Precision PCB Fabrication, High-Frequency PCB, High-Speed PCB, Standard PCB, Multilayer PCB and PCB Assembly.
The most reliable PCB & PCBA custom service factory.
1., high-frequency and high-speed board material introduction
When selecting the substrate used for high-frequency PCB, the variation characteristics of the material DK at different frequencies should be specially investigated. For the requirements of high-speed signal transmission or characteristic impedance control, DF and its performance under the conditions of frequency, temperature, and humidity are mainly investigated.
Under the condition of frequency change, the DK and DF values of the general substrate material change greatly. Especially in the frequency from 1MHz to 1GHz, their DK and DF values change more obviously. For example, the general type epoxy resin glass fiber fabric substrate material (general type FR-4) has a DK value of 4.7 at the frequency of 1MHz, and a DK value change of 4.19 at the frequency of 1GHz. Above 1GHz, its DK value changes gently. For example, at l0GHz, the DK value of fr-4 is 4.15. For substrate materials with high-frequency and high-speed board, the DK value changes little when the frequency changes. When the frequency changes from 1MHz to 1GHz, the DK value mostly keeps changing in the range of 0.02. The DK value has a slightly decreasing tendency in different frequency conditions from low to high.
The medium loss factor (DF) of the general substrate material is influenced by the frequency change (especially the change in the high-frequency range), and the change of DF value is larger than that of DK. The change rule tends to increase. Therefore, when evaluating the high-frequency characteristics of substrate material, the focus of the investigation should be the change of its DF value. For the substrate materials with high speed and high-frequency characteristics, there are two different types of general substrate materials in terms of the variation characteristics at high frequency. One type of substrate material has a very small change in (DF) value with the change of frequency. There is also a kind of substrate material in the range of change that is similar to the general type, but its (DF) value is lower.
2, high-frequency PCB and high-speed substrate glass fiber cloth introduction
Glass fiber reinforced material is the main undertaker of the mechanical strength of composite materials. Generally speaking, its dielectric constant is higher than that of the resin matrix and occupies a higher volume content in the composite materials, so it is the main factor determining the dielectric properties of composite materials. In the production of the FR-4 copper-clad plate, the traditional E-glass fiber cloth has been used. Although the comprehensive performance of E-glass fiber cloth is good and the performance price is ideal, the dielectric property is poor and the dielectric constant is high (6.6), which affects its application in the field of high frequency and high speed.
At present, the composition of glass fiber fabric with silicate composition produced in all countries in the world is roughly the same, its basic composition is SiO2, A1203, CaO ternary system, the weight percentage fluctuates in a small range. At room temperature, silicon-oxygen, boron oxygen, aluminum oxygen skeleton ions hardly conduct electricity. However, when the network is filled with cations, especially alkali metal ions, the lattice structure is interrupted at the alkali metal ions, forming weakly linked ions and generating thermionic polarization. This is the main factor affecting the dielectric properties of glass. At present, alkali-free glass fiber E glass fiber is usually used, whose dielectric constant is 7.2 (1 MHz), which cannot meet the requirements of high frequency and high-speed board.
The first option is confounding. In addition to E glass fiber, there is also D glass fiber (DK= 4.7, L MHz), Q glass fiber (DK= 3.9, L MHz), D glass fiber, and Q glass fiber. Although they have excellent dielectric properties, they have two major disadvantages: (1) poor machinability, wear on the bit (2) high cost, quite E glass cloth more than 10 times the price, use alone is not appropriate. Through the reasonable selection of different types of glass fiber, it is required not only to ensure excellent low dielectric properties and processing properties but also to solve the cost problem of industrial production.
3, high-frequency and high-speed substrate packing introduction
Filling materials in the manufacture of high-frequency substrate materials refer to chemical materials used as resin fillers in addition to reinforced fiber materials in the composition of substrate materials. The proportion of filling material in the resin of the whole substrate material, variety, surface treatment technology, and so on, all affect the dielectric constant of the substrate material.
Inorganic fillers are commonly used: talc, kaolin, magnesium hydroxide, aluminum hydroxide, silica powder, and alumina, etc. The addition of filler can effectively reduce the hygroscopicity of the product, to improve the heat resistance of the plate, at the same time, it can also reduce the coefficient of thermal expansion of the plate. The heat resistance, particle size distribution, hardness, surface treatment, the use of dispersion, and other factors should be considered in the selection of filler. In this regard, Hitachi Chemical has developed and applied a new interface control system technology, which enables the interface between the filler and resin to achieve high dispersion and high bonding. It overcomes the problems of packing in resin, such as agglomeration, low dispersity, and void after plate forming.
4., high-frequency and high-speed substrate resin introduction
Glass fiber reinforced material is the main undertaker of the mechanical strength of composite materials. Generally speaking, its dielectric constant is higher than that of the resin matrix and occupies a higher volume content in the composite materials, so it is the main factor determining the dielectric properties of composite materials. In the production of the FR-4 copper-clad plate, the traditional E-glass fiber cloth has been used. Although the comprehensive performance of E-glass fiber cloth is good and the performance price is ideal, the dielectric property is poor and the dielectric constant is high (6.6), which affects its application in the field of high frequency and high speed.
At present, the composition of glass fiber fabric with silicate composition produced in all countries in the world is roughly the same, its basic composition is SiO2, A1203, CaO ternary system, the weight percentage fluctuates in a small range. At room temperature, silicon-oxygen, boron oxygen, aluminum oxygen skeleton ions hardly conduct electricity. However, when the network is filled with cations, especially alkali metal ions, the lattice structure is interrupted at the alkali metal ions, forming weakly linked ions and generating thermionic polarization. This is the main factor affecting the dielectric properties of glass. At present, alkali-free glass fiber E glass fiber is usually used, whose dielectric constant is 7.2 (1 MHz), which cannot meet the requirements of high frequency and high-speed circuit board.
Cyanate ester (CE) resin is a kind of high-performance resin matrix developed in the late 1970s. Under the action of heat or catalyst, CE resin is cyclized and trimmed to form a network structure macromolecule containing triazine ring with a high crosslinking degree. Curable CE resin has many excellent properties: low dielectric coefficient (2.8-3.2) and minimal dielectric loss Angle tangent (0.002 ~ 0.008); High heat resistance (Tg is 240 degree Celsius-290 degree Celsius); Low moisture absorption (< 1.5%); Small coefficient of thermal expansion; Excellent mechanical properties and bonding properties. But the toughness of CE resin is poor and the curing temperature is too high. The modification of CE resin with bismaleimides resin is the most successful example of CE resin modification applied tohigh frequency and high-speed boardcoppered plate, which is usually called BT resin.
Traditional epoxy resin has a large content of polar groups, and its dielectric properties are poor. The usual modification methods include: increasing the number of branch chains, increasing the free volume of the material, and reducing the concentration of polar groups; The double bond structure is added to the epoxy resin to make the resin molecules not easy to rotate; Or introduce the group occupying a large space volume or non-polar polymer resin and high-frequency PCB and high-speed board material to reduce the polar group content, improve its dielectric properties.
