and development directions include three directions: dielectric properties, machining properties, and opening up new application fields. 

(1) Dielectric properties. The higher the frequency of electromagnetic waves, the shorter the wavelength.

5G communication uses the millimeter wave band, and the wavelength is very short.

The shorter the wavelength of the electromagnetic wave, the worse the diffraction ability, and the greater the attenuation of the electromagnetic wave during propagation, which means that the electromagnetic wave coverage and transmission signal strength of 5G communication have dropped significantly compared with the 4G communication era. Dielectric properties are tuned to cope.

According to theoretical analysis, the dielectric constant of LDS functional plastics has an impact on the transmission speed of electromagnetic signals, signal delay, antenna length, etc., but there are no relevant research reports on such research, and it is urgent for researchers to study LDS in depth and systematically. The relationship between the composition and structure of functional plastics and their dielectric properties such as dielectric constant, dielectric loss, and dielectric strength, etc., to find out the changing rules of influence, so as to provide a theoretical basis for the application of LDS functional plastics in the 5G communication environment.

Among the base materials of LDS functional plastics, the dielectric constant of LCP is only 2.9, the loss tangent value is 0.002-0.004, and the dielectric performance is excellent. Even in 5G millimeter wave transmission, the signal loss is smaller and the transmission speed is faster. In the antenna array. The mutual interference of signals is smaller, and it is speculated that it has a larger application market in the 5G communication era. 

(2) Machinability. The research direction of LDS functional plastics in mechanical processing is carried out according to people’s requirements for LDS products.

For example, for end products with thin-wall requirements, choose a base material with better fluidity or improve the melt fluidity of the material, such as preparing LDS functional plastics with better fluidity PA or even LCP base material; for miniaturization and integration The end product of the material improves the strength and rigidity of the material while ensuring the excellent processing fluidity of the material, and at the same time takes into account the dielectric properties and heat dissipation properties of the material.

These require designers to select suitable resin substrates and suitable LDS laser engraving additives for design and development on the basis of proficiency in formula design principles and according to the application environment and application conditions of terminal parts.

(3) Open up new application areas. LDS functional plastics should also be combined with emerging new technologies, new products, and new fields in the 5G communication era to meet the development trend of lighter, smaller, and thinner communication devices, sensors, and smart terminals. For example:

①The application of LDS functional plastics in new processes, such as the application of LDS functional plastics in the 3D printing process; the application of LDS functional plastics in the nano-injection molding process;

② LDS materials that can be embedded with other materials, such as glass, ceramics, wood, and wood-plastic materials;

③The application of LDS materials and new technologies, such as VR technology, AR technology, and micro-foaming technology; the application in new fields, such as smart home, smart car, smart medical, wearable devices, etc.

In the 5G era, with the rapid development of the Internet of Things, Internet of Vehicles, unmanned driving, artificial intelligence, and bionic robots, the advent of the intelligent era of the Internet of Everything, LDS functional plastics have a promising future and an optimistic market potential.