亚洲 欧美 图片 自拍 视频

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          Introduction of electronic components and circuit assembly technology (2)

          1. Leakage board design and printing

          In advanced assembly technology, solder paste is the main soldering material widely used, and solder paste deposition uses drain plate printing technology. In the stencil printing process, the squeegee blade pushes the solder paste into the drain plate opening and transfers it to the circuit board. The four factors that affect the solder paste printing performance are: (1) the size of the drain plate opening, which determines the printing paste. (2) solder paste demoulding, in the case of a specific solder paste, the opening wall and geometry and finish affect the demolding of the solder paste; (3) the aspect ratio and area ratio of the opening, the width and length of the opening Ratio, the ratio of the opening area to the area of a€?a€?the opening wall; usually the design rule is 31.5 in the aspect ratio and the area is 0.66; but for the smooth tapered opening wall, the two ratios are 1 and 0.44 respectively to obtain a good solder paste release. . These two ratios are important design rules when designing the thickness of the drain. When the opening length is greater than 5 times its width, the aspect ratio is the main design rule (QFP). When the opening length is equal to the width, the area ratio is a more precise design rule (when the ball grid array pad is used). (4) Solder paste printing accuracy, when solder paste is printed on the circuit board, the pad pattern on the circuit board and the opening on the drain board must be exactly the same in size and position, and the missing solder paste cube must be Deformation. BGA, CSP and FCOB are plate-level assembled with eutectic solder alloy. BGA can meet the requirements with ordinary SMT solder paste, but the CSP and FCOB I/O terminals are smaller than the SMT package. The plate opening is smaller, and a fine solder paste of less than 40 um in particle size must be used. Their design and manufacturing requirements for drain plates are as stringent as for narrow-pitch devices. BGA, CSP, and flip-chip assembly of drain plates typically require laser or electroforming processes, followed by electropolishing, although the manufacturing cost is high, but the consistency is greater than Chemically etched the drain plate; sometimes it is required to cross the nickel and use a tapered opening to improve the hole wall finish, which is beneficial to the demolding of the solder paste. The size of the drain plate opening is generally smaller than the pad size on the circuit board. The opening slightly increases the amount of solder paste printed.

          The thickness of the drain plate is the main indicator of the design of the drain plate. The thickness of the drain plate used for BGA is 0.13-0.15mm, and the thickness of the drain plate for CSP is 0.10-0.13mm. Since the drain plate is thin, it is necessary to prevent the solder paste from being ejected from the opening during printing. When assembling BGA and CSP, it is usually printed in a ratio of 1 to 1; but for CSP, the actual printing is 0.05-0.076 mm larger than the protrusion size, so that the device support height is slightly higher after reflow soldering to improve thermal adaptability. And can continue to use three types of solder powder. For CSPs with 0.3mm diameter projections, a 0.3-0.6mmr rectangular opening is recommended. The 0.36mm opening is the smallest possible opening size for the three types of solder powder for consistent and repeatable printing. If a rectangular or circular opening of 0.25 mm is printed, a Class IV solder powder will be required.

          In order to meet the requirements of thin and light, high density, multi-function and high reliability of electronic products, hybrid assembly technology is still one of the trends in circuit assembly development in the early 21st century. Not only the through-hole device and SMD hybrid assembly, but with the popularization and application of flip-chip-based direct energy assembly technology, the assembly of through-hole components, SMD or flip-chip on the same circuit board will occur. New challenges are presented to the design and printing of the missing plates. Different assembly processes are used to complete the assembly of hybrid circuit components. Reflow soldering technology is an ideal process to fully utilize the SMT production line, reduce costs and increase productivity. There are several types of slab design and printing methods available. Choice, which is ideal for double-drain printing.

          Advanced packaging has stricter printing accuracy requirements than soldering pastes, so the high-precision printing machine of the vision system should be used to complete the solder paste printing operation. This kind of printing also has high-end and mid-range, and should be purchased according to the needs of users. At the beginning of the printing operation, the alignment of the drain plate and the circuit board is first completed, and the precise alignment of the drain plate opening and the pad pattern on the circuit board can be conveniently realized by the visual system. The advanced alignment system features fully integrated image recognition processing for fast and accurate image alignment, ensuring high quality solder paste printing and high productivity. Printing and the second problem is based on the type of board, the type of squeegee and the solder paste used to set the printing height, squeegee pressure and angle, printing and other printing parameters; in addition, the ambient temperature and relative humidity are also important printing parameters; advanced The package has strict requirements on the deviation of the printing parameters and must be accurately and strictly controlled by means of the computer control system of the printing press. The higher-end presses are also equipped with 2D and 3D laser inspection systems to check the print quality and meet the requirements of advanced packaging for printing accuracy.

