When dealing with complex circuit boards, various components such as ICs, resistors, capacitors, connectors, etc., are often encountered. Although they all fall under the category of electronic components, different soldering techniques are required for assembly due to differences in pin density and packaging forms. With the strengthening trend of electronic miniaturization, the pin density of components becomes higher, and SMT soldering has become the most ideal SMT assembly.

What is SMT Soldering

We know that the PCBA board consists of two parts: PCB and components. The two are closely integrated, with the PCB providing support and connections for the components, and the components providing functionality for the electronics. The process of this integration is assembly and soldering.

In modern PCB assembly processes, soldering can include two main techniques: reflow soldering and wave soldering. Generally, reflow soldering is used for Surface Mount Device components, while wave soldering is employed for Through-Hole Device components. Although there can be some flexibility in certain cases, the cross-use of these techniques is not common due to significant differences in their working principles and designs.

SMT soldering, which is more focused on surface-mount components, typically refers to reflow soldering. Surface mount components, such as ICs, diodes, and transistors, have short, dense pins and small dimensions. Considering these characteristics, manual operations are evidently limited in large-scale production. Therefore, automated processes become the inevitable choice. SMT technology, using pick-and-place robots and SMT soldering machines, can efficiently and accurately complete this assembly process.

Four Stages of SMT Soldering

Stage 1: Preheat

After the SMT placement, the PCBA board enters the baking stage, marking the official start of surface mount soldering. The first stage is the preheating phase, where the oven gradually heats up, and the solvent with a lower melting point in the solder paste begins to evaporate. In this phase, precise control of the temperature ramp is crucial. Too rapid heating may lead to two main issues: damage to the components under thermal stress and potential collapse of the solder paste, resulting in short circuits. It is recommended to increase the temperature at a rate of two to three degrees per second to ensure the robustness of the soldering process.

Stage 2: Thermal Soaking

After the preheating ramp, the soldering process enters the soak stage, during which the oven maintains a constant temperature, ensuring the complete removal of volatiles from the solder paste. The crucial objective at this stage is to heat the metallic components in the solder paste to a sufficient temperature, causing them to melt. This allows the solder paste to provide the necessary wetting and surface tension in the subsequent soldering process, ensuring the formation of uniform and reliable solder joints between the pins and PCB pads. Additionally, the soak stage contributes to the even heating of the entire PCBA board, preventing issues such as component warping or cold spots caused by uneven heat distribution.

Stage 3: Reflow

Reflow is the most critical step in soldering and is also the stage most prone to soldering defects. Manufacturers set the oven’s maximum temperature based on the specifications of temperature-sensitive components and immediately begin the cooldown when the peak temperature is reached. The entire duration of the reflow process typically falls between 30 to 60 seconds. The key lies in strictly controlling the temperature ramp-up and ramp-down rates to avoid applying excessive heat to the components, thus preventing damage caused by thermal shock.

In practical SMT soldering processes, PCB flux is often applied to reduce the surface tension at the metal joints, facilitating the formation of a well-bonded metallurgical connection when the solder reaches its melting point. This provides the necessary conditions for the mixing and melting of various solder powder balls, ensuring the quality of the solder joints.

Stage 4: Cooling

Following the reflow step, the PCBA board moves on to the cooling stage. At this point, the molten solder cools and solidifies, securing the PCB components. The temperature of the PCBA board as it cools down is typically between 30 and 100 degrees, and its rate of cooling is around 3 degrees per second on average.

After SMT Soldering

While SMT soldering primarily relies on automated equipment, capable of handling intricate projects with fine pitch and high-density boards, uncertainties in the production process may lead to unexpected defects. SMT, as an intermediate step in PCB assembly services, requires detection and correction of errors rather than post-assembly inspection, as this not only increases the difficulty of rework but may also result in significant economic losses. Therefore, post-soldering inspection is imperative!

Taking FS Technology’s assembly line as an example, our SMT line includes a loader, pick-and-place machine, printer, SPI inspection equipment, reflow oven, and AOI inspection equipment. SPI ensures the quality of solder paste printing and component placement before soldering, while AOI is employed for inspecting the quality of SMT soldering.

For most manufacturers, AOI is provided as part of internal quality control, helping reduce assembly defects and detect and eliminate problems early on. It captures images using cameras, compares them with standard images, providing a visual understanding of soldering quality. Only products that pass AOI inspection proceed to the next production stage, while boards with issues undergo rework.

Summarize

In this article, FS Technology elaborates extensively on the entire process of SMT soldering using automated equipment in commercial projects. This includes crucial details about PCBs in the soldering oven and the necessary testing procedures post-soldering. If you are seeking a reliable PCBA solution provider, FS Technology is the ideal choice. We not only offer efficient SMT soldering services but also provide comprehensive turnkey PCBA solutions, offering convenient and reliable support for your projects. Feel free to contact us anytime for the best quotes!