SMT Technology Overview
SMT is known as Surface Mount Technology. It originated in the 1960s, developed in the 1970s and 1980s, and was perfected in the 1990s. Surface mount technology (SMT) is a method of producing electronic circuits in which components are mounted or placed directly on the surface of a printed circuit board (PCB).
Advantages of SMT assembly:
Assembling electronic products with SMT has the advantages of small size, good performance, full functionality and low cost. Widely used in aviation, communications, medical electronics, automotive and home appliances.
High assembly density, small size and light weight of electronic printed circuit boards.
The volume and weight of the chip assembly is only about 1/10 of the traditional plug-in assembly.
SMT is generally used to reduce the volume of electronic PCBAs by 40% to 60% and weight by 60% to 80%.
High reliability, high shock resistance, low solder joint defect rate.
Good high-frequency characteristics, reducing electromagnetic and radio frequency interference.
Easy to automate and increase productivity. Reduce costs by 30% to 50%. Saves material, energy, equipment, labor, time, etc.
Introduction of the SMD process
1. Paste mixing
The paste is thawed from the refrigerator and then stirred by hand or machine for printing and soldering.
2. Solder Paste Printing
Solder paste is placed on the stencil,
Solder paste is printed on the PCB pads by squeegee. Printing is the first process of the entire production, the quality of printing directly affects the entire production process of the pass rate. In the general PCBA industry, 60% of defective products are attributed to printing problems.
The surface finish process of PCB is immersion gold, and the gray part is the pads brushed with solder paste. This is a picture of a failure, and you can see that the solder paste is actually off the pads, so the engineer needs to realign the program and the stencil.
3. SPI
SPI is a solder paste thickness tester that detects solder paste printing and controls the results.
4. Feeder
Place the patch kit on the feeder, prepare the pick and place program, and then install the installer engineer into the computer. Based on the precise X and Y coordinate data in Pick and Place, the machine will refer to the marked points on the PCB, pick up the appropriate components with the nozzle, and place them in the appropriate locations.
5. Reflow Soldering
The mounted PCB is then passed through a reflow oven, where the paste is heated to a liquid at high internal temperatures, and then cooled and cured to complete the soldering.
The graph below shows the operating curve of the oven
6. Automatic Optical Inspection
AOI is an automated optical inspection that scans and detects soldering results and defects on PCBs.
7. Repair
Repair defects detected by AOI or manually.
Introduction of SMD
SMD is an acronym for Surface Mount Device, meaning: Surface Mount Device, which is one of the SMT (Surface Mount Technology) components, including CHIP, SOP, SOJ, PLCC, LCCC, QFP, BGA, CSP, FC, MCM and so on.
Simply put, SMT is a technology and SMD is a component that can be used for SMT. We can simply see from the package whether the component is SMD or not.
Types of SMD packages
1) Chips
The most common are resistors and capacitors. We can distinguish whether a component is a patch or not simply from the four numbers of the package. As 0201, he represents the length and width of that resistor or capacitor.
2) TO
This type of package can also be realized with three plug-ins. It is characterized by one pin and a heat sink at the other end. Usually, the number of pins does not exceed two.
3) SOT
The most common type of SMD has pins on both ends. The number of pins is usually between 3-7.
4) SOP
SO in SOP means small outline. The pins are three L-shaped and drawn on either side of the component body. It is denser and neater than SOT, and the number of pins is about 8-32.
5) QFP
The most common IC package with relatively high usage. Because of the high density pins are L-shaped and exposed outside the IC, it is easy to detect the soldering status and easy to maintain. the AOI inspection machine has a high degree of recognition.
6) QFN
QFN is also used for IC packages, which is similar to the former QFP. It is similar to the former QFP, except that the QFN pins are located below the IC body and do not extend. It is not possible to solder or visually inspect SMDs in this package because the pins in contact with the PCB pads are blocked by the component itself.
7) PLCC
This package was very common in early SMD designs because the pins of this package were in the shape of a J around the bottom of the SMD and were used in conjunction with the corresponding IC holder. It is easy to replace and insert the corresponding IC holder during pre-design testing. However, due to their large size, they are replaced later in production.
8) BGA
This package is by far the most complex and has a very high pin density. Since the pins are spherical and have a very small contact surface with the PCB pads, the SMT requirements are more precise. The minimum distance between two pins is 0.4mm, so no deviation is allowed. Since all pins are under the component, X-rays are required to check the soldering status. There is no way to repair it by hand.
Passive and active components
SMDs can also be categorized as active and passive components in terms of whether they depend on an energy source.
Active components internal pass active component is in the direction of the current is active or depends on the direction of current flow component. For example: transistors, ICs, diodes, crystals, etc.
A passive component is a specific function that does not require energy. Such as a large number of resistors, capacitors, inductors and other components of information.
Passive Component Characteristics:
1. Consumes its own power, or converts power to other forms of other energy sources
2. Works without an external power source as long as the input signal.
Active Part Characteristics:
1. Self-consumption of electrical energy
2. Must have an external power source in addition to an input signal to function properly.
Diodes and tertiary tubes are specific active passive, depending on external conditions:
A. Coordinating varactor diodes are passive devices
B. Varactor diodes used as parametric amplifiers are active devices
C. Triode is a passive device when power supply bias switching adjustment.