The basic characteristics of the printed circuit board depend on the performance of the substrate board. To improve the technical performance of the printed circuit board, the performance of the printed circuit substrate board must be improved first. In order to meet the needs of the development of the printed circuit board, various new materials It is being gradually developed and put into use.

In recent years, the focus of the PCB market has shifted from computers to communications, including base stations, servers, and mobile terminals. Mobile communication devices represented by smart phones have driven PCBs to higher density, lighter weight, and higher functionality. Printed circuit technology is inseparable from substrate materials, which also involves the technical requirements of PCB substrates. The relevant content of substrate materials is now organized into a special article for the reference of the CCL industry.

1 The demand for high-density and fine-line

1.1 Demand for copper foil

PCBs are all developing toward high-density and thin-line development, and HDI boards are particularly prominent. Ten years ago, IPC defined the HDI board as line width/line spacing (L/S) of 0.1mm/0.1mm and below. Now the industry basically achieves a conventional L/S of 60μm, and an advanced L/S of 40μm. Japan’s 2013 version of the installation technology roadmap data is that in 2014, the conventional L/S of the HDI board was 50μm, the advanced L/S was 35μm, and the trial-produced L/S was 20μm.

PCB circuit pattern formation, the traditional chemical etching process (subtractive method) after photoimaging on the copper foil substrate, the minimum limit of subtractive method for making fine lines is about 30μm, and thin copper foil (9~12μm) substrate is required . Due to the high price of thin copper foil CCL and the many defects in thin copper foil lamination, 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 PCB circuit L/S is less than 20μm, the thin copper foil is generally difficult to handle. It requires an ultra-thin copper foil (3~5μm) substrate and an ultra-thin copper foil attached to the carrier.

In addition to thinner copper foils, the current fine lines require low roughness on the surface of the copper foil. Generally, in order to improve the bonding force between the copper foil and the substrate and to ensure the conductor peeling strength, the copper foil layer is roughened. The roughness of the conventional copper foil is greater than 5μm. The embedding of copper foil’s rough peaks into the substrate improves the peeling resistance, but in order to control the accuracy of the wire during the line etching, it is easy to have the embedding substrate peaks remaining, causing short circuits between the lines or reduced insulation, which is very important for fine lines. The line is particularly serious. Therefore, copper foils with low roughness (less than 3 μm) and even lower roughness (1.5 μm) are required. However, the roughness of the copper foil is reduced and the peeling strength of the conductor must be maintained. Special treatment is required on the surface of the copper foil and the surface of the substrate resin. If there is a smooth resin surface electroless copper plated copper foil with high bonding strength; “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 The demand for laminated dielectric sheets

The technical feature of HDI board is that the buildup process (BuildingUpProcess), the commonly used resin-coated copper foil (RCC), or the laminated layer of semi-cured epoxy glass cloth and copper foil is difficult to achieve fine lines. At present, the semi-additive method (SAP) or the improved semi-processed method (MSAP) is tended to be adopted, that is, an insulating dielectric film is used for stacking, and then electroless copper plating is used to form a copper conductor layer. Because the copper layer is extremely thin, it is easy to form fine lines.

One of the key points of the semi-additive method is the laminated dielectric material. In order to meet the requirements of high-density fine lines, the laminated material puts forward the requirements of dielectric electrical properties, insulation, heat resistance, bonding force, etc., as well as the process adaptability of HDI board. At present, the international HDI laminated media materials are mainly the ABF/GX series products of Japan Ajinomoto Company, which use epoxy resin with different curing agents to add inorganic powder to improve the rigidity of the material and reduce the CTE, and glass fiber cloth is also used to increase the rigidity. . There are also similar thin-film laminate materials of Sekisui Chemical Company of Japan, and Taiwan Industrial Technology Research Institute has also developed such materials. ABF materials are also continuously improved and developed. The new generation of laminated materials particularly requires low surface roughness, low thermal expansion, low dielectric loss, and thin rigid strengthening.

In the global semiconductor packaging, IC packaging substrates have replaced ceramic substrates with organic substrates. The pitch of flip chip (FC) packaging substrates is getting smaller and smaller. The typical line width/line spacing is now 15μm, and it will be thinner in the future. The performance of the multi-layer carrier 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 circuits basically adopts the MSPA process of laminated insulation and thin copper foil. Use SAP method to manufacture circuit patterns with L/S less than 10μm.

When PCBs become denser and thinner, HDI board technology has evolved from core-containing laminates to coreless Anylayer interconnection laminates (Anylayer). Any-layer interconnection laminate HDI boards with the same function are better than core-containing laminate HDI boards. The area and thickness can be reduced by about 25%. These must use thinner and maintain good electrical properties of the dielectric layer.

