Basic Content of SMT And SMT Process Technology

1.Main contents of SMT

SMT is a complex system engineering.It includes surface assembly, completed parts, completed materials, completed processes, completed designs, testing technologies, completed and testing equipment, control and management, and other emergent contents and technologies. It is a comprehensive engineering science and technology.

1.1 Design of surface production parts (1). Structural size, terminal form, soldering heat resistance, etc. (2) Manufacturing. Manufacturing technology for various parts. (3) Packaging. Tape type, tube type, pallet, bulk, etc.

1.2 Circuit substrate single (multi-layer) PCB, ceramic, enamel metal plate, etc.

1.3 Complete circuit design, thermal design, component layout, motherboard graphic wiring design, etc.

1.4 Construction technology (1) Construction materials. Adhesives, solders, fluxes, cleaning. (2) Construction technology. Coating technology, mounting technology, welding technology, cleaning technology, and detection technology. (3) Construction equipment. Coating laying equipment, placement machines, welding machines, cleaning machines, testing equipment, etc. (4) Establish system control and management. Establish production line or system composition, control and management, etc.

1.5 Detection technology (1) Visual inspection. (2) Automatic optical inspection (AOI). (3) Automatic X-ray detection (X-Ray). (4) Probe detection. (5) Online testing (ICT) and functional testing (FCT), etc.

2. Basic content of SMT process technology

The main contents of SMT process technology can be divided into four parts: assembly material selection, assembly process design, assembly technology and assembly equipment application. SMT process technology involves chemical and material technology (such as various solder pastes, fluxes, cleaning agents), coating technology (such as solder paste printing), precision machining technology (such as template making), automatic control technology (such as equipment and production line control) ), welding technology (such as reflow soldering), testing and inspection technology (such as automatic optical inspection, automatic X-ray inspection), assembly equipment application technology and many other technologies. It has the comprehensive engineering technology characteristics of SMT and is the core technology of SMT.

3. SMT process technical specifications

With the rapid development and popularity of SMT, its process technology has become increasingly mature and began to be standardized. The United States, Japan and other countries have formulated corresponding standards for SMT process technology. Our country has also formulated Chinese electronics industry standards such as "General Technical Requirements for Surface Assembly Processes", "Technical Requirements for Assembly of Printed Board Assemblies", and "Requirements for Surface Mounting of Electronic Components". Among them, "General Technical Requirements for Surface Assembly Processes" The classification of SMT production lines and assembly processes, basic requirements for components, substrates and process materials, basic requirements for each production process, basic requirements for storage and production environment and electrostatic protection are standardized.In SMT process design and management, some technical requirements can be standardized based on the above standards. Due to the rapid development of SMT, its process technology will be constantly updated. Therefore, in practical applications, attention should be paid to the applicability of the above-mentioned standard references.

4. Line assembly method of SMT production system

The line assembly method of the SMT production system. The SMT production system formed by surface assembly equipment such as surface coating equipment, placement machines, welding machines, cleaning machines, and testing equipment is customarily called an SMT production line. At present, the variety and specifications of surface-mounted components are not yet complete, so some through-hole components (THT) are sometimes still needed in surface-mounted components (SMA). Therefore, generally speaking, surface mount components often include both plug-in components and SMD components, and only some of them use SMC/SMD. The assembly that uses both plug-ins and mount parts is called hybrid assembly, and the assembly that uses all SMC/SMD is called full surface assembly. Depending on the assembly objects, assembly processes and assembly methods, SMT production lines have a variety of assembly methods.The most basic component of the SMT production line using reflow soldering technology is generally used for surface assembly of SMC/SMD on one side of PCB, also known as single-line form. If SMC/SMD is assembled on both sides of the PCB, a two-wire assembly line is required. When both plug-ins and mount parts are used, a plug-in assembly line and corresponding equipment are added to the production line. When a non-clean-free assembly process is used, additional post-weld cleaning equipment is required. At present, some large enterprises have set up SMT product integrated assembly systems equipped with feeding trolleys and controlled and managed by computers. It is an advanced organizational form of automatic assembly and production of SMT products. (1) Print solder paste or patch tape onto the pads of the PCB to prepare for the welding of components. The equipment used is a solder paste printing machine, located at the front end of the SMT production line.(2) Glue dispensing is to drop glue onto the fixed position of the PCB. Its main function is to fix the components to the PCB when using wave soldering. The equipment used is a dispensing machine, located at the front end of the SMT production line or behind the testing equipment. (3) Mounting: Accurately install the surface-mounted components to the fixed position of the PCB. The equipment used is a placement machine, located behind the solder paste printing machine in the SMT production line. (4) SMD adhesive curing When using SMD adhesive, the SMD adhesive is cured so that the surface assembly components and the PCB are firmly bonded together. The equipment used is a curing oven, located behind the placement machine in the SMT production line. (5) Reflow soldering melts the solder paste so that the surface-mounted components and the PCB are firmly bonded together. The equipment used is a reflow oven, located behind the placement machine in the SMT production line. (6) Clean to remove welding residues, such as flux, that are harmful to the human body or products on the assembled PCB.(7) Inspection: Test the welding quality and assembly quality of the assembled SMA (surface assembly components). The equipment used includes magnifying glass, microscope, online tester (ICT), flying probe tester, automatic optical inspection (AOI) instrument, X-Ray detector, functional tester, etc. The location can be configured at a suitable place in the production line according to the needs of detection. (8) Repair: Repair the SMA that has detected a fault. The tools used are electric soldering iron, rework workstation, etc. Configured anywhere in the production line.

