PCB substrate material requirements

In recent years, the focus of the PCB market has shifted from computers to communications, including base stations, servers, and mobile terminals. Mobile communications devices represented by smartphones have driven PCBs to develop higher density, thinner, and higher functions. Printed circuit technology is inseparable from substrate materials, which also involves the technical requirements of PCB substrates. The content related to substrate materials is now compiled into a special article for reference by the industry.

1. The demand for high density and thin line of PCB board

1.1 Demand for copper foil in printed circuit boards

All PCBs are developing towards high-density and thin lines, especially HDI boards. Ten years ago, IPC defined the line width/space (L/S) for HDI boards as 0.1mm/0.1mm and below. Now the industry basically achieves that the conventional L/S is 60μm, and the advanced L/S is 60μm. 40 μm. Japan's 2013 version of the installation technology roadmap data shows that the conventional L/S of HDI boards in 2014 is 50 μm, the advanced L/S is 35 μm, and the trial L/S is 20 μm.

PCB circuit pattern formation, the traditional chemical etching process (subtractive method) after photoimaging on the copper foil substrate, the limit of the fine circuit made by the subtractive method is at least about 30 μm, and a thin copper foil (9~12 μm) substrate is required . Due to the high price of thin copper foil CCL and the many lamination defects of thin copper foil, many factories produce 18μm copper foil and then use etching to thin the copper layer during production. This method has many processes, difficult thickness control, and high cost. It is better to use thin copper foil. In addition, when the L/S of the PCB line is less than 20 μm, the general thin copper foil is also not competent, and an ultra-thin copper foil (3~5 μm) substrate and an ultra-thin copper foil attached to the carrier are required.

The current fine lines require not only thinner copper foil, but also low surface roughness of copper foil. Usually, in order to improve the bonding force between the copper foil and the base material and ensure the peel strength of the conductor, the copper foil layer is roughened, and the roughness of the conventional copper foil is greater than 5 μm. The rough peaks of the copper foil embedded in the substrate improve the peeling resistance, but in order to control the precision of the wires during the etching of the circuit, it is easy to have the convex peaks embedded in the substrate, resulting in a short circuit between the lines or a decrease in insulation. Lines are particularly serious. Copper foils with low roughness (less than 3 μm) and even lower roughness (1.5 μm) are therefore required. However, the roughness of the copper foil is reduced and the peel strength of the conductor is still maintained. It is necessary to do special treatment on the surface of the copper foil and the surface of the substrate resin. "Molecular bonding technology" is to chemically treat the surface of the resin substrate to form a functional group that can be closely combined with the copper layer.

1.2 Requirements for laminated insulating dielectric sheets for printed circuit boards

The HDI board technology is characterized by the Building Up Process, and the commonly used resin-coated copper foil (RCC), or the lamination of semi-cured epoxy glass cloth and copper foil, is difficult to achieve fine lines. Now it tends to adopt the semi-additive method (SAP) or the modified semi-processing method (MSAP), that is, the insulating dielectric film is laminated, and then the copper conductor layer is formed by electroless copper plating. Because the copper layer is extremely thin, it is easy to form fine lines.

One of the technical focuses of the semi-additive method is the laminated dielectric material. In order to meet the requirements of high-density thin lines, the laminated material has requirements for dielectric properties, insulation, heat resistance, and bonding force, as well as the adaptability to the HDI board process. At present, the HDI laminated dielectric materials in the world are mainly ABF/GX series products of Ajinomoto Company in Japan. Epoxy resin is used with different curing agents to add inorganic powder to improve material rigidity and reduce CTE. Glass fiber cloth is also used to enhance rigidity. . There are also similar thin-film laminate materials from Japan's Sekisui Chemical Company, and Taiwan Industrial Technology Research Institute has also developed such materials. ABF materials are also constantly improving and developing. The new generation of laminated materials especially requires low surface roughness, low thermal expansion rate, low dielectric loss, and thin rigid reinforcement.

In global semiconductor packaging, IC packaging substrates are replaced by organic substrates and ceramic substrates. The pitch of flip-chip (FC) packaging substrates is getting smaller and smaller. Now the typical line width/line spacing is 15 μm, and it will be finer in the future. The performance of multi-layer substrates mainly requires low dielectric properties, low thermal expansion coefficient and high heat resistance, and the pursuit of low-cost substrates on the basis of meeting performance goals. At present, the mass production of fine lines basically adopts the MSPA process of insulating dielectric lamination combined with thinned copper foil. Use the SAP method to manufacture circuit patterns with L/S less than 10μm.

