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Electronic communication technology ranges wired to wireless, low frequency low speed to high frequency high speed. The performance of mobile phones has now entered 4G will move towards 5G, which means faster transmission speed larger transmission capacity. The advent of the global cloud computing era has doubled data traffic, high-frequency high-speed communication equipment is an inevitable trend. PCB is suitable for the needs of high-frequency high-speed transmission. In addition to reducing signal interference losses in circuit design, maintaining signal integrity, PCB manufacturing to meet design requirements, it is important to have a high-performance substrate.
Design engineers increase the speed signal integrity of the PCB to address the electrical signal loss attributes. The key factors of substrate selection are dielectric constant (Dk) dielectric loss (Df). When Dk is less than 4 Df is less than 0.010, it is a medium Dk / Df grade laminate. When Dk is less than 3.7 Df is less than 0.005, it is low Dk / Df grade laminates are now available on the market in a variety of substrates.
The currently used high frequency circuit board substrates are mainly three types of materials: fluorine resin, polyphenylene ether (PPO PPE) resin modified epoxy resin. Fluorine-based dielectric substrates, such as polytetrafluoroethylene (PTFE), have low dielectric properties are typically used above 5GHz. There are also modified epoxy FR-4 PPO substrates.
In addition to the special requirements for the properties of insulating materials such as resins mentioned above for high-frequency copper-clad boards, the surface roughness (contour) of the conductor copper is also an important factor affecting signal transmission loss, which is affected by the Skin Effect. The skin effect is that electromagnetic induction occurs on the wire during high-frequency signal transmission, the inductance is larger at the center of the cross section of the wire, so that the current signal tends to concentrate on the surface of the wire. The surface roughness of the conductor affects the transmission signal loss, the surface smoothness loss is small.
At the same frequency, the greater the roughness of the copper surface, the greater the signal loss, so we control the roughness of the copper thickness of the surface as much as possible in actual production, the smaller the roughness without affecting the bonding force, the better. Especially for signals above 10GHz. The copper foil roughness needs to be less than 1 μm at 10 GHz, the use of super-flat copper foil (surface roughness 0.04 μm) is more effective. The surface roughness of the copper foil also needs to be combined with a suitable oxidation treatment adhesive resin system. In the near future, there will be a copper foil coated with resin with almost no contour, which can have higher peel strength without affecting the dielectric loss.
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