As a key process in surface mount technology production, reflow soldering is the key to ensure the quality of PCB board and electronics product. Reflow soldering is mainly used to solder the circuit board that has been mounted on the component, melt the solder paste by heating to fuse and solder the patch component and the circuit board pad, and then cool the solder paste through the cooling of reflow soldering to solidify the components and pads together. Improper temperature curve will cause PCB board to have incomplete soldering, missing soldering, component offset, solder balls and other soldering defects, which affecting product quality.
This article will introduce about the definition, technological process, consideration, defects and comparison with wave soldering.
What is reflow soldering?
Reflow soldering is a process used to attach electronic components to a printed circuit board (PCB) by heating solder paste, which is applied to the board where components will be placed. Initially, the solder paste, a mixture of solder powder and flux, is printed onto the PCB. Components are then positioned on top of the paste, and the entire board is heated in a reflow oven or furnace. As the temperature rises, the solder paste melts and forms a solid connection between the component leads and the PCB pads once it cools and solidifies. This method is widely used in electronics manufacturing for its precision and efficiency in assembling complex, densely packed circuits.
What is reflow soldering used for
The term “reflow” is used to refer to the fact that the solder alloy is solid but when the temperature is higher than the melting point of the solder, the solder begins to melt and resume flow, hence the term “reflow”.
After one or more electronic components are connected to the contact pad using solder paste (a mixture of solder and flux), the solder is melted by controlled heating to achieve a permanent bond, and soldering can be performed by different heating methods such as a reopening oven, infrared heating lamp or heat gun.
Reflow soldering in PCB assembly
Reflow soldering in PCB assembly is a process where solder paste is used to attach electronic components to a printed circuit board; the paste, consisting of solder and flux, is first applied to the board, components are placed on top, and then the entire assembly is heated in a reflow oven. As the temperature increases, the solder paste melts and flows, creating strong electrical connections between the component leads and the PCB. Once cooled, the solder solidifies, securing the components firmly in place. This method is favored for its precision and effectiveness in assembling high-density and complex electronic circuits.
Solder Reflow Process Overview
The solder reflow process is a key technique in electronics assembly, ensuring precise and reliable connections between components and a printed circuit board (PCB). Here’s an overview of the process, broken down into detailed steps:
1.Solder Paste Application: Solder paste, a mix of solder powder and flux, is applied to the PCB’s pads where components will be placed. This is usually done through a stencil that helps deposit the paste accurately onto the designated areas.
2.Component Placement: Electronic components are then carefully placed onto the PCB, aligning their leads or pads with the solder paste. This placement is often done using automated pick-and-place machines for high precision and efficiency.
3.Preheat Stage: The PCB is gradually heated in a preheat zone of the reflow oven to bring it up to a temperature that helps to activate the flux in the solder paste, which helps to clean the surfaces and prepare them for soldering.
4.Reflow Stage: The PCB moves through the reflow oven’s reflow zone, where it is subjected to a carefully controlled temperature profile. This profile heats the solder paste to its melting point, allowing it to flow and create strong connections between the component leads and PCB pads.
5.Cooling: After the reflow stage, the PCB moves into a cooling zone where it is gradually cooled down. This cooling process allows the solder to solidify, forming durable and reliable solder joints.
6.Inspection and Testing: Once cooled, the assembled PCB is inspected for soldering quality, checking for issues such as solder bridges or insufficient solder. Automated Optical Inspection (AOI) and other testing methods ensure that the solder joints meet the required specifications.
7.Post-Processing: Any additional processes, such as cleaning to remove flux residues or further testing to verify electrical performance, are completed before the PCB is ready for integration into the final product.
Each of these steps is crucial for ensuring high-quality solder joints and reliable electronic assemblies, making the reflow soldering process essential for modern electronics manufacturing.
What is the advantage and disadvantage of reflow soldering
● Advantages of reflow soldering
1. The temperature of reflow oven is easy to control, and oxidation is avoided during reflow soldering.
2. It has a heating circuit inside that heats the nitrogen to a high enough temperature and blows it to the circuit board where the component has been attached, so that the solder on both sides of the component melts and bonds with the motherboard.
