What are SMT and SMD?

Using SMT to assemble electronic products has the advantages of small size, good performance, full function and low cost. Widely used in aviation, communications, medical electronics, automobiles and household appliances.

Overview of surface mount technology

SMT stands for surface mount technology. Originated from 1960s, developed in 1970s and 1980s and perfected in 1990s. Surface mount technology (SMT) is a method of producing electronic circuits, in which components are directly mounted or placed on the surface of printed circuit board (PCB).

Advantages of SMT assembly:

Using SMT to assemble electronic products has the advantages of small size, good performance, full function and low cost. Widely used in aviation, communications, medical electronics, automobiles and household appliances.

Electronic printed circuit boards have high assembly density, small volume and light weight.

The volume and weight of the chip module are only about110 of the traditional plug-in module.

After the widespread use of SMT, the volume of electronic PCBA is reduced by 40% ~ 60%, and the weight is reduced by 60% ~ 80%.

High reliability, strong impact resistance and low solder joint defect rate.

Good high frequency characteristics, reduce electromagnetic and radio frequency interference.

Easy to automate production and improve productivity. Reduce the cost by 30% to 50%. Save materials, energy, equipment, manpower, time, etc.

Brief introduction of patch technology

1. Solder paste mixing

After the solder paste is taken out of the refrigerator and thawed, it is stirred by hand or machine to adapt to printing and welding.

2. Solder paste printing

Place that solder paste on a steel mesh,

Solder paste is printed on PCB pads by scraper. Printing is the first process of the whole production, and the quality of printing directly affects the qualification rate of the whole production process. In the general PCBA industry, 60% of defective products are attributed to printing problems.

As shown in the following figure, the surface finish process of PCB is gold sinking, and the gray part is the pad for brushing solder paste. This is a failed picture. You can see that the solder paste actually deviates from the pad, so the engineer needs to reposition the program and template.

Step 3: energy

SPI is a solder paste thickness detector, which can detect solder paste printing and control the printing effect.

4. tributaries

Put the patch component on the paper feeder, prepare to pick up and place the program, and then install the installation engineer into the computer. According to the accurate X and Y coordinate data in "Pick and Place", the machine will refer to the marked points on the PCB, pick up the corresponding components with the suction nozzle and place them in the corresponding position.

5. Reflow welding

Then, the installed PCB board passes through a reflow oven, and the paste solder paste is heated into liquid at high internal temperature, and finally cooled and solidified to complete the welding.

The following figure shows the furnace temperature working curve.

6. Automatic optical freedom

AOI is an automatic optical inspection, which detects the welding effect of PCB and the defects of PCB by scanning.

7. Maintenance

Fix defects detected by AOI or manually.

Patch introduction

SMD is the abbreviation of surface mount device, which means: surface mount device is a kind of SMT 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 whether the components are SMD from the packaging.

Patch package type

1) chip

The most common are resistors and capacitors. We can simply distinguish whether a component is a patch or not from the four numbers of the software package. For example, 020 1 indicates the length and width of the resistor or capacitor.

2) to

This type of package can also be implemented by three plug-ins. It has a pin and a radiator at the other end. Usually, the number of pins does not exceed two.

3) SOT

The most common SMD has pins at both ends. The number of pins is usually between 3 and 7.

4) Standard operating procedures

So it means small outline in SOP. The pins are three L-shaped, which are drawn on both sides of the assembly. It is more dense and tidy than SOT, with about 8-32 pins.

5) QFP

The most common IC package has a relatively high utilization rate. Because the high-density pin is L-shaped and exposed outside the IC, it is easy to detect the welding state and maintain. AOI detector has a high degree of recognition.

6) QFN

QFN is also used for integrated circuit packaging. It is similar to the previous QFP. The difference is that the QFN pin is located under the IC body and does not extend. The SMD of this package cannot be soldered or visually inspected. Because the pins in contact with the PCB pads are blocked by the components themselves.

7) PLCC

This kind of package is very common in early SMD design, because the pins of this package are J-shaped around the bottom of SMD and used together with the corresponding IC bracket. During the pre-design test, it is easy to replace and insert the corresponding IC bracket. However, due to its large size, it was replaced in future production.

8) BGA

This is the most complicated package at present, and the pin density is very high. Because the pins are spherical, the contact surface with PCB pads is small, so the requirements for SMT are more accurate. The minimum distance between the two pins is 0.4 mm, so no deviation is allowed. Since all pins are under the component, X-ray is needed to detect the welding state. There is no way to repair it manually.

Passive and active components

SMD can also be divided into active components and passive components from the perspective of whether it depends on energy.

Among the active elements, the active element is an element that is active in the current direction or depends on the current flow direction. Such as transistors, integrated circuits, diodes, crystals, etc.

Passive components are a special function that does not require energy. Such as resistance, capacitance, inductance and other components.

Passive device characteristics:

1. Consume electric energy by itself, or convert electric energy into other forms of energy.

2. As long as the signal is input, it can work without external power supply.

Characteristics of moving parts:

1. self-consumption of electric energy

2. In addition to the input signal, there must be an external power supply to work normally.

Diodes and diodes are specific active and passive, depending on external conditions:

A. the coordinated varactor diode is a passive device.

B. The varactor diode used as a parametric amplifier is an active device.

C. When adjusting the power supply bias switch, the triode is a passive device.

These are just the most common and commonly used SMD components. It is neither practical nor entirely possible to explain all possible SMD components in one article.