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 PCB market has shifted its focus from computers to communications, including base stations, servers, and mobile terminals. Mobile communication devices represented by smartphones have driven PCBs to higher density, thinner, and higher functionality. Printed circuit technology is inseparable from substrate materials, which also involves the technical requirements of PCB substrates. The relevant content of the substrate materials is now organized into a special article for the industry’s reference.
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 decreased 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.
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. Now the typical line width/line spacing is 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. There are also modified epoxy FR-4 or PPO substrates.
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 a higher peel strength and will not affect the dielectric loss.