3. Dont need to immerse the whole printed circuit board in molten solder, but to complete the soldering task by local heating, so the soldered components are subject to little thermal shock and will not be damaged by overheating.
4. The solder is soldered at the soldering site and with local heating, avoiding welding defects such as bridging.
5. The solder is used once, and there is no reuse, so the solder is pure and has no impurities, ensuring the quality of the solder joint.
● Disadvantages of reflow soldering
Temperature gradients are not easy to grasp (specific temperature ranges for the four operating areas).
What are the stages of reflow soldering
1. Check and make sure that the inside of the equipment keeps clean ( no debris).After confirming cleaning, check the line, ensure safety after booting, select the production program to open the temperature setting.
2. Turn on the air transport, mesh belt transportation, cooling fan.
3. Turn on the temperature zone switch in order, wait for the temperature to rise to the set value, then you can start to pass the PCB. Pay attention to the direction of the board, and ensure that the distance between the two consecutive PCB boards of the conveyor belt is greater than 10mm.
4. The flux in the solder paste wets the pads, component ends, and pins. The solder paste softens, slumps, and then covers the pads, component ends, and pins, isolating them from oxygen.
5. When the PCB enters the insulation area, the PCB and components are fully warmed up to prevent the PCB from suddenly entering the soldering high temperature area and damaging the PCB and electronic components.
6. When the PCB enters the soldering area, the temperature rises rapidly to make the solder paste reach the melting state, and the liquid solder wets, diffuses or reflows the pads, component ends and pins of the PCB to form solder joints.
7. The PCB enters the cooling zone to solidify the solder joint. Reflow soldering is completed at this point.
8. Check and electrical test the soldered circuit board, clean and dry it according to the single number, color temperature and voltage classification.
What is the difference between wave soldering and reflow soldering
Wave soldering can basically be understood as it only solders for relatively small components, which is different from reflow soldering, and reflow soldering heats the components of the board, in fact, the original brushed solder paste is liquefied, in order to achieve the purpose of connecting the components with the board.
Working process
How wave soldering works
The board enters the machine port –> The sensor senses the rear spray FLUX (flux) –> The preheating zone starts to warm –> The tin spray starts to spray –> Cooling.
How reflow soldering works
Several temperature zones are heated -> tin liquefaction –> cooling.
Advantage and disadvantages
Wave soldering advantages
(1) Its solder is in a flowing state, so that the solder surface of the printed circuit board can fully contact with the solder, and the thermal conductivity is good.
(2) The contact time between the solder and the printed circuit board can be significantly shortened.
(3) The transmission system of the printed circuit board only does linear motion, which is simple to make.
Disadvantages of wave soldering
Solder is sprayed into the air at a very high temperature at a high speed, oxidizing more, and the resulting oxides often cause various forms of soldering defects.
（★ The advantages and disadvantages of reflow soldering are described above.）
Application
Wave soldering is mainly used in perforated or hybrid technology circuit boards, so there is generally a process of cutting off excess wire feet.
Reflow soldering is generally used for SMD packaging, to meet the needs of continuous miniaturization of PCB boards, and is used for SMT technology.
What are the reflow solder defects
Bridging
Manhattan Effect
Wicking
Poor Wetting（solder joint）
Desoldering
Solder balls
Tin whisker
Offset or reversed (Electronic components)
Flux residue
Pseudo soldering
Missing solder
How to control reflow soldering process
① It is necessary to understand the quality and soldering requirements on the PCBA, such as high temperature requirements and solder joints and devices that need to be taken care of in terms of life.
② Understand the soldering difficulties on the PCBA, such as the part where the solder paste is printed larger than the pad, and the part with a particularly small spacing.
③ Find out the hot and cold points on the PCBA, and weld the temperature measurement coupling on the point.
