What is SMT? Compare the pros and cons of through-hole technology

SMT or SMT technology is a term commonly used in the production of electronic circuit boards and electronic component assembly lines. The article below, Hapoin will help you better understand what SMT is? What are the advantages and disadvantages of this technology compared to traditional techniques?

What is SMT?

What is SMT? SMT’s full English name is Surface Mount Technology which means surface mounting technology. In particular, electronic and electrical components are mounted directly on the printed circuit board (PCB) surface. This method of manufacturing circuit boards is very popular in semi-automatic and automatic electronic component assembly lines.

Before the appearance of SMT technology, people had to do mechanical processing to attach a piece of metal to both ends to be able to solder directly onto the printed circuit surface. Since the birth of SMT, picking up electronic components from the bar feeding conveyor, vibrating bar, vibrating hopper, storage tray, etc. and placing them into position on the printed circuit board can be done with precision. High precision and small error.

SMT technology is applied to create device assemblies for most electronic devices today. Some products where SMT processing circuit boards can be found include: Laptops, desktop computers, smartphones, tablets, electronic toys, smart TVs, VR headsets, robot,…

What is SMT machine? Basic knowledge about SMT (Photo: Internet)

SMT technology line

SMT is a modern line that is very commonly used in the electronic circuit production process. Today, when the requirements for portability and compactness of electronic products are increasing, the SMT line can optimize the size of the PCBs. Along with that, it also helps to attach more devices such as resistors, diodes, capacitors.

SMT lines can be divided into 2 types based on the level of automation including:

• Automatic SMT transfer line
  Semi-automatic SMT line

What is the device used in SMT?

Passive SMT

SMT has many different concepts, however, for consistency in the manufacturing industry, engineers have agreed to call passive SMT as SMT resistors or also known as SMT capacitors with standard package sizes. standardized.

Capacitors and resistors will come in a variety of sizes, including: 0201, 0402, 0603, 0805, 1206 and 1812. The sizes mentioned can range up to hundreds of inches. Currently, they are not commonly used because many smaller components appear. Even so, passive SMT can still be found in applications that require larger power sources.

Tubes and Diodes

Usually, diodes and bulbs are contained in relatively small plastic packages. The Diodes will allow current to flow in one direction, drawing on one end. Each transistor is the basic building block for computer circuits and other electronic devices. It has fast response, using the same functions of voltage regulation, oscillation, amplification, switching and signal modulation.

Integrated Circuits

Integrated circuits in SMT technology are equipped with integrated packages and are custom designed depending on the level of connectivity required. Depending on each business and each type of chip, the SMT system will have certain changes such as:

• The small chip will use SOIC integrated packages or small sketch integrated circuits, used for familiar 74 series logic chips. Smaller version packages include SSOP and TSOP.
• Larger chips that require direct access will use a VLSI integrated package that is characterized by a rectangular or square footprint.
• Besides, the BAG chip or Ball Grid Array is used in many applications. Instead of connecting on the side, they move down below the pack. Because the entire underside of the package can be used, the connections are more widely spaced, so it is more reliable.

Integrated circuit is one of the devices used in the SMT line (Photo: Internet)

What is the SMT technology line process?

Normally, the PCB electronic circuit board assembly process will go through the following steps:

Material preparation and examination: Prepare the PCB and SMC and check if there are any errors. PCBs will usually have flat copper pieces of tin-lead, gold or silver, without holes called solder pads.

Stencil preparation: Stencil is used to provide a fixed position during the solder paste printing process. The stencil will be produced according to the designed position of the solder pad on the PCB.

Solder paste printing: solder paste is usually composed of a mixture of flux and tin, it is used to connect the solder paste and SMC on the PCB. They are applied to the PCB with Stencil using a squeegee with an angle range of  45 – 60 degrees.

SMC placement: The printed PCB will be transferred to the pick and place machines. There they are loaded onto a conveyor belt with electronic components placed on it.