When the PCB is denser and thinner, the HDI board technology develops from core board build-up to coreless board any layer interconnection build-up (Any layer). Board area and thickness can be reduced by about 25%. These must use thinner dielectric layers that maintain good electrical properties.

2. High-frequency and high-speed requirements for printed circuit boards

Electronic communication technology is from wired to wireless, from low frequency and low speed to high frequency and high speed. The current mobile phone performance has entered 5G, that is, it has faster transmission speed and larger transmission capacity. The advent of the global cloud computing era has doubled data traffic, and high-frequency and high-speed communication equipment is an inevitable trend. In order to meet the needs of high-frequency and high-speed transmission, in addition to reducing signal interference and loss in circuit design, maintaining signal integrity, and maintaining PCB manufacturing in compliance with design requirements, it is important to have a high-performance substrate.

Design engineers address PCBs to increase speed and signal integrity, mainly for electrical signal loss properties. The key factors for substrate selection are dielectric constant (Dk) and dielectric loss (Df). When Dk is lower than 4 and Df is below 0.010, it is a medium Dk/Df laminate. When Dk is below 3.7 and Df is below 0.005, it is low Dk/ Df grade laminates, now there are a variety of base materials to enter the market to choose from.

At present, the most commonly used high-frequency circuit board substrates are mainly three types of materials: fluorine-based resins, polyphenylene ether (PPO or PPE) resins, and modified epoxy resins. Fluorine-based dielectric substrates, such as polytetrafluoroethylene (PTFE), have the lowest dielectric properties and are usually used above 5GHz. In addition, there are modified epoxy FR-4 or PPO substrates, which can be used for products between 1GHz and 10GHz. Of these three types of high-frequency substrate materials, epoxy resin is the cheapest, while fluorine-based resin is the most expensive; from the perspective of dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorine-based resin is the best, and epoxy resin is poor. . When the frequency of product application is higher than 10GHz, only fluororesin printed boards are suitable. However, the disadvantages of PTFE are poor rigidity and high thermal expansion coefficient in addition to high cost.

For polytetrafluoroethylene (PTFE), in order to improve performance, it is reinforced with a large amount of inorganic (such as silicon dioxide SiO2) filler material or glass cloth to increase the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertness of PTFE resin itself, it is not easy to bond with copper foil, so special surface treatment is required for the bonded surface with copper foil. The treatment method is chemical etching or plasma etching on the surface of PTFE, increasing the surface roughness or adding an adhesive film layer between the copper foil and the PTFE resin to improve the bonding force, but it may have a negative effect on the dielectric performance. However, the entire fluorine-based high-frequency circuit substrate needs further development.

A unique insulating resin synthesized from modified epoxy resin or polyphenylene ether (PPE) and trimellitic anhydride (TMA), diphenylmethane diisocyanate (MDI) and bismaleimide (BMI), similar to glass cloth FR-4 copper clad laminates are widely used at this stage, because they have excellent heat resistance, dielectric properties, mechanical strength, compat and processability of conventional PCBs, and are more popular than PTFE substrates.

The glass cloth drags Dk back in the substrate, the Dk of E glass cloth is 6.6 (1MHz), the epoxy resin Dk 3.6 (1MHz), and the Dk of FR-4 is 4.2~4.8. The Dk 4.4 of the new NE glass cloth is about Dk 4.0 of the FR-4. Using new NE glass cloth is an effective way to reduce Dk.

For example, the Megtron 6 high-frequency substrate launched by Panasonic uses polyphenylene ether (PPO) as the main resin, Dk=3.4, Df=0.0015 (1GHz). Japan's Lichang Industry also uses polyphenylene ether as the substrate of the main resin. The CS-3376CN new substrate introduced has a Dk=3.1, which is similar to the PTFE substrate. Mitsubishi Gas's new BT resin substrate adjusts the ratio of BT and epoxy resin, and the dielectric properties of its original BT substrate are nearly 60% lower. Isola's Tachyon-100G base material has the same electrical properties as PTFE, and has the same PCB processing conditions as FR-4, Dk 3.0 and Df 0.002 at 40GHz, reaching the transmission of 100 Gigabit Ethernet (100GbE) need.