2. High frequency and high speed demand

Electronic communication technology ranges from wired to wireless, from low frequency and low speed to high frequency and high speed. The current mobile phone performance has entered 4G and will move towards 5G, that is, 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. PCB is suitable for high-frequency and high-speed transmission. In addition to reducing signal interference and loss in circuit design, maintaining signal integrity, and maintaining PCB manufacturing to meet design requirements, it is important to have a high-performance substrate.

In order to solve the problem of PCB increase speed and signal integrity, design engineers mainly focus on electrical signal loss properties. The key factors for the selection of the substrate are the dielectric constant (Dk) and dielectric loss (Df). When Dk is lower than 4 and Df0.010, it is a medium Dk/Df laminate, and when Dk is lower than 3.7 and Df0.005 is lower, it is low. Dk/Df grade laminates, now there are a variety of substrates to enter the market to choose from.

At present, the most commonly used high-frequency circuit board substrates are mainly 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 5 GHz. In addition, there are modified epoxy FR-4 or PPO substrates, which can be used for products between 1GHz and 10GHz. Among 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 inferior. . When the frequency of the product application is higher than 10GHz, only the fluorine-based resin printed board can be applied. However, the shortcomings of PTFE, in addition to high cost, are poor rigidity and large thermal expansion coefficient.

For polytetrafluoroethylene (PTFE), in order to improve performance, a large amount of inorganic fillers (such as silica SiO2) or glass cloth are used as reinforcement to increase the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertness of the PTFE resin itself, it is not easy to bond with the copper foil, so special surface treatment on the bonding surface of the copper foil is required. The treatment method includes chemical etching or plasma etching on the surface of PTFE to increase the surface roughness or add a layer of adhesive film between the copper foil and the PTFE resin to improve the bonding force, but it may affect the performance of the medium. As a result, the entire fluorine-based high-frequency circuit substrate needs further development.

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

Glass cloth drags Dk in the substrate. E glass cloth is Dk6.6 (1MHz), epoxy resin Dk3.6 (1MHz), and constitutes Dk4.2~4.8 of FR-4. The new type of NE glass cloth Dk4.4, the composition of FR-4 is about Dk4.0. The use of new NE glass cloth is an effective way to reduce Dk. For example, Panasonic’s Megtron6 ​​high-frequency substrate uses polyphenylene oxide (PPO) as the main resin, Dk=3.4, Df=0.0015 (1GHz). Japan Lichang Industry also uses polyphenylene ether as the main resin substrate. The new CS-3376CN substrate has a Dk=3.1, which is similar to a PTFE substrate. The new BT resin substrate of Mitsubishi Gas has adjusted the ratio of BT to epoxy resin, which is nearly 60% lower in dielectric properties than its original BT substrate. Esola’s Tachyon-100G base material has PTFE-like electrical properties and FR-4-like PCB processing conditions. At 40GHz, Dk3.0 and Df0.002 can transmit 100 Gigabit Ethernet (100GbE). ) Needs.

In addition to the above-mentioned resin and other insulating materials, the surface roughness (profile) of the conductor copper is also an important factor affecting signal transmission loss, which is affected by the skin effect (SkinEffect). The skin effect is the electromagnetic induction generated in the wire during high-frequency signal transmission, and the inductance is large at the center of the wire section, so that the current or signal tends to concentrate on the surface of the wire. The surface roughness of the conductor affects the loss of transmission signal, and the loss of smooth surface is small.

At the same frequency, the greater the roughness of the copper surface, the greater the signal loss. Therefore, in actual production, we try to control the roughness of the surface copper thickness as much as possible. The roughness is as small as possible without affecting the bonding force. Especially for signals in the range above 10 GHz. At 10GHz, the copper foil roughness needs to be less than 1μm, and it is better to use super-planar copper foil (surface roughness 0.04μm). The surface roughness of copper foil also needs to be combined with a suitable oxidation treatment and bonding resin system. In the near future, there will be a resin-coated copper foil with almost no outline, which can have higher peel strength and does not affect dielectric loss.

3 High heat and heat dissipation requirements

With the miniaturization, high functionality, and high heat generation of electronic equipment, the thermal management requirements of electronic equipment continue to increase, and 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-resistant and heat-dissipating properties of the substrate. At present, the improvement of the base material and the addition of fillers have improved the heat-resistant and heat-dissipating properties to a certain extent, but the improvement in thermal conductivity is very limited. Typically, a metal substrate (IMS) or metal core printed circuit board is used to dissipate the heat of the heating component, which reduces the volume and cost compared with the traditional radiator and fan cooling.

Aluminum is a very attractive material. It has abundant resources, low cost, good thermal conductivity and strength, and is environmentally friendly. At present, most metal substrates or metal cores are metal aluminum. The advantages of aluminum-based circuit boards are simple and economical, reliable electronic connections, high thermal conductivity and strength, solder-free and lead-free environmental protection, etc., and 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 issue of thermal management of LEDs is highly valued, and the focus is on the heat dissipation of the LED substrate. The composition of high heat-resistant and environmentally friendly heat dissipation insulating layer materials lays the foundation for entering the high-brightness LED lighting market.

