海角社区

In today's electronics industry, SMT soldering is the core technology in the PCB assembly process. Manufacturers with SMT can achieve high-density integration within a limited space and directly mount various complex and precise SMD components onto the PCB surface. SMT technology has greatly enhanced the flexibility and electrical performance of PCB design, which is highly in line with the current development direction of electronic products towards miniaturization, lightweight, high speed and high performance.

Today, SMT technology is widely applied across various industries. From consumer electronics such as smartphones, laptops and wearable devices, to medical devices like high-precision sensors and monitoring equipment, the core components of almost all modern electronic products rely on the high efficiency and reliability of SMT technology. However, the stability and consistency of the SMT soldering process directly affect the performance and reliability of the final product. Today, our topic is SMT soldering. Let’s first understand what SMT soldering involves.

What is SMT Soldering？

SMT soldering is a method of directly soldering electronic components onto the surface of a printed circuit board. Unlike through-hole soldering, which requires inserting the component pins into the holes on the PCB, SMT assembly directly places the components on the pads and completes the connection through soldering. The following table summarizes the differences between SMT soldering and through-hole soldering.

SMT Soldering vs. Through-Hole Soldering

Advantages of SMT soldering

1. SMT soldering enables high-density mounting of SMD components on both sides of compact SMT PCBs. This is highly suitable for the demand for compact design in modern SMT manufacturing, such as portable devices, wearable devices, etc.

2. The SMT assembly process is fully compatible with automated SMT equipment and production lines. This can ensure the consistency of high speed and high mounting accuracy in SMT assembly.

3. The SMT process requires no drilling, and involves fewer steps in the SMT soldering process. This point has a significant cost advantage and is suitable for reducing the overall cost during large-scale SMT manufacturing and SMT PCB assembly.

4. SMT soldering has higher reliability. With fewer mechanical solder joints, it reduces the risk of failure in SMD soldering and provides excellent shock and impact resistance for lightweight SMT components.

5. The design of SMT PCB is more flexible. This technology not only supports the design of complex multilayer boards, but also enables the high-density circuit layout in SMT manufacturing.

SMT Soldering Process

Although SMT soldering has many advantages, to truly achieve these advantages, precise process control must be maintained throughout the entire SMT soldering process flow. From solder paste printing to the final inspection, every link in the process directly affects the quality, reliability and performance of SMT PCB assembly. Next, we will introduce in detail the complete SMT soldering process.

1. Solder Paste Printing

The SMT soldering process begins with solder paste printing. First, the solder paste is precisely printed onto the PCB pads using a pre-made steel mesh. The steel mesh must be precise so that the amount of solder paste can be well controlled. The quality of the solder paste has a direct impact on the overall SMT soldering effect. Only when the solder paste is well controlled can the components be soldered well.

2. SMT Components Placement

After the solder paste printing is completed, the PCBs will reach the position of the fully automatic SMT pick-and-place machines along the production line. Then, the SMT placement machine will precisely place the SMD components on the corresponding solder paste according to the modulated program.

3. Dispensing Process (if necessary)

In some SMT assembly scenarios, dispensing treatment is carried out on some components before soldering. Dispensing is suitable for double-sided SMT PCB assembly or components that require additional fixation during the SMT soldering process. In complex SMT manufacturing, dispensing helps improve production yield.

4. Solder Paste Inspection (SPI)

After the mounting is completed, it will enter the solder paste inspection stage. The SPI system measures the height, volume, area and position of the solder paste. This step is to detect any printing defects before reflow soldering. Early SPI detection helps to avoid subsequent SMT soldering failures.

5. Reflow Soldering

The core process in SMT soldering is reflow soldering. At this stage, the PCB is gradually heated and cooled through an SMT reflow soldering machine according to a strictly set temperature curve. The entire temperature curve typically consists of four stages: preheating, constant temperature, reflow, and cooling. The temperature and time control in each stage directly affects the welding quality.

