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Classification and selection of PCB materials

Classification and selection of PCB materials

Classification and selection of PCB materials

1. The FR-4 we often choose is not a material name

What we often refer to is “FR-4” is the code name of a kind of flame-resistant material grade. It represents a material specification that the resin material must be able to extinguish by itself after burning. It is not a material name, but a kind of material. Material grade, so there are many types of FR-4 grade materials used in general circuit boards, but most of them are made of so-called Tera-Function epoxy resin plus filler (Filler) and glass fiber The composite material made.

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For example, the FR-4 water-green fiberglass board and black fiberglass board that our family makes now have functions such as high temperature resistance, insulation, and flame retardancy. So when choosing materials, everyone must figure out what characteristics they need to achieve. In this way, you can buy the products you need.

Flexible Printed Circuit Board (Flexible Printed Circuit Board, abbreviated FPC) is also called flexible printed circuit board, or flexible printed circuit board. The flexible printed circuit board is a product that is designed and manufactured on a flexible substrate by a printing method.

There are two main types of printed circuit board substrates: organic substrate materials and inorganic substrate materials, and organic substrate materials are the most used. The PCB substrates used for different layers are also different. For example, prefabricated composite materials are used for 3- to 4-layer boards, and glass-epoxy materials are mostly used for double-sided boards.

2. When choosing a sheet, we need to consider the impact of SMT

During the lead-free electronic assembly process, due to the increase in temperature, the degree of bending of the printed circuit board when heated is increased. Therefore, it is required to use as small a degree of bending as possible in the SMT, such as FR-4 and other types of substrates. Since the expansion and contraction stress of the substrate after heating affects the components, it will cause the electrode to peel off and reduce the reliability. Therefore, you should also pay attention to the material expansion coefficient when selecting the material, especially when the component is larger than 3.2×1.6mm. PCB used in surface assembly technology requires high thermal conductivity, excellent heat resistance (150℃, 60min) and solderability (260℃, 10s), high copper foil adhesion strength (1.5×104Pa or more) and bending strength (25 ×104Pa), high electrical conductivity and small dielectric constant, good punchability (accuracy ±0.02mm) and compatibility with cleaning agents, in addition, the appearance is required to be smooth and flat, without warpage, cracks, scars, and rust spots.

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3, PCB thickness selection

The thickness of the printed circuit board is 0.5mm, 0.7mm, 0.8mm, 1mm, 1.5mm, 1.6mm, (1.8mm), 2.7mm, (3.0mm), 3.2mm, 4.0mm, 6.4mm, of which 0.7mm and 1.5 The PCB of mm board thickness is used for the design of double-sided board with gold fingers, and 1.8mm and 3.0mm are non-standard sizes. From the perspective of production, the size of the printed circuit board should not be less than 250×200mm, and the ideal size is generally (250~350mm)×(200×250mm). For PCBs with long sides less than 125mm or wide sides less than 100mm, Use the jigsaw method. The surface mount technology stipulates the bending amount of the substrate with a thickness of 1.6mm as upper warpage ≤0.5mm and lower warpage ≤1.2mm. Generally, the allowable bending rate is below 0.065%. It is divided into 3 types according to metal materials, as shown in typical PCBs; 3 types according to structural softness and hardness. Electronic plug-ins are also developing towards high pin count, miniaturization, SMD and complexity . The electronic plug-in is installed on the circuit board through the pins and the pins are soldered on the other side. This technology is called THT (Through Hole Technology) plug-in technology. In this way, a hole must be drilled for each pin on the PCB, which shows the typical application of PCB.

4, drilling

With the rapid development of SMT chip technology, multi-layer circuit boards need to be connected to each other, which is guaranteed by electroplating after drilling, which requires various drilling equipment. In order to meet the above requirements, at present, PCB CNC drilling equipment with different performance is launched at home and abroad. The production process of printed circuit boards is a complex process, which involves a wide range of processes, mainly involved in photochemistry, electrochemistry, and thermochemistry; there are more process steps involved in the manufacturing process, with more rigid The layer circuit board is taken as an example to illustrate the processing procedure. Drilling is a very important process in the entire process, and the processing of the hole takes the longest time. The position accuracy of the hole and the quality of the hole wall directly affect the subsequent process of hole metalization and patching, and also directly affect the printed circuit. The processing quality and processing cost of the board The principle, structure and function of the CNC drilling machine The common methods of drilling on the circuit board include the CNC mechanical drilling method and the laser drilling method. At this stage, the mechanical drilling method is the most used.

12月-24-2021

FASTPCBA