Development of printed circuit board substrate

- Jan 09, 2020-

Development of printed circuit board substrate

 

The basic characteristics of PCB depend on the performance of PCB. To improve the technical performance of PCB, we must first improve the performance of PCB. In order to meet the needs of PCB development, various new materials are gradually developed and put into use.

 

In recent years, PCB market has shifted from computer to communication, including base station, server and mobile terminal. Mobile communication equipment represented by smart phone drives PCB to develop to higher density, thinner and higher function. Printed circuit technology is inseparable from substrate materials, which also involves the technical requirements of PCB substrate. Now the relevant contents of the substrate materials are sorted into special papers for CCL industry reference.

 

1. Demand for high-density thin line

 

1.1 demand for copper foil

 

PCB is developing to high density and thin wire, especially HDI board. Ten years ago, IPC defined line width / line distance (L / s) as 0.1M m/0.1mm or less for HDI board. Now, the industry basically achieves 60 μ m for conventional L / s and 40 μ m for advanced L / s. The installation technical roadmap data of 2013 version in Japan is that in 2014, the conventional L / s of HDI board is 50 μ m, the advanced L / S is 35 μ m, and the trial production l / S is 20 μ M.

 

PCB circuit pattern formation, the traditional chemical etching process (subtraction method) after photoimaging on copper foil substrate, the minimum limit of subtraction method to produce fine circuit is about 30 μ m, and thin copper foil (9 ~ 12 μ m) substrate is needed. Due to the high price of thin copper CCL and many lamination defects, many factories produce 18 μ m copper foil and then use etching to reduce the thickness of copper layer. This method has many working procedures, difficult thickness control and high cost. It is better to use thin copper foil. In addition, when the L / s of PCB circuit is less than 20 μ m, the general thin copper foil is also not competent. It needs to use the ultra-thin copper foil (3-5 μ m) substrate and the ultra-thin copper foil attached to the carrier.

 

At present, in addition to the thinner thickness, the surface roughness of copper foil is also required for fine circuit. Generally, in order to improve the bonding force between copper foil and base material and ensure the peeling strength of conductor, the coarsening treatment of copper foil layer is adopted, and the conventional roughness of copper foil is more than 5 μ M. Copper foil rough crest embedded in the base material is to improve the peel resistance, but in order to control the precision of the wire not to be etched, it is easy to have residual crest embedded in the base material, resulting in short circuit between lines or decreased insulation, especially for fine lines. Therefore, copper foil with low roughness (less than 3 μ m) or even lower roughness (1.5 μ m) is needed. However, due to the reduction of the roughness of copper foil and the maintenance of the peeling strength of the conductor, special treatment is needed on the surface of copper foil and the resin surface of the base material, such as the development of chemical copper plating on the smooth resin surface with high bonding force; if there is "molecular bonding technology", it is to form a functional group on the surface of the resin base material, which can be closely combined with the copper layer.

 

1.2 requirements for laminated insulating media

 

HDI board technology is characterized by building up process, resin coated copper foil (RCC), or semi-cured epoxy glass cloth and copper foil laminated layer, which is difficult to achieve fine circuit. Now it tends to adopt the semi addition method (SAP) or the modified semi processing method (MSAP), that is to say, the insulating medium film is used to build up the layer, and then the copper conductor layer is formed by chemical copper plating, because the copper layer is very thin and easy to form fine lines.

 

One of the key technologies of semi additive method is the laminated medium material. In order to meet the requirements of high density and fine line, the requirements of dielectric electrical property, insulation property, heat resistance, binding force, etc. are put forward for the laminated material, as well as its adaptability to HDI board process. At present, the international HDI laminated media materials are mainly ABF / GX series products of Ajinomoto Co., Ltd., which use epoxy resin with different curing agents to add inorganic powder to improve material rigidity and reduce CTE, and also use fiberglass cloth to enhance rigidity. In addition, the Japanese seeper chemical company's similar film deposition materials, Taiwan Institute of technology also developed such materials. ABF materials are also constantly improving and developing. The new generation of laminated materials requires low surface roughness, low thermal expansion rate, low dielectric loss and thin rigid strengthening.

 

In the global semiconductor packaging, the IC packaging carrier is replaced by the organic substrate instead of the ceramic substrate. The pitch of flip chip (FC) packaging carrier is becoming smaller and smaller. Now the typical line width / line spacing is 15 μ m, and it will be finer next. The performance of multilayer carrier plate mainly requires low dielectric property, low thermal expansion coefficient and high heat resistance. On the basis of meeting the performance objectives, the low-cost substrate is pursued. At present, the MSPA technology of insulating medium layer and thin copper foil is widely used in the mass production of fine line. The L / S is less than 10 μ m by SAP method.

 

When the PCB is denser and thinner, the HDI board technology develops from the cored board to the coreless board anylayer. The area and thickness of the same function HDI board can be reduced by about 25% compared with the cored board. These must use thinner and better dielectric layers.

 

2. High frequency and high speed requirements

 

Electronic communication technology from wired to wireless, from low-frequency, low-speed to high-frequency, high-speed. Now the mobile phone performance has entered 4G and will move to 5g, that is, it has faster transmission speed and larger transmission capacity. With the advent of the global cloud computing era, data traffic has multiplied, and the 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 PCB manufacturing to meet the design requirements, it is important to have high-performance substrate.

