Introduction to assembly technology of electronic component circuit boards
Electronic components are the cells of electronic information devices, and board-level circuit assembly technology is the basis for manufacturing electronic devices. The emergence of different types of electronic components has always led to a revolution in board-level circuit assembly technology. Through-hole insertion technology (THT), which appeared at the same time as the rise of integrated circuits in the 1960s, was replaced by the first-generation SMT debuted in the 1980s with the booming LSI in the latter half of the 1970s. The peripheral terminal package represented by QFP It has become the mainstream packaging of today; in the 1990s, with the narrow spacing of QFP, board-level circuit assembly technology is facing challenges. Despite the development of the narrow-pitch assembly technology (FPT), there are still many board-level circuit assemblies with a pitch of 0.4mm or less. The process is facing a solution. As the most ideal solution in the first half of the 1990s, the United States proposed the second package assembly technology of the IC package side array package (BGA), and its next small package is the chip size package (CSP) was in the 1990s. It has not become the focus of attention. For example, a Q-pin package of more than 400 pins that is difficult to assemble and assemble is replaced by PBGA and TBGA with easy-to-assemble terminal spacing of 1.0-1.5 mm, achieving group reflow of such devices. In particular, flip chip connection technology is used in the connection between the chip and the package substrate, so that thousands of pins of PCBA are used in supercomputers and workstations, called FCBGA, and are beginning to be put into practical use. The third representative surface assembly technology is directly chip-level assembly, but due to reliability, cost and KGD constraints, only in special fields, further development of IC packaging, chip-wrapped wafer package (WLP) surface array bump type at the end of 1999 In 2014, FC is expected to become the third representative package for the multi-needle and high-performance requirements of semiconductor devices.
IC packaging has lagged behind the inherent capabilities of IC chips. We hope that the performance gap between the bare chip and the packaged chip is reduced, which promotes the development of new designs and new packaging technologies. In the new package design, the multi-chip package (CSP) contains more than one chip, stacked on top of each other, through wire bonding and flip chip design (on-chip soldering on flip-chip, flip-chip on-line soldering, Or wire bonding wire bonding) to achieve inter-chip interconnection, further reducing the weight and space occupied by the device.
Due to size and cost advantages, wafer-level CSP (Wafer-level cap) will be further developed to form first-level interconnect and package I/O terminals on the chip before the wafer is cut into small blocks (chips). This not only shortens the manufacturing cycle, but also has I/O terminals divided into surface array type and peripheral type (depending on the distribution of I/O terminals); the former, the EIAJ has a terminal pitch of 0.8 mm or less, and the external size is 4 mm- The 21mm ultra-small package is standard, mainly for logic and memory devices. The latter is a leadless miniaturized package with peripheral terminals such as SON and QFN, which is mainly suitable for memory and low-level logic devices. Since the advent of the CSP in the early 1990s, various structural forms have been proposed. The FBGA with face array type is now the mainstream. The first generation FBGA is a plastic type face down type. The second generation FBGA is a carrier type. The face-down type uses lead frame plastic modules and packages, and the new generation FBGA uses crystal as the carrier for transmission, and the final assembly process of cutting (dashing), ie WLP method, replaces the connection technology used in the previous package. (Wire welding, TAB, and flip chip bonding), but before the scribe line separation, the wiring technology of the pre-semiconductor process is used to connect the chip pad to the external terminal, and the solder ball connection and electrical test are followed. The wafer state is completed, and finally the line division is forced. Obviously, the FBGA of the actual chip size is made by the WLP method, and the appearance is indistinguishable from the FC.
In short, PBGA, TBGA, FBGA, (CSP) and FC are the current trends in IC packaging. In the first 15 years of the 21st century, the third representative package will be rapidly developed. Around high-density assembly, the diversity of package structure will be the most prominent feature of IC packaging in the early 21st century. LSI chip's stacked package, ring package: In addition, there will be a new 3D package, optical-electronic interconnection, light surface assembly technology will also flourish. System-on-a-chip (SOC) and MCM system-in-package (MCM/SIP) will be further developed and put into practical use with improved design tools, increased wiring density, adoption of new substrate materials, and economical KGD supply. stage.
