When choosing pcb board , we must strike a balance between meeting design requirements, mass production and cost. In short, design requirements include electrical and structural reliability. This issue is usually more important when designing high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material has a large dielectric loss DF (dielectric loss) at several GHz frequencies, which may not be applicable.
For example, the 10Gb/s high-speed digital signal is square wave , which can be regarded as the superposition of sine wave signals of different frequencies. Therefore, 10Gb/s includes many different frequency signals: 5GHz base frequency signal, third-order 15GHz, fifth-order 25ghz, seventh-order 35GHz signal, etc. Maintaining the integrity of the digital signal and the steepness of the upper and lower edges is the same as the low-loss and low-distortion transmission of radio frequency microwaves (the high-frequency harmonic part of the digital signal reaches the microwave band). Therefore, in many ways, material selection for high-speed digital circuit PCBs is similar to that of radio frequency microwave circuits.
In actual engineering operations, the selection of high-frequency boards seems simple, but there are still many factors to consider. Through the introduction of this article, as a PCB design engineer or high-speed project leader, I have a certain understanding of the characteristics and selection of board materials. Understand the electrical performance, thermal performance and reliability of the board. Use lamination rationally to design products with high reliability and good processing performance, and optimize and consider various factors.
The following will be introduced separately. The main considerations for choosing a suitable plate are:
1. Manufacturability:
For example, multiple pressing properties, temperature performance, CAF/heat resistance, mechanical toughness (adhesion) (good reliability) and What is the fire protection level?
2. Timely availability of materials:
Many high-frequency plates have a very long procurement cycle, even 2-3 months; except for the conventional high-frequency plates RO4350, which are in stock, many high-frequency plates need to be provided by customers. Therefore, high-frequency plates need to be communicated with manufacturers in advance and prepare materials as early as possible;
3. Cost factor Cost:
It depends on the price sensitivity of the product, whether it is a consumer product or a communication, medical, industrial, military application;
4. The applicability of laws and regulations, etc.:
needs to be integrated with environmental protection regulations in different countries and meet the requirements of RoHS and halogen-free.
Among the above factors, the operating speed of high-speed digital circuits is the main factor considered in PCB selection. The higher the circuit speed, the smaller the chosen pcbdf value should be. Medium and low-loss circuit boards are suitable for 10Gb/s digital circuits; low-loss boards are suitable for 25gb/s digital circuits; ultra-low-loss flat panels will adapt to faster high-speed digital circuits, and their speeds can reach 50GB/s or higher.
From the material Df point of view:
Df is between 0.01 and 0.005. The circuit board is suitable for digital circuits with an upper limit of 10Gb/S;
Df is between 0.005 and 0.003. The circuit board is suitable for digital circuits with an upper limit of 25Gb/S;
Df is not more than 0.0015 for circuit boards. Suitable for 50Gb/S or higher high-speed digital circuits.
Commonly used high-speed plates are:
1), Rogers: RO4003, RO3003, RO4350, RO5880, etc.
2), TUC: Tuc862, 872SLK, 883, 933, etc.
3), Panasonic: Megtron4, Megtron6, etc.
4), Isola: FR408HR, IS620, IS680, etc.
5), Nelco: N4000-13, N4000-13EPSI, etc.
6), Dongguan Shengyi, Taizhou Wangling, Taixing microwave, etc.
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