SMT Technology Overview
SMT full name Surface Mount technology. Originated in the 1960s, developed in the 70s and 80s, perfected in the 1990s. Surface-mount technology (SMT) is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards (PCBs). (Sourcing from en.wikipedia.org )
Advantages of SMT Assembly:
The assembly of electronic products with SMT has the advantages of small size, good performance, full functionality and low cost. Widely used in aviation, communications, medical electronics, automobiles and household appliances.
- The assembly density is high, the size of the electronic printed circuit board is small, and the weight is light.
- The volume and weight of the chip components are only about 1/10 of that of the conventional plug-in components.
- After the SMT is generally used, the volume of the electronic PCBA is reduced by 40% to 60%, and the weight is reduced by 60% to 80%.
- High reliability, strong seismic resistance, low solder joint defect rate.
- High frequency characteristics are good, reduce electromagnetic and radio frequency interference.
- Easy to automate produce and increase productivity. Reduce the costs by 30% to 50%. Save materials, energy, equipment, manpower, time, etc.
SMT Process Introduction
- 1. solder paste mixing
After the solder paste is taken out of the refrigerator and thawed, it is stirred by hand or machine to suit printing and soldering.
- 2. solder paste printing
The solder paste is placed on the steel mesh, and the solder paste is printed on the PCB pad by a doctor blade. Printing is the first process of the entire production, and the quality of the printing directly affects the pass rate of the entire production process. In the general PCBA industry, 60% of defective products are attributed to printing problems.
As shown in the figure below, the surface finish process of the PCB is immersion gold, the gray part is the pad that has been brushed past the solder paste. This is a picture of failure, you can see that the solder paste is actually deviating from the pad, so the engineer needs to re-adjust the program and the position of the stencil.
SPI is a solder paste thickness detector that can detect the solder paste printing and control the solder paste printing effect.
- 4. Feeder
Place the patch components on the feeder, prepare the Pick and place program, and then engineer install program into the computer. According to the precise X and Y coordinate data in “pick and place”, the machine will refer to the Mark point on the PCB, pick up the corresponding components with the nozzle, and place them in the corresponding positions.
- 5. Reflow Soldering
Then the mounted PCB board go through re-flow oven, and the paste-like solder paste is heated to a liquid after passing through the high temperature inside, and finally cooled and solidified to complete the soldering.
The figure below is the furnace temperature working curve
- 6. AOI
AOI is an automatic optical inspection, which can detect the welding effect of the PCB by scanning, and can detect the defects of the PCB board.
- 7. Repair
Repair the AOI or manually detected defects.
SMD is the abbreviation of Surface Mounted Devices, which means: surface mount device, which is one of SMT (Surface Mount Technology) components, including CHIP, SOP, SOJ, PLCC, LCCC, QFP, BGA, CSP, FC, MCM and so on.
Simply speaking, SMT is a technology, SMD is a component that can be used for SMT. We can simply see from the package whether the component is SMD.
SMD Footprint Type
The most common are resistors and capacitors. We can simply distinguish whether the component is a patch from the four numbers of the package. As 0201, he represents the length and width of this resistor or capacitor.
This type of package is also possible with three plug-ins. It is characterized by one pin and the other end is a heat sink. Generally, the number of pins does not exceed two.
The most common type of SMD, with pins on both ends. The number of pins is generally between 3-7.
The SO in the SOP is the meaning of the Small Outline. The pins are three L-shaped and are drawn on both sides of the component body. More dense and neat than SOT, the number of pins is around 8-32.
The most common IC package, the usage rate is relatively high. Because the high-density pin is L-shaped and exposed outside the IC, it is easy to detect the soldering state and facilitate maintenance. The AOI detection machine has a high degree of recognition.
QFN is also used for IC packaging. It is similar to the previous QFP. The difference is that the QFN pin is below the IC body and does not extend. There is no way to solder or visually inspect this packaged SMD. Because the pins that are in contact with the PCB pads are blocked by the components themselves.
This package is very common in early SMD designs because the pins of this package are J-shaped around the bottom of the SMD and used with the corresponding IC holder. It is easy to replace and insert in the corresponding IC holder during the pre-design test. However, because it is large in size, it is replaced in later production.
This package is currently the most complex and has a very high pin density. Because the pins are spherical and have a small contact surface with the PCB pads, the requirements for SMT are more precise. The minimum distance between the two pins is 0.4mm, so no deviation is allowed. And because all pins are below the component, X-ray is required to detect the soldering status. There is no way to repair it by hand.
Passive and active components
SMD can also be divided into active components and passive components from the perspective of whether it depends on the source of energy.
The active component internal-pass active component is a component that is active in the direction of current or depends on the direction of current flow. Such as: transistors, IC, diodes, crystal, etc.
Passive components are a specific function that does not require an energy source. Such as a lot of resistance, capacitor, inductance and other components of the information.
Passive device features:
1. Consumption of electricity by itself, or conversion of electrical energy into other forms of other energy
2. As long as the signal is input, it can work without external power supply.
Active parts features:
1. Self-consumption of electrical energy
2. In addition to the input signal, you must have an external power supply to work properly.
Diodes and tertiary tubes are specific active passive depending on external conditions:
A. Coordinating varactors are passive devices
B. The varactor diode used as a parametric amplifier is an active device
C. The triode is a passive device when there is a power supply bias switch adjustment.
These are just the most commonly found and used SMD components. Fully explaining all possible SMD components in a single article is neither practical nor entirely possible. We list some other components packages for SMT as below for your reference.
SMT surface mount technology packages are used for most printed circuit designs that are going to be manufactured in any quantity. Although it may appear there is a relatively wide number of different packages, the level of standardisation is still sufficiently good.
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