          2. Mounting technology

          Although array packages significantly use the placement location specification to limit widening, since the I/O terminals of such packages are arrayed under the package, the most prerequisite for proper placement of such devices is to check for the presence or absence of solder balls. And spacing, check the solder spherical state. This requires the placement machine's vision system to perform this function based on the ball's shape quality factor and the build-up of solder ball distortion recognition levels. . Two-dimensional width and shape quality factor testing is a reliable method of checking the volume and distortion of the entire ball, so the vision system of the placement machine should have the appropriate and resolution to collect and form the best impact; for this purpose appropriate external lighting must be used And the telecentric optical system, and provides a constant magnification through large depth focusing to determine the presence and precise size of the ball; LEDs (light-emitting diodes) provide optimum illumination conditions, especially for back-illumination and reasonable selection of contour alignment For field of view and front illumination, the front illumination should use three programmable light sources to provide special ideal illumination for each package form to provide proper contrast between the solder ball structure and the backing environment, providing precise alignment optical conditions. The field of view should be adapted to the subtle and bit error requirements of the object so that the difference between good, defective, damaged solder balls can be determined. A high-performance placement machine that handles all advanced packages must have two component cameras (one standard and one flip-chip camera). The BGA device's precise determination can find the overall position and orientation of the ball grid based on 5 balls in each corner, and according to the BGA tree retrieval algorithm and the position determined by the template comparison algorithm; then by means of gray level machine vision system and computer control Finally, the precise alignment and placement of the BGA is achieved. In addition, device local fiducial markers can be placed on the PCB to improve placement accuracy. The mounting error of the BGA mainly comes from the non-coplanarity of the contact surface, so the coplanarity of the contact surface must be established and maintained during the mounting operation, and the automatic collimator is used to keep the movement of the placement machine coplanar.

          Although the CSP is a more compact package, it is flatter than the BGA, so it is easier to perform accurate placement. Like the BGA, the above method can be used to check the presence or absence of the ball, the pitch and the deformation state, but without the use of a grayscale vision system, it is only necessary to use a binary camera for observation and alignment, so it can be more than the QFP and BGA. High speed placement CSP.

          The popularization and application of advanced packaging technology requires that the placement machine can adapt to the accuracy requirements of the IC chip, especially the flip-chip placement, the repeatability is less than 4um, and the positioning system with high stability and high resolution can be used. The vision system can check 0.10-0.127. The mm pad and the 0.05mm high bump, so the flip-chip vision system must have different light source facilities and a higher resolution camera than the standard camera, with high precision for bump recognition and alignment. The placement machine should also have a certain feeder company (suitable for different feeding methods) and the ability to replace the placement tool. In addition, it should be equipped with a flux coating tool to meet the requirements of flip chip placement.

          The promotion of advanced packaging and the development of hybrid technology require the formation of flexible SMT production lines. According to the needs of electronic products, different types of placement machines and other assembly equipments are selected to form flexible production lines. When conditions permit, they should be upgraded to CIMS, so as to continuously meet the needs of various electronic equipment circuit components in the knowledge economy.

          3. Welding technology

          The practical use of advanced IC packaging, the increasing density of board-level circuit assembly, the use of double-sided assembly and hybrid assembly PCB components, put forward new requirements for reflow soldering technology, easy to set welding process parameters, easy to use, furnace Uniform temperature distribution, good repeatability of process parameters, suitable for BGA and other advanced IC package materials, adapt to different substrate materials, can be nitrogen-filled, suitable for double-sided SMT soldering and mounting adhesive curing, suitable for high-speed stickers The unit line can be installed to realize microcomputer control. Reflow soldering techniques that meet these requirements are primarily hot air circulation plus far infrared, heated reflow furnaces and reheating furnaces for full hot gas circulation heating.

          The remarkable feature of the full hot air reflow furnace is the use of a multi-nozzle heating assembly. The heating element is enclosed in the assembly, avoiding the adverse effects of the heating components and the PCB assembly. The heated gas is sprayed from the multi-nozzle system into the furnace chamber by a blower. It ensures uniform temperature in the width of the working area, and can control the hot gas flow and temperature of the top and bottom areas respectively to realize double-sided reflow soldering. The main problem is the control of the circulating wind speed and the adhesion of the flux fumes to the substrate. Also, since the air is a poor conductor of heat and has poor thermal conductivity, a large amount of circulating hot air is required in the hot air circulation reflow furnace, which is a complex component. The quality of welding undoubtedly has an impact.

          In the reflow furnace with hot air circulation and far infrared heating, the electromagnetic wave not only can effectively activate the flux activity, but also can decompose the flux resin component in the circulating air, effectively preventing the adhesion of the flux to the internal parts of the mechanism and the connector; The air circulation improves the uniformity in the furnace. Compared with the full hot air circulation, the wind speed is easy to control, and the position of the device is shifted. This furnace increases the heating zone under the same length as the standard reflow furnace, and the temperature of each zone can be controlled separately. It is easy to obtain the heating curve suitable for BGA and CSP welding requirements. The temperature inside the furnace is uniform and there is no overheating. Nitrogen protection can be used. The heating body can be installed to meet the heat requirements of the multilayer substrate and ensure excellent welding quality.

          Obviously, the above two reflow soldering techniques will be used in different application fields, so the convection heating-based reflow soldering technology will become the mainstream of board-level circuit assembly soldering technology in the early 21st century.

          亚洲 欧美 图片 自拍 视频
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