In an automatic insertion line, if the printed board is not flat, it will cause inaccuracies, components cannot be inserted into the holes and surface mount pads of the board, and even damage the automatic insertion machine. The board on which the components are mounted is bent after welding, and it is difficult to cut the component feet neatly. The board cannot be installed on the chassis or the socket in the machine, so it is also very troublesome for the assembly factory to encounter board warping. At present, printed boards have entered the era of surface mount and chip mount, and the requirements of assembly plants for board warpage must be more and more strict.

According to US IPC-6012 (1996 edition) (Qualification and Performance Specification for Rigid Printed Boards), the maximum allowable warpage and twist is 0.75% for surface mount printed boards and 1.5% for other boards. This raises the requirements for surface mount printed boards over IPC-RB-276 (1992 edition). At present, the warpage allowed by various electronic assembly plants, whether it is a double-sided circuit board or a multi-layer circuit board, is 1.6mm thick, usually 0.70~0.75%, and many SMT and BGA boards require 0.5%. Some electronics factories are agitating to raise the warpage standard to 0.3%, and the warpage test method is in accordance with GB4677.5-84 or IPC-TM-650.2.4.22B. The warpage of the printed board can be calculated by placing the printed board on a certified platform, inserting the test pins into the places where the warpage is greatest, and dividing the diameter of the test pins by the length of the curved edge of the printed board . bending.

For all PCB substrates, a thermosetting resin or thermoplastic polyester (A STAGE) is applied to the substrate reinforcement - paper, cloth, glass cloth or glass mat, and then goes into a drying chamber to dry to remove the resin or most of the volatile components in The polyester reaches a semi-cured state, the so-called B stage (B STAGE), also known as prepreg. Then select the number of layers of prepreg material according to the required thickness of the substrate, place copper foil des on the upper and lower silicon wafers according to single-sided and double-sided selection, and then enter the laminator together to further cure the material in a high temperature and high pressure environment. The material is cured into a C stage (C STAGE), so the substrate is generally also called a copper clad laminate. Class C materials generally have good stability, electrical insulation and chemical resistance.

PCB Design Requirements Metalized Via Interconnect Introduction

Substrate: Generally, it is an organic insulating material, and a ceramic material is used as a substrate for special purposes. Organic insulating materials can be classified as thermosetting resins or thermoplastic polyesters for rigid or flexible PCBs, respectively. Commonly used thermosetting resins are phenolic resins and epoxy resins, and thermoplastic polyesters are polyimide and polytetrafluoroethylene.

Combined with the requirements of high performance, multi-function, high reliability, low loss, amplitude and phase consistency, miniaturization and light weight, it brings great difficulty to the PCB design and manufacture of multilayer microwave printed circuit boards. To this end, PCBs with different functions are designed on different layers, such as microstrip lines, strip lines, low-frequency control lines and other mixed signal lines are combined in the same multi-layer structure, through various types of metallized holes. Manufactured to implement DC interconnects.

Vertical interconnection is the main way to realize the connection between different layers of circuits in microwave multilayer circuits. Vertical interconnection is mainly achieved through metallized blind and buried vias. In view of the needs of PCB design, the circuits of the same layer in the inner layer will be interconnected with the circuits of different layers above and below at the same time. Therefore, the research on controlled depth drilling technology becomes inevitable.

Chemical Immersion Tin

    ① Electroless tin plating, also known as immersion tin. The electroless tin plating process is to deposit tin on the surface of the PCB by chemical deposition. Its tin thickness is 0.8μm ~ 1.2μm, and it is gray-white to bright color, which can well ensure the flatness of the PCB surface and the coplanarity of the connection pads. Since the electroless tin layer is the main component of the solder. Therefore, the electroless tin layer is not only a protective coating for the connection pads, but also a direct solder layer. Because it is lead-free and complies with today's environmental requirements, it is also the predominant surface finish in lead-free soldering.

7. People

    ①Since the character accuracy requirement is lower than that of the circuit and solder mask, the characters on the PCB are basically screen printed at present. In this process, the screen of the printing plate is first made according to the character film, and then the screen is used to print the character ink on the plate, and finally the ink is dried.

Eight, milling

    ①So far, the PCB we have produced has always been in the form of PANEL, which is a large board. Now because the whole board has been completed, we need to separate the delivery mode from the big board according to (UNIT delivery or SET delivery). At this time, we will use the CNC machine tool to process according to the pre-written program. Profile edge and strip milling is done in this step. If there is a V-CUT, the V-CUT process needs to be added. Capability parameters involved in this process include external tolerances, chamfer dimensions and internal corner dimensions. The design also needs to consider the safe distance from the graphics to the edge of the board.

Nine, electronic test

    ①Electronic test is the electrical performance test of PCB, also known as "on" and "off" test of PCB. Among the electrical test methods used by PCB manufacturers, bed of needles test and flying probe test are the most commonly used.

    (1) Needle beds are divided into ordinary mesh needle beds and special needle beds. Universal needle beds can be used to measure PCBs with different network structures, but the equipment is relatively expensive. A special needle bed is a needle bed specially formulated for a certain type of PCB, and is only suitable for the corresponding PCB.

    (2) Flying probe test Use a flying probe tester to test the continuity of each network by moving probes (multiple pairs) on both sides. Since the probe can move freely, the flying probe test is also a general purpose test.