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SMT Reflow Oven History and Trends

The technology of reflow soldering is not new in the field of electronics manufacturing. The components on various boards used in our computers are soldered to the circuit board through this process. This device has a heating circuit inside, which connects air or nitrogen Heat it to a high enough temperature and blow it to the circuit board where the component has been attached, so that the solder on both sides of the component melts and adheres to the motherboard. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is easier to control.

    In the development process of reflow soldering, it has been subdivided into various categories for various applications. For details, please refer to the classification and working principle of reflow soldering. The following describes the development history and development trend prediction of reflow soldering.

    Due to the need for miniaturization of electronic product PCB boards, chip components have appeared, and traditional soldering methods have been unable to meet the needs. First of all, the reflow soldering process is used in the assembly of hybrid integrated circuit boards. Most of the components assembled and soldered are chip capacitors, chip inductors, surface mount transistors and diodes. With the development of SMT technology, the emergence of a variety of chip components (SMC) and SMD (SMD), the reflow soldering process technology and equipment as part of the mounting technology has also been developed accordingly, and its application is becoming more and more extensive, Almost all electronic products have been applied, and reflow soldering technology, around the improvement of equipment, has also experienced the following development stages.

Reasons for the evolution of reflow soldering equipment: the heat transfer efficiency and the reliability of soldering continue to improve!
First generation reflow soldering: hot plate conduction reflow soldering equipment

(The slowest heat transfer efficiency: 5-30 W/m2K (the heating efficiency of different materials is different), there is a shadow effect)

Second generation reflow soldering: infrared heat radiation reflow soldering equipment
The heat transfer efficiency is slow: 5-30W/m2K (the infrared radiation efficiency of different materials is different), there is a shadow effect, and the color of the components has a great influence on the heat absorption.

Third generation reflow soldering: hot air reflow soldering equipment
The heat transfer efficiency is relatively high: 10-50 W/m2K, no shadow effect, color has no effect on heat absorption.

Fourth generation reflow soldering: gas-phase reflow soldering system
High heat transfer efficiency: 200-300 W/m2K, no shadow effect, the welding process needs to move up and down, and the cooling effect is poor.

Fifth generation reflow soldering: Vacuum steam condensing welding (vacuum vapor phase welding) system
Welding without voids in confined spaces, the heat transfer efficiency is highest (300 W-500W/m2K). The welding process remains static and vibration-free. Excellent cooling effect, color has no effect on heat absorption, is the most perfect welding system at present!
    
    In recent years, with the development of many electronic products in the direction of small, light, and high density, especially the large number of handheld devices, in the process of component materials, they have presented severe challenges to the original SMT technology, which has enabled SM to obtain The opportunity for rapid development. lC pin pitch has developed to 0.5mm, 0.4mm, 0.3mm, BGA has been widely used, CSP has also emerged, and shows a rapid upward trend, no-clean, low-residue solder paste on materials has been widely used. All of these have put forward new requirements for the reflow soldering process. A general trend is to require more advanced heat transfer methods for reflow soldering to achieve energy saving and uniform temperature. It is suitable for the welding requirements of double-sided PCBs and new device packaging methods, and Gradually realize the full replacement of wave soldering. Generally speaking, the reflow soldering furnace is developing in the direction of high efficiency, multi-function and intelligence. The main development paths are as follows. Reflow soldering in these development fields has led the future development of electronic products.

1. Reflow soldering technology

Inert gas protection has been used in reflow soldering for some time, and has been applied in a large range. Due to price considerations, nitrogen protection is generally selected. Nitrogen reflow soldering has the following advantages.

(1) Prevent reduction of oxidation.

(2) Improve welding wetting power and speed up wetting.

(3) Reduce the generation of solder balls, avoid bridging, and get good welding quality.

2. Double-sided reflow technology

The double-sided PCB has become quite popular, and it is gradually becoming more complicated. CY7C374-83A is becoming more and more popular. The main reason is that it provides designers with a very good elastic space, thereby designing a more compact, compact and low-cost product. As of today, double-sided boards are generally soldered to the top (component side) by reflow, and then soldered to the bottom (pin side) by wave soldering. A current trend is toward double-sided reflow soldering, but there are still some problems with this process. The bottom component of the large board may fall during the second reflow process, or the bottom solder joint is partially melted, causing solder joint reliability problems.

3. Through-hole reflow soldering technology

Through-hole reflow soldering is sometimes called reflow soldering of classified components. It is gradually emerging. It can remove the wave soldering link and become a process link in the PCB mixed packaging technology. One of the biggest benefits of this technology is that it can play the surface. At the same time, the advantages of the mounting manufacturing process use through-hole plug-ins to obtain better mechanical connection strength. The flatness of the larger-sized PCB board cannot make the pins of all surface-mount components contact the pads. Even if the pins and pads can be in contact, the mechanical strength provided by it is often not large enough, and it is easy to be disconnected during the use of the product and become a point of failure.