With the demand for miniaturization, high performance, high reliability, safety and electromagnetic compatibility of electronic equipment in the industrial and consumer electronics markets, new demands are placed on the performance of electronic circuits, since the 1990s, The components are further developed in the direction of miniaturization, multi-layering, large capacity, high voltage resistance and high performance. At the same time, with the popularization and application of SMT in all electronic devices, the use of chip components worldwide has increased rapidly. The number of components reaches 1 megabit, and the ratio of passive components to IC is generally greater than 20. Since such a large number of discrete components are required, the discrete components dominate the size of the final PCB component; in addition, the increase in the amount of chip passive components enables placement. The bottleneck in the process is more difficult to solve by the placement of chip components, resulting in a loss of balance in the production line, a decrease in equipment utilization rate, an increase in cost, and a 30% of the production time of the chip component, which seriously affects the production volume. An effective way to solve these problems is. Achieve the integration of passive components.
The integrated passive components are available in the following packages:
Array: integrates many types of passive components together and encapsulates them in the form of planar array terminals;
Network: integrates many hybrid resistors and capacitors and encapsulates them in the form of peripheral terminals;
Hybrid: a mixture of some passive components and active devices for packaging;
Embedding: embedding passive components in a PCB or other substrate;
Integrated Hybrid: The integrated passive components are packaged in QFP or TSOP format.
The popularization and application of these passive packages can effectively solve the placement: bottleneck, improve the balance of SMT production line, reduce the cost, increase the output, and increase the assembly density.
Development of advanced board level circuit assembly technology
The development of circuit assembly technology is largely restricted by the assembly process. If there is no advanced assembly process, advanced packaging is difficult to promote and apply, so the emergence of advanced packaging will inevitably put new requirements on the assembly process. In general, BGA, CSP, and MCM can be assembled using standard surface-mounting equipment processes. However, due to the miniaturization of the package terminal surface array, stricter requirements are imposed on the assembly process, thereby facilitating SMT assembly equipment and processes. development of.
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 release, 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 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. The board-level assembly of BGA, CSP and FCOB uses a eutectic solder alloy. The BGA can meet the requirements with ordinary SMT solder paste. However, the CSP and FCOB I/O terminals have a small soldering area compared to the SMT package, so leakage is required. The plate opening is smaller and a fine solder paste of less than 40 um in particle size must be used. Their drain plate design and manufacturing requirements are as stringent as narrow-pitch devices. BGA, CSP, and flip-chip assembly of the drain plate generally require a laser or electroforming process, followed by electropolishing, although the manufacturing cost is high, but the consistency exceeds the chemically etched plate; sometimes it is required to cross nickel and use a cone Opening the hole to improve the wall finish of the hole is beneficial to the demolding of the solder paste. The size of the opening of the drain plate is generally smaller than the size of the pad on the circuit board, and 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. For the BGA, the thickness of the drain plate 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 should be prevented from opening when printing. The solder paste is removed from the middle. 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. A 0.3-0.6 mmr rectangular opening is recommended for CSPs with 0.3 mm diameter projections. 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, small, high-density, multi-functional 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 is more stringent than conventional SMT in soldering paste printing, 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 packaging 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 distributed in an array under the package, the most prerequisite for accurate 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 achieve this function based on the ball's shape quality factor and the established solder ball distortion approval level... Two-dimensional width and shape quality factor testing is a reliable way to check the entire ball volume and distortion, so The placement system's vision system should have the appropriate and resolution to collect and shape the best impact; for this purpose, appropriate external illumination and telecentric photographic optics must be used and a constant magnification can be provided by large depth focusing to determine the ball's Existence and exact dimensions; LEDs (light-emitting diodes) provide optimum illumination conditions, especially for back-illumination with contour alignment and reasonable choice of brightfield and front illumination. The front illumination should use three programmable light sources to provide special for each package. Ideal illumination to create an appropriate contrast between the solder ball structure and the backing environment to provide precise alignment optical conditions. The field of view should be adapted to the subtle and bit error requirements of the object to determine the difference between good, defective, damaged solder balls. 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 tools. 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, in the furnace Uniform temperature distribution, good repeatability of process parameters, suitable for BGA and other advanced IC package materials, suitable for different substrate materials, nitrogen-fillable, suitable for double-sided SMT soldering and mounting adhesive curing, suitable for high-speed stickers Install the unit line, which can realize microcomputer control. The reflow soldering technology that can meet these requirements is mainly hot air circulation plus far infrared, heated reflow furnace and reheating furnace with 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 complicated for components. The quality of welding is undoubtedly influential.
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.