④ Determine other places where thermal coupling temperature measurement is necessary, such as BGA package and bottom solder joints, heat-sensitive device body, etc. (try to use all temperature measurement channels to obtain more information).
⑤ Set the initial parameters, and compare and adjust with the process specifications.
⑥ Carefully observe the PCBA after welding under the microscope, observe the shape and surface condition of the solder joint, the degree of wetting, the direction of tin flow, the residue and the solder ball on the PCBA.
The above six steps are the setting and modulation of the process, and when we are satisfied with its effect, we can enter mass production. Once the welding parameters (temperature, time, air volume, wind speed, load factor, exhaust, etc.) have been determined, it is the goal of process monitoring to ensure that these parameters have some stability.
What temp and time does solder reflow
The reflow furnace has 4 zones, which are divided into preheating area, constant temperature zone, tin melting zone and cooling zone.
The heating slope of the preheating zone should be less than 3 °C/sec, and the setting temperature should be ~130 °C at room temperature. The residence time is calculated as follows: let the ambient temperature be 25 °C, if the heating rate is calculated at 3 °C/sec, (150-25)/3 is 42s, if the heating rate is 1.5 °C/s, then (150-25)/1.5 is 85s.
The set temperature of the constant temperature zone is 130°C~160°C, and the constant temperature time is 60~120s.
The heating rate of the reflux zone is controlled at 2.5-3 °C/s, and the peak temperature should generally be reached within 25s-30s.
The cooling rate of the cooling section is generally 3~4 °C/s and can be cooled to 75 °C, and the cooling slope is less than 4 °C/s.
Will the solderability of the gold-plated PCB board be reduced after multiple reflow ovens
The reliability welding of printed plates, aerospace (QJ) and national military standards (GJB) are not allowed to exceed three times, and it is also strictly stipulated that the process parameters of each welding must not exceed the requirements of the standard specifications. On the one hand, this is based on the fact that multiple soldering will greatly reduce the adhesion (tensile strength) of the printed board pads; On the other hand, multiple welding will also cause poor embrittlement of intermetallic compounds (IMC) in the solder joint, increasing the risk of solder joint breakage. This understanding applies to both leaded and lead-free hot air leveling (HASL) printed boards and chemical nickel-gold (ENIG PCB) printed boards.
Challenges and solutions of reflow soldering process
The reflow soldering process, while crucial for assembling PCBs, comes with several challenges that can impact the quality and reliability of electronic assemblies. Here’s an overview of these challenges along with potential solutions:
1.Challenge: Solder Paste Quality
Solution: Using high-quality solder paste and storing it correctly can prevent issues such as poor flux activity or inconsistent solder deposition. Regularly inspecting and testing solder paste for viscosity and consistency ensures optimal performance during the reflow process.
2.Challenge: Temperature Profile Management
Solution: Maintaining a precise and consistent temperature profile throughout the reflow oven is critical. Implementing a well-calibrated thermal profiling system can help monitor and adjust temperatures to avoid issues like insufficient soldering or component damage. Regular maintenance and calibration of the reflow oven are essential.
3.Challenge: Component and PCB Design Issues
Solution: Proper design and layout of components and PCB pads are vital. Designing for manufacturability, ensuring adequate spacing between components, and using appropriate pad sizes can reduce soldering problems. Simulation tools and design rule checks can help address these issues early in the design phase.
4.Challenge: Soldering Defects
Solution: Common defects include solder bridges, cold solder joints, and insufficient solder. Addressing these requires careful inspection and control over the reflow process. Using automated optical inspection (AOI) systems can quickly identify and correct these defects. Adjusting solder paste application and reflow parameters can also mitigate these issues.
5.Challenge: Flux Residue
Solution: Flux residue can affect the reliability of the solder joints and cause corrosion or electrical failures. Employing appropriate cleaning methods, such as solvent-based or water-soluble cleaning processes, can effectively remove residues. Selecting low-residue fluxes can also minimize post-soldering cleanup requirements.
