technology), or surface-mount technology, is an electronic circuit production technology in which components are mounted or placed directly on the surface of a printed circuit board. In this industry, there are two methods, SMD (surface-mount
device) and THT (through-hole
technology). Both technologies can be applied on the same PCB, except that through-hole technology is used for components that are not suitable for surface mount (e.g., large transformers, connectors, electrolytic capacitors, etc.) SMT components are usually smaller than through-hole devices because they have smaller or even no pins, and they may have short pins or leads of various types and contact patterns, or a ball-grid arrangement (BGA), etc. ), etc.
SMT Advantages
SMT has the following key advantages over traditional through-hole insertion technology:
(1) Smaller components. 0.4*0.2mm (0.016*0.008 in: 01005) was achieved in 2012, and there is a trend toward more miniaturization
(2) Higher component density (number of components per unit area). components per unit area) and more connections per component
(3)Higher connection density
(4)Lower cost and time (go-live production)
(5)Fewer holes in PCB design and fabrication
(6)Easier and faster placement
(7)Minor errors in component placement are automatically stretched and repaired due to the surface tension of the melted solder paste
(8) The surface tension of the solder paste is not as strong as the surface tension of the solder paste. (8) Components can be mounted and soldered on both the top and bottom sides of the board
(9) Lower resistance and inductance effects, resulting in less RF signal effects
(10) Better mechanical properties under vibration and drop conditions
(11) Many SMT components are cheaper than plug-in components
(12)Better EMC performance, given smaller solenoid coils resulting in lower electromagnetic radiation
SMT Disadvantages
(1)Because of the smaller size and SMD lead spacing, manual repairs at the placement or component level are much more difficult and require specialized skilled workers and more expensive rework tools to perform the operation
(2)SMD components cannot be used directly on inserted motherboards (which can be used in the case of plug-in components). For plug-in motherboards (a rapid test prototyping tool), you need to customize a PCB or solder SMD components to the pin carrier.
(3) SMD solder connections can be damaged by potting components during thermal cycling
(4) SMT solder connections are becoming smaller and less spaced, resulting in the SMT process requiring greater precision
(5) SMT is not suitable for large, high-energy, high-voltage components, such as transformers in power supply circuits, etc., and the integration of SMT and plug-in processes is together is more common.
(6) SMT is not suitable for frequent mechanical stress in the application, such as some connectors, as an interface with the external connection, frequent pulling and inserting for the stability of the soldering challenges
SMT rework
In the whole PCBA processing process, SMT problem components are often used soldering iron or non-contact rework system for repair. In general, the rework system is a better choice, because SMD component repair requires considerable proficiency skills, and less easy. Non-contact soldering rework methods: infrared soldering and hot gas soldering.
Infrared method
By infrared welding method, the weld is melted by heat through electromagnetic induction of long and short waves. Advantages are:
(1) Recommended installation
(2) No need for pressurized air
(3) No need for nozzles for different components, reducing the cost of replacing the nozzles
(4) Fast infrared source response
Disadvantages
(1) The central area is subjected to more heat compared to the peripheral area
(2) Temperature control Not precise enough, very easy to peak
(3) Surrounding elements need to be covered to avoid damage, takes more time
(4) Surface temperature depends on the reflectivity of the element
(5) Temperature depends on the shape of the surface, convective energy loss will reduce the temperature of the element
(6) No possibility of refluxing the ambience
Hot air method< /p>
In the hot gas method of welding, the energy heated at the joint is transferred through the hot gas, usually with the help of air or nitrogen. Advantages
(1) Mimics the scenario in reflow soldering
(2) Some systems allow switching between hot air and nitrogen
(3) Standard component nozzles for greater stability and quicker processing
(4) Allows for reproducible soldering
(5) Sufficient thermal energy to heat a large number of components
(6) Sufficient thermal energy for a large number of components
(7) Sufficiently high heat capacity. >(6) Uniform heating
(7) Components are heated to temperatures that do not exceed set gas temperatures
(8) Rapid cooling after reflow results in smaller weld lines
Disadvantage: The thermal properties of the heat generator result in a slower response. Rework often corrects errors caused by manual or machine error and consists of the following steps
Melting the solder and moving the component
Moving the residual solder
Directly or dispenser-printing the solder paste on the PCB
Placing the new component and reflowing it
In some cases, hundreds or thousands of the same components need to be repaired. Such errors often occur and are caught during the placement process. However, when they are caught too late, they are subject to high-volume repairs, which require a targeted repair strategy to ensure the quality of the repair.
SMT Packaging
SMT mounted components are typically smaller than other pin components and are designed to be easily mass-produced by machines rather than by hand. The electronics industry has its standard package forms and sizes (the industry-leading standard is JEDEC) that contain:
The codes in the charts usually define the length and width of components in inches or millimeters. For example, a metric 2520 component is 2.5mm*2.0mm, which in inches would be roughly