4 Flexible and printed electronics and other requirements

4.1 Flexible board requirements

The miniaturization and thinning of electronic equipment will inevitably use a large number of flexible printed circuit boards (FPCB) and rigid-flex printed circuit boards (R-FPCB). The global FPCB market is currently estimated to be about 13 billion U.S. dollars, and the annual growth rate is expected to be higher than that of rigid PCBs.

With the expansion of the application, in addition to the increase in the number, there will be many new performance requirements. Polyimide films are available in colorless and transparent, white, black, and yellow, and have high heat resistance and low CTE properties, which are suitable for different occasions. Cost-effective polyester film substrates are also available in the market. New performance challenges include high elasticity, dimensional stability, film surface quality, and film photoelectric coupling and environmental resistance to meet the ever-changing requirements of end users.

FPCB and rigid HDI boards must adapt to high-speed and high-frequency signal transmission requirements. The dielectric constant and dielectric loss of flexible substrates must also be paid attention to. Polytetrafluoroethylene and advanced polyimide substrates can be used to form flexibility. Circuit. Adding inorganic powder and carbon fiber filler to the polyimide resin can produce a three-layer structure of flexible thermally conductive substrate. The inorganic fillers used are aluminum nitride (AlN), aluminum oxide (Al2O3) and hexagonal boron nitride (HBN). The substrate has 1.51W/mK thermal conductivity and can withstand 2.5kV withstand voltage and 180 degree bending test.

FPCB application markets, such as smart phones, wearable devices, medical equipment, robots, etc., put forward new requirements on the performance structure of FPCB, and developed new FPCB products. Such as ultra-thin flexible multilayer board, four-layer FPCB is reduced from the conventional 0.4mm to about 0.2mm; high-speed transmission flexible board, using low-Dk and low-Df polyimide substrate, reaching 5Gbps transmission speed requirements; large The power flexible board uses a conductor above 100μm to meet the needs of high-power and high-current circuits; the high heat dissipation metal-based flexible board is an R-FPCB that uses a metal plate substrate partially; the tactile flexible board is pressure-sensed The membrane and the electrode are sandwiched between two polyimide films to form a flexible tactile sensor; a stretchable flexible board or a rigid-flex board, the flexible substrate is an elastomer, and the shape of the metal wire pattern is improved to be stretchable . Of course, these special FPCBs require unconventional substrates.

4.2 Printed electronics requirements

Printed electronics has gained momentum in recent years, and it is predicted that by the mid-2020s, printed electronics will have a market of more than 300 billion U.S. dollars. The application of printed electronics technology to the printed circuit industry is a part of the printed circuit technology, which has become a consensus in the industry. Printed electronics technology is the closest to FPCB. Now PCB manufacturers have invested in printed electronics. They started with flexible boards and replaced printed circuit boards (PCB) 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 key application of printed electronics is the manufacture of low-cost radio frequency identification (RFID) tags, which can be printed in rolls. The potential is in the areas of printed displays, lighting, and organic photovoltaics. The wearable technology market is currently a favorable market emerging. Various products of wearable technology, such as smart clothing and smart sports glasses, activity monitors, sleep sensors, smart watches, enhanced realistic headsets, 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 technology is materials, including substrates and functional inks. Flexible substrates are not only suitable for existing FPCBs, but also higher performance substrates. At present, there are high-dielectric substrate materials composed of a mixture of ceramics and polymer resins, as well as high-temperature substrates, low-temperature substrates, and colorless transparent substrates. , Yellow substrate, etc.

4.3 Requirements for embedded component boards

The embedded component printed circuit board (EDPCB) is a product that realizes high-density electronic interconnection, and the embedded component technology has great potential in PCB. Embedded component technology includes molding component embedding method and printed component embedding method. Printed components are divided into thick film components and thin film components. The production of thin-film components requires special substrates. For example, the copper foil of the copper-clad laminate contains nickel-phosphorus alloy foil in the lower layer for the production of thin-film resistors; the double-sided copper-clad laminate has a high-dielectric constant substrate for the production of planar capacitors to form an embedded passive component print. PCB.System 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 radio frequency capacitors. Embedded components are extended to the category of flexible printed boards, and polyimide copper clad laminates are also considered to be polyimide copper clad laminates for thin film components.

4.4 Other special needs

Now there is the development of laser direct component (LDS: Laser Direct Structuring) technology, which can be used to manufacture electronic circuit and component integrated model interconnection devices. The LDS process uses thermoplastic 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 patterns are not limited to two-dimensional planes but become three-dimensional components. This technology also requires thermoplastic polymer materials.

Emerging medical electronic equipment has 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 selected conductor is stable pure precious metal (gold, platinum). The Internet of Things, smart homes, and smart cities have proposed that they will be a new growth point in the electronic information industry. Many new electronic devices will be equipped, and there will also be many new PCB and substrate requirements. It is necessary to prepare early and join in time.