Whether it is led or lead-free SMT soldering, precise thermal control management is required to ensure the good formation of solder joints and reliable soldering strength. Modern SMT reflow soldering equipment is equipped with a highly stable and consistent temperature control system, which can effectively reduce soldering defects and ensure soldering consistency and long-term reliability in mass production.

6. AOI Inspection

After the reflow soldering is completed, PCBs enter the AOI inspection stage. Through the AOI inspection system, engineers can confirm the quality of solder joints and identify possible defects. For hidden solder joints (such as BGA, QFN, etc.), X-ray inspection is often necessary to ensure the quality of SMT soldering.

7. Cleaning process (if necessary)

In some SMT soldering processes, the cleaning procedure is a necessary step. Using PCB cleaner to remove the residual flux after soldering can ensure the electrical performance and reliability of the product during long-term use. Reasonable selection and management of PCB cleaning processes is also one of the important guarantees for improving the quality of SMT manufacturing.

Common SMT Soldering Defects and 海角社区s

Even with advanced SMT soldering machines and fully automated SMT lines, various defects can still occur in the SMT soldering process. The following are some common problems we have summarized during the SMT soldering process:

1. Tombstoning

Tombstoning refers to the lifting of one end of an SMD component during surface mount soldering. Usually, this kind of problem occurs because:

Reflow soldering does not heat evenly

Uneven printing of solder paste causes imbalance in the amount of solder

The stencil design of the steel mesh is unreasonable

The solutions are:

Optimize the temperature curve of the SMT reflow soldering machine.

Monitor paste volume consistency using precise paste Inspection.

Improve the design of steel mesh and the control of solder paste printing in SMT manufacturing.

2. Solder Bridging

In the SMT soldering process, when too much solder paste causes a short circuit between adjacent pads, this phenomenon is called solder connection. This problem often occurs due to excessive solder paste, equipment misalignment, or positioning errors in the SMT soldering machine. The opening size of the steel mesh can be adjusted. The mounting accuracy is confirmed through AOI. Regular cleaning and calibration of SMT soldering equipment and other methods can be adopted to prevent the occurrence of solder adhesion.

3. Insufficient Solder

Insufficient tin can lead to false soldering or open circuits at the solder joints, affecting the reliability of SMT PCB assembly connections. Common causes of insufficient solder include insufficient volume of solder paste printing, unstable quality, dry or clogged solder paste. So we need to check the steel mesh regularly and use SPI to detect areas with low solder content.

4. BGA Voids

In soldering BGA or QFN packages, moisture in the PCB or component, improper reflow atmosphere, or excessive solder paste can cause air bubbles to form inside the solder balls. This phenomenon is called voids. To avoid these problems, humidity-sensitive components can be pre-baked, the openings of the steel mesh can be controlled, and the amount of solder paste can be reasonably distributed.

5. Component Misalignment

In SMT assembly, the phenomenon where SMT components are not accurately mounted at the center of the pad is called component offset. To solve this kind of problem, we need to calibrate the SMT placement machine regularly, use the visual positioning system in the SMT Equipment and optimize the solder paste printing accuracy.

Quality Control and Testing in SMT Soldering

In the SMT soldering process, strict quality control is essential to ensure stable production and long-term reliability of SMT PCB assembly. From visual inspection to automated testing, modern SMT manufacturing production lines have multiple verification methods in each process.

1. Visual inspection

Preliminary inspection is usually carried out manually or semi-automatically to check the quality of visible solder joints. The test is based on the IPC-A-610 standard, which sets out the acceptable quality criteria for SMT soldering. The inspection contents include typical defects such as cold soldering, offset, continuous soldering and insufficient soldering.

2. Automatic optical inspection(AOI)

AOI is a very important quality control method in automated SMT assembly, which is usually directly integrated into the SMT manufacturing production line to intercept defects in real time. The high-speed camera will scan each solder joint and the mounting position of the SMD component one by one. AOI inspection can identify defects such as monument erection, missing parts, tin connection, and reverse polarity in real time.