5.SMT production environment and personnel quality requirements

Production environment requirements SMT production equipment is a high-precision mechatronics equipment. The equipment and process materials have certain requirements for the cleanliness, humidity, and temperature of the environment. In order to ensure the normal operation of the equipment and assembly quality, the following requirements are imposed on the working environment: There are fixed area lines at the production site, and layout plans for floors (teams). The drawing of the layout map meets the requirements of the regulations; the layout is reasonable, and the signs are applied correctly; warehouse materials and in-process products are stored in categories, and all items are stacked neatly, reasonably, and in designated areas, racks, and shelves. The positioning must be consistent with the location number and ledger; all items placed in the stagnation area must have fixed location marks and must not be mixed. In terms of cleanliness and civilization, we should ensure that: there is no dust on the material racks, transport racks, and turnover boxes; the public walkways in the jurisdiction are smooth and free of debris, the stairs and floors are smooth and free of garbage, and the doors and windows are clean and dust-free; the work is civilized, and there is no barbarism or violence. orderly operation behavior; implement the "daily sweep" and "weekly sweep" systems. There are systems, inspections, assessments, and records for on-site management; the three-dimensional contract area (including the four parts of the line body, equipment, and the ground) is clean and dust-free, and there are no unnecessary items; it can achieve "one inspection a day" and "daily inspection" Nissin". The auxiliary environment of the production line is a necessary condition to ensure the normal operation of the equipment. It mainly includes the following aspects:

5.1 Dynamic factors:

The power required for SMT equipment is usually divided into two parts: electrical energy and compressed air. Its quality not only affects the normal operation of the equipment, but also directly affects the service life of the equipment. (1) Compressed air. On the SMT production line, the power of the equipment is compressed air. One piece of equipment can have at least a few cylinders and solenoid valves, or up to more than twenty cylinders and solenoid valves. Compressed air should be introduced into the corresponding equipment of the production line through a uniformly equipped air source pipe network. The air compressor should be at a certain distance from the factory building; the air pressure is usually 0.5 to 0.6 MPa. When introducing it from outside the wall, the pipeline loss should be taken into account; the compressed air should be removed Oil, water and dust removal, the oil content is less than 0.5×10-6. (2) Adopt three-phase five-wire AC power supply. The so-called three-phase five-wire AC power supply means that in addition to the three-phase U, V, and W phase lines connected to the power grid, the working neutral line and protective ground wire of the power supply must be strictly connected separately; in front of the transformer of the machine, If a line filter or AC voltage regulator is installed, the power supply voltage is unstable and the power supply is not purified well, the machine may suffer data loss and other damage.

5.2 Normal environment of SMT workshop:

SMT production equipment is high-precision mechatronics equipment with relatively high environmental requirements and should be placed in a clean factory (not lower than Class 100000 in the "GB73-84 Clean Factory Design Specification"). Temperature: 20 ~ 26℃ (can be relaxed when storing solder paste and patch tape in the refrigerator); in an air-conditioned environment, there must be a certain amount of fresh air, try to control the CO2 content below 1000 PPM, and the CO content below 10 PPM , to ensure human health. Relative humidity: 40% ~ 70%; noise: ≤70 dB; cleanliness: particle size ≤5.0≥0.5 (μm), dust concentration ≤3.5×105≥2.5×104 (particles/m2). Curtains should be added to the windows on the wall to prevent sunlight from shining directly on the machine, because SMT production equipment is basically equipped with photoelectric sensors, and strong light will cause the machine to malfunction.

5.3 Anti-static system:

There should be an anti-static system at the SMT site, and the system and anti-static ground wire should comply with national standards. Production equipment must be well grounded and should adopt the three-phase five-wire grounding method and be independently grounded. (1) Set up an anti-static safety workbench. It consists of a workbench, anti-static table mat, wrist strap connector and grounding wire. (2) There should be more than two wrist strap connectors on the anti-static desk mat, one for operators and the other for technicians and inspectors. Personnel who come into direct contact with static-sensitive devices must wear an anti-static wrist strap. (3) Anti-static safety It is not allowed to place debris that easily generates static electricity on the workbench. Plastic boxes, rubber, cardboard, glass, drawings and materials, etc. should be placed in anti-static bags. (4) Anti-static capacitor. Component bags, turnover boxes, PCB loading and unloading racks, etc. in the production site should be equipped with electrostatic protection. Metal and ordinary containers are not allowed, and all containers must be grounded. (5) Personnel entering the electrostatic work area and those in contact with SMD devices must wear anti-static work clothes, especially in dry environments with relative humidity less than 50% (such as winter). Anti-static work clothes should comply with relevant national standards.

5.4 Process discipline:

SMT computer rooms must have strict access systems, strict operating procedures, and strict process discipline. For example: Personnel other than those in this position are not allowed to enter without authorization. Only two people during the learning period can operate on the machine. Untrained personnel are strictly prohibited from operating on the machine. All equipment must not operate with faults. If any fault is found, stop it immediately and report it to the technical person in charge. Report and troubleshoot before starting the machine; all equipment and other components are not allowed to be disassembled without permission, and indoor equipment is not allowed to be taken out of the workshop, etc.