Surface mount technology (SMT) allows electronic components to be mounted directly onto printed circuit boards (PCBs), offering advantages such as reduced size, low cost, and improved performance. The SMT process includes solder paste preparation, printing, component placement, reflow soldering, and inspection, with various types of surface-mounted devices (SMDs) available for different applications. SMDs can be categorized into passive and active components, with distinct features and functionality.

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Surface Mount Technology (SMT) and SMD Packages
1. 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
https://en.wikipedia.org/wiki/Surface-mount_technology#History )
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.
1. The assembly density is high, the size of the electronic printed circuit board is small, and the
weight is light.
2. The volume and weight of the chip components are only about 1/10 of that of the
conventional plug-in components.
3. 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%.
4. High reliability, strong seismic resistance, low solder joint defect rate.
5. High frequency characteristics are good, reduce electromagnetic and radio frequency
interference.
6. Easy to automate produce and increase productivity. Reduce the costs by 30% to 50%. Save
materials, energy, equipment, manpower, time, etc.
2. 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.

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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.
3) SPI
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.

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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.

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3. SMD Introduce
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
4. SMD Footprint Type
1) CHIP
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.
2) TO
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.
3) SOT
The most common type of SMD, with pins on both ends. The number of pins is generally
between 3-7.

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4) SOP
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.
5) QFP
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.
6) QFN
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.

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7) PLCC
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.
8) BGA
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.

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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.

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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.