6.Challenge: Component Warping and Damage
Solution: The high temperatures involved in reflow soldering can cause sensitive components to warp or get damaged. Using components rated for high temperatures and optimizing the reflow profile to minimize thermal stress can help reduce this risk. Preheating the PCB and gradually ramping up temperatures can also mitigate thermal shock.
7.Challenge: Thermal Gradient and Uneven Heating
Solution: Uneven heating can result in inconsistent soldering. Ensuring proper airflow and temperature distribution within the reflow oven is crucial. Conducting regular thermal profiling and adjusting oven settings to achieve a uniform temperature gradient can help address this challenge.
Addressing these challenges with the appropriate solutions results in better performing, more reliable electronic assemblies.
Conclusion
With the wide application of surface assembly technology, everyone is more and more concerned about the quality of SMT, as an important part of it, we should improve the process quality control technology, so as to reduce or avoid the defects that often occur in reflow soldering, so as to ensure the final quality of electronic products.
Reflow soldering is a key process used in electronics manufacturing to attach surface-mounted components to printed circuit boards (PCBs); it involves applying solder paste to the board, placing components on the paste, and then heating everything in a reflow oven to melt the solder, which cools to create strong, reliable electrical connections. This method is essential for producing complex electronic devices with precise and durable connections, as it ensures that components are securely attached while maintaining the integrity of the delicate circuit board. The process allows for high-density assemblies and is used in making everything from smartphones to medical devices, ensuring that electronic components function properly and reliably in their intended applications.
Soldering and reflow soldering are both methods for joining electronic components to a circuit board, but they differ in their approach: traditional soldering usually involves manually melting solder with a soldering iron to make connections, which is suited for individual or small-scale work, whereas reflow soldering is a more automated process used for mass production, where solder paste is first applied to the board, then components are placed on the paste and heated in a reflow oven to melt the solder uniformly, creating precise and consistent connections for complex and high-density assemblies. Essentially, soldering is more hands-on and ideal for smaller projects, while reflow soldering is efficient and effective for producing large quantities of electronics with consistent quality.
To perform reflow soldering, start by applying solder paste to the printed circuit board (PCB) where you want to attach components, then place the components onto the paste, ensuring they are correctly aligned; next, carefully transfer the board to a reflow oven, which heats the assembly in a controlled manner to melt the solder paste, creating strong electrical connections as the solder cools and solidifies; finally, inspect the board to ensure all components are securely attached and the solder joints are free of defects. This method allows for precise and efficient assembly of complex electronics by leveraging the even heating provided by the reflow oven to achieve consistent results.
The reflow soldering mechanism involves applying solder paste to the circuit board, placing surface-mounted components on top, and then passing the board through a reflow oven that gradually heats the assembly to melt the solder paste, which then flows around the component leads and solidifies as it cools, forming strong and reliable electrical connections. This process ensures precise and uniform solder joints by using controlled heat profiles to carefully melt and solidify the solder, making it ideal for assembling complex electronic devices with many small, closely spaced components.
Reflow soldering can generally be done multiple times, but it’s not ideal to repeat the process too often; typically, each reflow cycle is used to initially solder components to a board, and while you can reflow a board a few times to make adjustments or correct errors, each cycle risks degrading the components or the board itself due to thermal stress or potential damage to the solder mask. In practice, reflowing a board more than once should be done cautiously and only if absolutely necessary, as excessive reflow cycles can lead to issues like weakened solder joints or damaged components.
IR reflow soldering, or infrared reflow soldering, is a technique where infrared heat is used to melt solder paste on a circuit board, allowing components to be securely attached; this process involves using infrared lamps to heat the board and components evenly, causing the solder paste to flow and form solid connections as it cools. This method is particularly useful for achieving precise temperature control and uniform heating, making it ideal for delicate or high-density electronic assemblies where accurate soldering is crucial. The result is strong, reliable solder joints that hold the components in place and ensure proper electrical connections.
Last updated on September 4th, 2024 at 07:40 am