Teach you how to select PCB board by hand (II)

3、 ... and 、 Model selection index
1.PCB Influencing factors ：
1.1 Dielectric constant Dk
The dielectric constant must be small and stable , Usually the smaller the better . The transmission rate of the signal is inversely proportional to the square root of the dielectric constant of the material , High dielectric constant is easy to cause signal transmission delay . The lower the dielectric constant , The faster the signal transmission .
1.2 Dielectric loss Df
The dielectric loss must be small , This mainly affects the quality of signal transmission , The smaller the dielectric loss , The smaller the signal loss , With the same length of signal line , The stronger the ability to reach the receiving end .
1.3 temperature
frequently-used PCB Media is FR4 Of materials , The dielectric constant relative to air is 4.2-4.7, This dielectric constant changes with temperature , stay 0-70 In degrees , The maximum variation range is 20%. The change of dielectric constant will cause line delay 10% The change of , The higher the temperature, the greater the delay .
1.4 frequency
The dielectric constant will also change with the signal frequency , The higher the frequency, the smaller the dielectric constant . General FR4 Of materials PCB The transmission speed of the inner signal in the board is ：

The surface layer generally depends on the situation , Generally in 140-170 Between .
2.Tg、Df：
In principle, choose Tg The higher the value , The smaller the dielectric constant , Plates with smaller dielectric loss .
The following figure shows different frequencies Dk、Df value ：

2.1 Tg The value is PCB The critical temperature at which the material melts from solid state to rubber state fluid , It's called Tg spot , Melting point .
（1）Tg The higher the point, the higher the temperature requirement of the plate during pressing , The pressed board will also be hard 、 brittle , To a certain extent, it will affect the quality of mechanical drilling in the subsequent process and the electrical characteristics during use .
（2）Tg The point is the maximum temperature at which the substrate remains rigid （℃）, That is, ordinary PCB Substrate material at high temperature , Not only produce softening 、 deformation 、 Melting and other phenomena , At the same time, it is also manifested in mechanical 、 A sharp decline in electrical characteristics .
（3） commonly Tg The plate of 130 Over degrees ,High-Tg It's usually 170-210 degree , secondary Tg about 150-170 degree , The lower Tg about 130-150 degree .

Of the substrate Tg Improved , Heat resistance of printed boards 、 Moisture resistance 、 Chemical resistance 、 Stability and other characteristics will be improved .Tg The higher the value , The better the temperature resistance of the plate , Especially in the lead-free tin spraying process , high Tg More applications .
2.2 Dielectric constant Dk It is the medium that will produce induced charge when applying an electric field and weaken the electric field , The original external electric field （ In a vacuum ） The ratio to the electric field in the final medium is the dielectric constant .