4. Green lead-free reflow oven manufacturing

Out of consideration for environmental protection, lead will be strictly restricted in the 21st century. Although the amount of lead used in the electronics industry is extremely small, less than 1% of the total amount, it is also prohibited. It will be restricted in the next few years. It has been phased out, and reliable and economical lead-free solders are being developed. Currently, a variety of alternatives are developed that generally have a melting point temperature of about 40 Tun higher than that of tin-lead alloys, which means that reflow soldering must be at a higher temperature. Carrying out under the nitrogen protection can partially eliminate the oxidation and the damage to the PCB itself due to the increase in temperature.

5. Flexible PCB continuous reflow soldering technology

Special furnaces have been developed to handle continuous flexible boards with SMT components attached to them. The biggest difference from ordinary reflow furnaces is that such furnaces require special rails to transfer the flexible boards. Of course, this furnace also needs to be able to deal with the problem of continuous boards. For the separated PCB board, the flow rate in the furnace has no dependence on the status of the first few stations, but for the continuous flexible board in rolls, the flexible board is The whole line is continuous, any special problem on the line, the pause means that the whole line must be paused, which creates a special problem. The part that is paused in the furnace will be damaged due to overheating. Therefore, such a furnace must have strain Ability to stop randomly, continue to process the section of flexible board, and return to normal working state when the whole line resumes continuous operation.

6. Vertical oven technology

The market's demand for shrinking the volume makes CSP, MPM and even POP more widely used, so that components have a smaller footprint and higher signal transmission rate after mounting. Filling or grouting is used to strengthen the solder joint structure, so that it can withstand the stress caused by the inconsistency of the thermal expansion coefficient of the silicon wafer and the PCB material. Generally, the chip is sealed with glue by the dripping or filling method. .

7. Simulation and optimization technology of reflow soldering temperature curve

The method of using computer technology to simulate the reflow soldering process has received extensive attention. This method can greatly shorten the process preparation time, reduce the experimental cost, improve the welding quality, and reduce the welding defects.

Through the establishment of product model structure using PCBCAD data, the reflow soldering process simulation model can replace the traditional online parameter setting process, and can even be used to ensure the compatibility of PCB design and reflow soldering process before production, and guide the design of manufacturability ( DFM), the simulation model can also eliminate the defects that cannot cover all product areas when using thermocouple testing. The PCB component model solver and the built-in reflow oven model can accurately predict the PCB component for specific process settings. Reflow soldering temperature curve. Using this method to optimize the process of new products in the PCB design stage can easily ensure the compatibility of product design and process equipment.

8. Replaceable assembly and reflow technology

In recent years, alternative assembly and reflow technology (Alternative Assembly and Reflow Technology, AART) has attracted interest in the PCB assembly industry. The AART process can perform reflow soldering of through-hole components and surface-mount components at the same time, eliminating the need for wave soldering and manual soldering. Compared with the traditional AART process, it has less cost, cycle and defect rate. Through the AART process, a complex PCB assembly process can be established. AART must consider materials, design, and process factors that affect it. A Decision Support System (DSS) can help engineers implement the AART process.

9. Reflow soldering process control technology

The built-in computer control system of the intelligent reflow furnace can easily input various data in the Window operating environment, and can quickly take out or replace the reflow soldering process curve from the memory, saving adjustment time and improving production efficiency.

The purpose of process control is to achieve the two goals of required quality and lowest possible cost. In the past, process control mainly focused on the detection of defects to improve quality; but now, the most fundamental connotation of control is to continuously monitor various processes and find out deviations that do not meet the requirements. Process control is the ability to obtain relevant data in a specific operation that affects the final result. Once a potential problem occurs, it can receive relevant information in real time, take corrective measures, and immediately adjust the process to the best condition. Monitoring the actual process data is the real process control. In the reflow soldering process control, it means monitoring the thermal curve of each board manufactured.

An automatic management system that can continuously monitor the reflow soldering furnace can indicate whether the process deviation is out of control before the actual process deviation occurs. This is the Automatic Reflow Management (ARM) system. This system uses continuous SPC histogram , Line balance network, documentation and product tracking constitute a complete software package, and can automatically measure process data in real time, and make judgments to affect product cost and quality. The basic function of the automatic reflow soldering management system is to accurately detect and automatically Collecting product data through the furnace, it provides the following functions: no need to verify the process curve; automatically collect reflow soldering process data; provide real-time feedback and alarm for zero-defect production; provide automatic SPC chart of reflow soldering process and correct process capability index (ComplexProcessCapabilityindex , Cpk) variable alarm.

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