3. Solder paste inspection

SPI is carried out immediately after solder paste printing to ensure consistency in the amount of solder paste at the beginning of the SMT soldering process. This inspection can detect printing problems at an early stage to effectively prevent subsequent SMT soldering defects.

4. In-circuit testing

ICT verifies the electrical functions of the completed SMT PCB assembly at the circuit level. It is used to detect whether the circuit board has on-off, short circuit or open circuit in the circuit, and whether the component parameters are normal.

5. Functional testing

Functional testing simulates the actual working environment of the product and is mostly implemented in the final stage of SMT manufacturing. This test can verify whether the overall performance of the PCB is normal and ensure the reliable operation of the SMT PCB assembly products in the actual application environment.

6. Reliability testing

Reliability testing verifies the long-term reliability of a product through various stress tests. These tests are:

Thermal Cycling test: It is used to evaluate the strength of solder joints under temperature changes.

Vibration Testing: It is used to detect the mechanical durability of SMT solder joints.

Wet heat and environmental testing: It is used to simulate the impact of harsh working environments on the long-term stability of SMT PCB assembly.

A Leading PCB Assembly Supplier - PCBasic

As a trusted and experienced PCB assembly supplier, PCBasic has always been at the forefront of SMT soldering and SMT assembly. With years of experience in SMT manufacturing, we offer full control over the SMT soldering process from sample development to mass production.

We have advanced SMT production lines and equipment

    High-speed surface mount technology (SMT) placement machines achieve high-precision positioning and mounting of SMT components.

    Precision solder paste printing with real-time solder paste inspection machine;

    Complete automatic optical inspection and X-ray inspection;

    Fully supports SMD soldering and surface mount soldering, suitable for difficult packages such as fine-pitch ICs, BGA, QFN, etc.

We have strict SMT soldering process control

    Throughout the entire SMT process, we implement full-process control:

    Stable wave control in SMT wave soldering, compatible with hybrid technology board;

    Optimize the temperature curve of the SMT reflow soldering machine;

    Strictly implement the quality management system under ISO and IPC standards;

    Real-time monitoring and continuous optimization to reduce defects in the SMT PCB assembly process.

We have comprehensive SMT manufacturing service capabilities

    PCBasic can provide overall solutions for SMT PCB assembly and SMT manufacturing covering multiple industries:

    It is widely applied in consumer electronics, medical devices, automotive electronics, industrial control, etc.

    Be flexible in responding to the demands of small-batch sampling and large-batch mass production;

    Professional support for SMD soldering and SMT soldering solution selection based on product characteristics.

At PCBasic, we combine advanced SMT equipment, deep process capabilities and strict quality system to provide you with world-class SMT soldering and PCB assembly integrated solutions. Whether you need complex multilayer boards or high-density SMT PCB assembly, PCBasic can be your reliable partner in SMT manufacturing.

Conclusion

The SMT soldering process is a highly engineered and precise system. In this article, we explore the meaning of SMT soldering, the process, and the methods of quality control in the SMT process. From solder paste printing, reflow soldering, inspection to functional verification, each link requires strict control and rich experience. By cooperating with experienced SMT assembly suppliers like PCBasic, enterprises can fully utilize advanced SMT Manufacturing capabilities, cutting-edge SMT Equipment and strict quality control systems. Welcome to contact!

About PCBasic

Time is money in your projects – and PCBasic gets it. PCBasic is a PCB assembly company that delivers fast, flawless results every time. Our comprehensive PCB assembly services include expert engineering support at every step, ensuring top quality in every board. As a leading PCB assembly manufacturer, we provide a one-stop solution that streamlines your supply chain. Partner with our advanced PCB prototype factory for quick turnarounds and superior results you can trust.