Reflow soldering:

• Soldering oven: Once the SMC is in place, the circuit boards will be transferred into the solder reflow oven.
• Pre-heat zone: The first zone in the annealing furnace, where the temperature of the circuit and components are gradually raised at the same time. The temperature increase rate is 1.0 – 2.0 degrees Celsius per second until it reaches 140 – 160 degrees Celsius.
• Soak zone: The circuit board will be kept in this area with a temperature of 140 – 160 degrees Celsius for about 60 – 90 seconds.
• Reflow zone: The circuit boards will enter a zone where the temperature increases by 1 – 2 degrees Celsius per second to reach a peak of 210 – 230 degrees Celsius to help melt the tin in the solder powder and bond. components lead to pads on the PCB. The surface tension of the molten solder will help the components stay in place.
• Cooling zone: Helps ensure that the solder solidifies when it exits the heating zone, thereby avoiding joint errors.

If the circuit board is double-sided, the placement, printing, and reflow process can be repeated using glue or solder paste to hold the components in place.

Clean and inspection: The boards are cleaned after soldering and checked for any flaws. From there, defects can be repaired or remade and the product can be preserved. Common SMT related equipment such as magnifying lenses, probe testing machines, AOI (automated optical inspection), X-ray machines,…

Each different company will have different chip mounting techniques to create chip mounting machines on SMT lines. However, those steps can be summarized through the following steps:

• Applying solder paste: Solder paste is applied through the hole of a metal mask (stencil or metal mask). Then, it will be placed on the PCB to avoid sticking to other parts of the component and move on to the component mounting stage.
• Attaching ICs, attaching chips: SMT technology will automatically remove components from trays or conveyor belts and place them in the corresponding positions where solder paste has been applied. After the solder paste layer is dried, the PCB will be turned over and continued to be mounted repeatedly.
• Heating and cooling: In each factory, the heating step will be performed in an oven or an area with gradually increasing temperatures to help components adapt. When the temperature is high enough, the solder paste layer will melt and firmly stick the component layer onto the PCB. They will then be cooled by some cleaning chemicals, water and solvents, followed by quick-drying compressed air.
• Checking and fixing errors: For workshops in the electronic components industry, optical or X-Ray AOI (Automated Optical Inspection) machines are used to check the quality of finished products. The above device will help detect contact errors, position errors of solder paste and components on the printed circuit surface.

Basic process for assembling electronic circuit boards (Photo: Internet)

What are the advantages and disadvantages of SMT technology compared to old technology?

Advantages of SMT

Infomation	SMT	Old Technology
Dimensions of machined components (including resistors, capacitors, other semiconductor devices,…)	Reduced component size on the board. The smallest components installed with SMT technology are 0.1×0.1mm in size	The components on the board need to be relatively large in size to perform manual piercing
Component density	The ability to connect components on components is relatively high. Higher connection density because the holes do not block routing space on the inner layers	The number of components per unit area will be less
Attachment ability	Components can be mounted on both sides of the circuit board	Components are mounted on only one side of the circuit board
Error problem	Small errors in component placement are automatically corrected when the surface tension of the solder melts. The components are dragged into position to align with the row piece	When piercing holes, great precision is required. Because if the wires pass through the holes, the components are perfectly aligned and do not move out of alignment. Therefore, when an error occurs, the components will be completely removed and will be costly
Mechanical performance	Better mechanical performance under vibration and impact conditions	No guarantee compared to SMT
Resistance and inductance compared to solder joints	Lower so less unwanted RF signal effects, better high frequency performance	Higher
Number of holes drilled	Less than	More costs time and production costs
Degree of specialization	Achieve 100% rate due to automatic implementation	Errors still occur and quality is not guaranteed

SMT technology helps users solder with small components (Photo: Internet)

Disadvantages of SMT

So what are the disadvantages of SMT technology? This technology requires much greater attention to detail. Even as the process becomes largely automated, solder paste filling remains a top issue as solder holes become smaller, requiring less solder. This leads to further difficulties as the welds can be damaged by the potting compounds undergoing thermal cycling.

In addition, businesses’ investment costs are also higher when implementing SMT. Because assembling and repairing components with many components is more difficult, this requires highly skilled operators, and tools are also more expensive because of the small size and narrow spacing of SMT.

>>> More: What is Solder Paste

Conclude

Surely readers have understood what SMT is through the above article. SMT technology allows automated production to complete many assembly steps. This helps minimize production costs and maximize output. That is why SMT technology should be widely applied in the electronic circuit board manufacturing industry.

Jasmine Wu – Hapoin