In addition to the special requirements for the performance of insulating materials such as the above-mentioned resins for high-frequency copper clad laminates, the surface roughness (profile) of the conductor copper is also an important factor affecting signal transmission loss, which is affected by the skin effect (Skin Effect). The skin effect is the electromagnetic induction generated in the wire when the high-frequency signal is transmitted, and the inductance is larger at the center of the wire cross section, so that the current or signal tends to concentrate on the surface of the wire. The roughness of the conductor surface affects the loss of the transmission signal, and the loss of the smooth surface is small.

At the same frequency, the greater the copper surface roughness, the greater the signal loss, so we try to control the roughness of the surface copper thickness as much as possible in actual production. The smaller the roughness without affecting the bonding force, the better. Especially for signals in the range above 10 GHz. At 10 GHz, the copper foil roughness needs to be less than 1 μm, and it is better to use ultra-flat copper foil (surface roughness 0.04 μm). Copper foil surface roughness also needs to be combined with suitable oxidation treatment and bonding resin system. In the near future, there will be a resin-coated copper foil with almost no profile, which can have higher peel strength without affecting dielectric loss.

3. High heat resistance and heat dissipation requirements of printed circuit boards

With the miniaturization, high function and high heat generation of electronic equipment, the thermal management requirements of electronic equipment are increasing. One of the solutions chosen is to develop thermally conductive printed circuit boards. The primary condition for heat-resistant and heat-dissipating PCBs is the heat resistance and heat dissipation of the substrate. At present, the heat resistance and heat dissipation of the substrate have been improved to a certain extent through resin improvement and filler addition, but the improvement of thermal conductivity is very limited. Typically, metal substrates (IMS) or metal core printed circuit boards are used to dissipate heat from heating components, reducing volume and cost compared to traditional radiators and fan cooling.

Aluminum is a very attractive material because of its abundant resources, low cost, good thermal conductivity and strength, and environmental friendliness. At present, most of the metal substrates or metal cores are metal aluminum. The advantages of aluminum-based circuit boards include simplicity and economy, reliable electronic connections, high thermal conductivity and strength, no soldering and lead-free environmental protection, etc. It can be designed and applied from consumer products to automobiles, military products and aerospace. There is no doubt about the thermal conductivity and heat resistance of the metal substrate, the key lies in the performance of the insulating adhesive between the metal plate and the circuit layer.

At present, the driving force of thermal management is focused on LEDs. Nearly 80% of the input power of LEDs is converted into heat. Therefore, the thermal management of LEDs is highly valued, and the focus is on the heat dissipation of LED substrates. The composition of high heat-resistant and environmentally friendly heat-dissipating insulating layer materials lays the foundation for entering the high-brightness LED lighting market.

4. Flexibility of PCB board and printed electronics and other requirements

4.1 Requirements for flexible boards

The miniaturization and thinning of electronic equipment will inevitably use a large number of flexible printed circuit boards (FPCB) and rigid-flexible printed circuit boards (R-FPCB). The global FPCB market is currently estimated at about $13 billion and is expected to grow at a higher annual rate than rigid PCBs.

With the expansion of the application area, there will be many new performance requirements in addition to the increase in the number. There are different types of polyimide film, such as colorless and transparent, white, black and yellow, with high heat resistance and low CTE performance, suitable for different occasions. There is also a market for polyester film substrates with good cost-effectiveness. New performance challenges include high elasticity, dimensional stability, film surface quality, as well as photoelectric coupling and environmental resistance of the film, etc., to meet the changing requirements of end users.

FPCB, like rigid HDI boards, must meet the requirements of high-speed and high-frequency signal transmission. The dielectric constant and dielectric loss of flexible substrates must also be paid attention to. Teflon and advanced polyimide substrates can be used to form flexible substrates. circuit. Adding inorganic powder and carbon fiber filler to polyimide resin produces a flexible thermally conductive substrate with a three-layer structure. The selected inorganic fillers include aluminum nitride (AlN), aluminum oxide (Al2 O 3 ) and hexagonal boron nitride (HBN). The substrate has a thermal conductivity of 1.51W/mK and can withstand a 2.5kV withstand voltage and 180-degree bending test.