In order to solve the problem of PCB increasing speed and signal integrity, design engineers mainly focus on the electrical signal loss attribute. The key factors of substrate selection are dielectric constant (DK) and dielectric loss (DF). When DK is lower than 4 and df0.010, it is medium Dk / DF level laminate. When DK is lower than 3.7 and df0.005, it is low Dk / DF level laminate. Now there are a variety of substrates available for selection in the market.

 

At present, the most widely used high-frequency circuit board base materials are mainly fluorine resin, polyphenylene oxide (PPO or PPE) resin and modified epoxy resin. Fluorinated dielectric substrates, such as polytetrafluoroethylene (PTFE), have the lowest dielectric properties and are usually used above 5GHz. In addition, modified epoxy FR-4 or PPO base material can be used for products between 1GHz and 10GHz. Among the three kinds of high frequency substrate materials, epoxy resin is the cheapest, while fluoro resin is the most expensive. Considering dielectric constant, dielectric loss, water absorption and frequency characteristics, fluoro resin is the best and epoxy resin is poor. When the frequency of product application is higher than 10 GHz, only fluororesin printed boards can be used. But the disadvantages of PTFE are not only high cost, but also poor rigidity and large coefficient of thermal expansion.

 

For polytetrafluoroethylene (PTFE), in order to improve the performance, a large number of inorganic materials (such as silica SiO2) or glass cloth are used as reinforcements to improve the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertia of polytetrafluoroethylene resin itself, it is not easy to combine with the copper foil. In the treatment method, polytetrafluoroethylene surface is chemically etched or plasma etched to increase the surface roughness or add a layer of adhesive film between copper foil and polytetrafluoroethylene resin to improve the bonding force, but it may have an impact on the dielectric properties. The whole fluorine system high frequency circuit board needs further development.

 

Unique insulating resin is synthesized by modified epoxy resin or by polyphenylene ether (PPE), trimellitic anhydride (TMA), diphenylmethane diisocyanate (MDI) and bismaleimide (BMI). FR-4 copper clad laminate, similar to glass cloth, is selected more at this stage because it has excellent heat resistance, dielectric property and mechanical strength, and compat has processing property of conventional PCB, which is better than PTFE Substrates are more popular.

 

The glass cloth drags the DK hind leg in the base plate. The dk6.6 (1MHz) of E glass cloth and the dk3.6 (1MHz) of epoxy resin make up the dk4.2-4.8 of FR-4. The new ne glass cloth dk4.4 consists of dk4.0 of FR-4. The new ne glass cloth is an effective way to reduce DK. For example, Panasonic launched Megtron 6 high frequency substrate using polyphenylene oxide (PPO) as the main resin, DK = 3.4, DF = 0.0015 (1GHz). Japan's Lichang industry also uses polyphenylene ether as the main resin substrate. The new cs-3376cn substrate is similar to PTFE substrate with DK = 3.1. By adjusting the ratio of BT to epoxy resin, the dielectric property of the new BT resin substrate of Mitsubishi gas is nearly 60% lower than that of the original BT substrate. The tachyon-100g base material of Isola has the same electrical properties as PTFE and the same PCB processing conditions as FR-4. At 40GHz, dk3.0 and df0.002 can meet the needs of transmitting 100 Gigabit Ethernet (100gbe).

 

In addition to the above special requirements for the performance of resin and other insulating materials, the surface roughness (profile) of copper conductor is also an important factor affecting the signal transmission loss, which is affected by skin effect. The skin effect is that the electromagnetic induction is generated in the conductor when the high frequency signal is transmitted, and the inductance is large in the center of the conductor section, which makes the current or signal tend to concentrate on the conductor surface. The surface roughness of conductor affects the transmission signal loss, and the loss of surface smoothness is small.

 

At the same frequency, the greater the surface roughness of copper, the greater the signal loss, so we try to control the surface roughness of copper as much as possible in the actual production, the smaller the roughness without affecting the binding force, the better. Especially for signals over 10GHz. The roughness of copper foil should be less than 1 μ m at 10 GHz, and the effect is better when using super plane copper foil (surface roughness 0.04 μ m). The surface roughness of copper foil also needs to be combined with appropriate oxidation treatment and bonding resin system. In the near future, there will be almost no profile of resin coated copper foil, which can have higher peel strength without affecting the dielectric loss.

 

3 high heat resistance and heat dissipation requirements

 

With the miniaturization, high function and high heat generation of electronic equipment, the thermal management requirements of electronic equipment are increasing. One solution is to develop thermal conductivity printed circuit board. The primary condition of heat-resistant and heat dissipation PCB is the heat-resistant and heat dissipation of substrate. At present, the heat-resistant and heat dissipation of substrate are improved to a certain extent by resin improvement and adding fillers, but the improvement of thermal conductivity is very limited. Typically, the metal substrate (IMS) or metal core printed circuit board is used, which plays the role of heat dissipation of heating components, and reduces the volume and cost compared with the traditional radiator and fan cooling.

 

Aluminum is a very attractive material, which is rich in resources, low cost, good thermal conductivity and strength, and environmentally friendly. At present, most of the metal substrates or metal cores are aluminum. The advantages of aluminum based circuit board are simple and economic, reliable electronic connection, high heat conduction and strength, no welding 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 led, and nearly 80% of the input power of LED is converted into heat. Therefore, the thermal management of LED is paid more attention to, especially the heat dissipation of LED substrate. High heat resistance and environmental protection

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