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PCB/PCBA knowledge
What is the SMT line process and what are the precautions

Surface Mount Technology is a surface mount technology for electronic components, and it is also the most popular technology in the current electronics manufacturing industry. Compared with traditional plug-in electronic components, SMT components are smaller, lighter, lower power consumption, and can achieve higher integration and performance. In the SMT line process, the components are directly soldered to the surface of the PCB (Printed Circuit Board), and the precise positioning and installation of the components are completed by a placement machine and other equipment.

smt process
smt process

SMT technology has the advantages of high production efficiency, stable process, and low cost. It has gradually become the mainstream technology in the electronics manufacturing industry and is widely used in various fields such as mobile phones, computers, home appliances, and automobiles. The development of SMT technology has also promoted the transformation and upgrading of the entire electronics manufacturing industry and promoted the rapid development of the electronics industry.

Table of Contents

What is the SMT line process?

1. Empty board loading

  • Confirm that the size and thickness of the empty board are consistent with the process requirements to avoid problems such as inaccurate installation positions or too-long pins.
  • Make sure that the surface of the empty board is in good condition to avoid problems such as unevenness or scratches on the surface.
  • During the production process of the empty board, avoid electrostatic interference as much as possible to prevent static electricity from damaging electronic components.
  • The empty board needs to be placed in a dry, dust-free, and oil-free environment to avoid dust, grease and other substances from adhering to the empty board.
  • When loading empty boards, it is necessary to ensure that the operator’s hands are clean and wear gloves to avoid contamination of the empty board surface with hands.
  • For different empty board types, different loading methods need to be selected according to the process requirements, for example, for thin board loading, a special carrier rack is required to avoid bending and deformation of the empty board.

In short, during the loading process of SMT empty boards, it is necessary to pay attention to the details to ensure that the production environment is clean and tidy, the operation of personnel is standardized, and the equipment inspection is strict, so as to ensure the quality and efficiency of the entire production process.

2. Solder paste printing

In this step, solder paste will be printed through the steel plate on the pads/pads of the parts to be soldered on the PCB. The position and volume of the solder paste will affect the subsequent soldering quality. These solder pastes will melt in the subsequent SMT process – the high temperature zone of the reflow oven, and solder the electronic components to the circuit board during the re-solidification process.

Solder paste printing
Solder paste printing

The main reasons for using solder paste as a combination of electronic parts and PCBs are as follows:

1) Before the soldering is completed, the electronic parts are pasted and fixed on the circuit board so that they will not be shifted due to the movement or vibration of the PCB.

2) Solder and fix the electronic components on the PCB after reflowing at high temperature, so that they will not fall during the end user’s use, and achieve the purpose of electronic signal transmission.

The selection and quality of solder paste have a great influence on the SMT line process. It is necessary to select the appropriate type and brand of solder paste according to the requirements of components and production requirements.

At the same time, it is necessary to pay attention to some details in SMT line process:

  • It is necessary to check whether the printing machine and scraper are in normal condition before printing to avoid poor printing quality.
  • The thickness of the solder paste needs to be adjusted according to the requirements of the components and the production process. Too thick or too thin will affect the installation quality of the components.
  • The quality of solder paste needs to be guaranteed by testing, such as testing the extrusion volume, viscosity and other indicators of solder paste.
  • When printing solder paste, it is necessary to keep the production environment clean and tidy to avoid dust, grease and other substances from adhering to the solder paste.
  • After the solder paste printing is completed, the printing quality needs to be inspected to avoid problems such as poor printing and short circuit.

3. Solder paste inspection

The quality of solder paste printing will directly affect the quality of subsequent parts welding, so in order to ensure stable quality, an additional optical instrument will be installed after solder paste printing to detect whether the solder paste printing is good or not and whether there is less tin, tin leakage, excess tin and other bad phenomena. After inspection, if you find a board with bad solder paste printing, you can pick it out first, wash off the solder paste on it and reprint the solder paste, or remove the excess solder paste by repairing it.

solder paste inspection
solder paste inspection

The reason why SPI is important is that after the solder paste is cured, there is a soldering problem founded in the part, then it must be repaired with a soldering iron. If the problem of solder paste printing can be found and improved or solved before early curing, the production defect rate can be greatly reduced and the cost of repair can be reduced.

4. SMD

High-speed patch: accurately install the electronic components SMD to the fixed position of the PCB. This type of SMD passive component (such as small resistors, capacitors, and inductors), also known as a Small Chip, is usually relatively small in size, and generally, only two terminals need to be soldered. Therefore, when placing such small parts on the circuit board, the relative position accuracy requirements are relatively low. A high-speed placement machine usually has several nozzle heads, and the speed is very fast, and several parts can be placed in one second.

However, it is not suitable for large parts or parts with a certain weight to be processed by a high-speed placement machine. On the one hand, it will slow down the speed of the small parts that were originally playing fast. Second, the parts will be shifted from their original positions due to the rapid movement of the board.

Multi-functional patch: it can be used for almost all SMD parts patching requirements. But because it is not speed-oriented, but precision-oriented, the slow machine is generally used to make some relatively large electronic parts, such as BGA integrated circuits, connectors, shielding frames/covers, etc., which need more accurate positions.

5. Pre-reflow AOI

An additional pre-reflow AOI will be conducted before the reflow soldering to confirm the quality of the patch before reflow. Another case is that some boards directly solder the shield to the circuit board at the SMT stage. Once the shield is placed on the circuit board, it is impossible to check the patch and solder quality through AOI or visual inspection. In this case, it is necessary to conduct the pre-reflow AOI test, and place a shield before the patch.

6. Reflow soldering

The solder paste is melted at a high temperature, and after cooling, the electronic component SMD and the PCB board are firmly welded together. The rising and falling temperature often affect the soldering quality of the entire circuit board. According to the characteristics of the solder, the general reflow oven will set the preheating zone, heat absorption zone, reflow zone, and cooling zone to achieve the best soldering effect.

Reflow soldering
Reflow soldering

In addition, the maximum temperature in the reflow oven should not exceed 250°C, otherwise many parts will be deformed or melted because they cannot withstand such a high temperature. Basically, after the circuit board passes through the reflow oven, the assembly of the entire circuit board is completed, except for the hand-soldered parts. The next step is to inspect and test the circuit board for defects or poor functions.

7. PCB cleaning

Its function is to remove the welding residues harmful to the human body such as flux on the assembled PCB board. The equipment used is a washing machine, and the location may not be fixed.

8. Post-reflow AOI

Post-reflow AOI has almost become the standard configuration of SMT line process today, but not every SMT production line will be equipped with an optical inspection machine (AOI). One of the purposes of setting up post-reflow AOI is because some circuit boards with too high density cannot be effectively carried out the follow-up open and short circuit test (ICT), then AOI comes.

PCB AOI test
PCB AOI test

However, because AOI is an optical interpretation, it has its inherent blind spots. AOI cannot judge the quality of components such as false soldering, BGA solderability, resistance value, capacitance value, and inductance value. Therefore, some advanced EMS factories will have X-RAY, which is used for products and customers with high-quality requirements and high-reliability requirements, to detect the soldering quality of pins.

9. Visual inspection

Regardless of whether there is a post-reflow AOI test, a common SMT line process will always set up a visual inspection area for the circuit assembly board. Key items of manual inspection: whether the version of PCBA is a modified version; whether the customer requires components to use substitute materials or components of designated brands; is the orientation of directional components such as IC, diodes, triodes, tantalum capacitors, aluminum capacitors, and switches correctly; whether there are short circuits, open circuits, false parts, false welding and other defects after welding.

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