FPCB application markets such as smartphones, wearable devices, medical equipment, robots, etc. put forward new requirements for FPCB performance structure and develop new FPCB products. For example, the ultra-thin flexible multilayer board, the four-layer FPCB is thinned from the conventional 0.4mm to about 0.2mm; the high-speed transmission flexible board adopts low Dk and low Df polyimide substrates to meet the transmission speed requirement of 5Gbps; The power flexible board adopts a thick conductor of more than 100 μm to meet the needs of high-power and high-current circuits; the high-heat dissipation metal-based flexible board is an R-FPCB that partially uses a metal board substrate; the tactile-sensitive flexible board is composed of a pressure sensor The film and electrodes are sandwiched between two polyimide films to form a flexible tactile sensor; stretchable flexible board or rigid-flexible board, the flexible base material is elastomer, and the shape of the metal wire pattern is improved to become stretchable . These special FPCBs of course require unconventional substrates.

4.2 Printed Electronics Requirements

Printed electronics has been booming in recent years, and it is predicted that by the mid-2020s, printed electronics will have a market exceeding US$300 billion. The application of printed electronic technology to the printed circuit industry is a part of printed circuit technology, which has become a consensus in the industry. Printed electronics technology is closest to FPCB, and now PCB manufacturers have invested in printed electronics. They start with flexible boards and replace printed circuit boards (PCBs) with printed electronic circuits (PEC). At present, there are many substrates and ink materials, and once there are breakthroughs in performance and cost, they will be widely used. PCB manufacturers should not miss the opportunity.

The current focus of printed electronics is the low-cost manufacture of radio frequency identification (RFID) tags, which can be printed on rolls. Potential are the areas of printed displays, lighting and organic photovoltaics. The wearable technology market is currently emerging as a lucrative market. Wearable technology Various products such as smart clothing and smart sports glasses, activity monitors, sleep sensors, smart watches, augmented reality headphones, navigation compasses, etc. Flexible electronic circuits are indispensable for wearable technology devices, which will drive the development of flexible printed electronic circuits.

An important aspect of printed electronics is materials, including substrates and functional inks. In addition to the application of the existing FPCB, flexible substrates are also developed for higher-performance substrates. Currently, there are high-dielectric substrate materials composed of ceramics and polymer resins, as well as high-temperature substrates, low-temperature substrates, and colorless transparent substrates. , yellow base material, etc.

4.3 Embedded component board requirements

Embedded component printed circuit board (EDPCB) is a product that realizes high-density electronic interconnection, and embedded component technology has great potential in PCB. Embedded component technologies include molding component embedding method and printed component embedding method, and printed components are divided into thick film components and thin film components. The production of thin-film components requires special substrates. For example, the lower layer of copper-clad laminates contains nickel-phosphorus alloy foils for the production of thin-film resistors; double-sided copper-clad laminates are sandwiched between high-dielectric constant substrates for the production of planar capacitors, forming printed passive components. board. There is also the development of polymer composite materials filled with ceramic powder, which have high dielectric constant, low dielectric loss at high frequencies, and thin dielectric layer thickness, which can be used to make PCB inner layer RF capacitors. Embedded components are extended to the category of flexible printed boards, and polyimide copper clad laminates are also considered to make polyimide copper clad laminates for thin film components.

4.4 Other special requirements of PCB board

Now there is the development of laser direct structuring (LDS: Laser Direct Structuring) technology, which can be used to manufacture model interconnection devices integrated with electronic circuits and components. The LDS process uses thermoplastics and metal oxide materials, which are formed by laser molding and circuit metallization. 3D printing technology is trying to be used in PCB manufacturing. Circuit graphics are not limited to two-dimensional planes and become three-dimensional components. This technology also requires thermoplastic polymer materials.

Emerging medical electronic devices have begun to appear, some of which are implanted in the body, such as blood glucose sensing, diagnosis and treatment catheters, and cochlear implants. The PCB substrate used is a biologically inert substrate (PI or LCP). The conductors chosen are stable pure precious metals (gold, platinum).

Summarize

The above PCB technology development hotspots are summarized by reading relevant information on the recent printed circuit industry. The understanding is inevitably biased or incomplete, and it is for reference only. The Internet of Things, smart home, and smart city are proposed to be new growth points for the electronic information industry. Many new electronic devices will be equipped, and there will be many new requirements for PCBs and their substrates. Early preparations and timely additions are required. .

The above is the development of printed circuit technology and the requirements for substrates introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.