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SMT reflow soldering detailed explanation for the development and process requirements

  • Time:2020-03-28
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SMT reflow soldering detailed explanation for the development and process requirements 

The lead-free reflow soldering process is currently the most important soldering process in surface mount technology, and it has been widely applied in many industries including mobile phones, computers, automotive electronics, control circuits, communication, LEO lighting, and more. More and more electronic components are transitioning from via to surface mount, and reflow soldering is a clear trend in the soldering industry to replace peak soldering within a considerable range. So what role will reflow soldering equipment play in the increasingly mature lead-free SMT process? From the perspective of the entire SMT surface mounting line, let me analyze it for you:

(1) The entire SMT surface mounting line generally consists of three parts: a solder paste red glue printing machine, a mounting machine, and a reflow soldering. For SMT machines, compared to those with lead, there are no new requirements for the equipment itself; For screen printing machines, due to the slight differences in physical properties between lead-free and lead-based solder paste, some improvement requirements have been put forward for the equipment itself, but there is no qualitative change; The challenge pressure of lead-free lies precisely in the reflow soldering furnace. The melting point of lead solder paste is 183 degrees. To form a good solder joint, intermetallic compounds with a thickness of 0.5-(3)5um must be formed during welding. The formation temperature of intermetallic compounds is 10-15 degrees above the melting point, which is 195-200 degrees for lead solder. The maximum temperature that electronic components on a circuit board can withstand is generally 240 degrees. Therefore, for lead welding, the ideal welding process window is 195-240 degrees. Lead free soldering has brought significant changes to the welding process due to the change in the melting point of lead-free solder paste. The currently commonly used lead-free solder paste is Sn96Ag0.5Cu(3)5, with a melting point of 217-221 degrees. Good lead-free welding also requires the formation of intermetallic compounds with a thickness of 0.5-(3)5um. The formation temperature of intermetallic compounds is also 10-15 degrees above the melting point, which is 230-235 degrees for lead-free welding. Due to the fact that the maximum withstand temperature of lead-free soldering electronic components does not change, the ideal soldering process window for lead-free soldering is 230-245 degrees.

The significant reduction in process windows poses great challenges to ensuring welding quality, and also puts higher demands on the stability and reliability of lead-free welding equipment. Due to the inherent lateral temperature difference in the equipment itself, and the difference in thermal capacity of electronic components during the heating process, the range of welding temperature window that can be adjusted in lead-free reflow soldering process control becomes very small. This is the true difficulty of lead-free reflow soldering.

The reflow soldering furnace plays a crucial role in the final product quality from the perspective of the entire lead-free process. However, from the perspective of investment in the entire SMT production line, the investment in lead-free soldering furnaces often only accounts for 10-25% of the total investment in the entire SMT line. That is why many electronic manufacturers immediately replace their existing reflow soldering furnaces with higher quality reflow soldering furnaces after transitioning to lead-free production.

(2) What new requirements have the increasingly mature lead-free process put forward for reflow soldering furnaces? We will analyze it from the following aspects:

Requirements for welding quality

·How to obtain a smaller lateral temperature difference

Due to the small window of lead-free welding process, the control of lateral temperature difference is very important. The temperature inside reflow soldering is generally affected by four factors:

(1) Transfer of hot air

At present, the mainstream lead-free reflow soldering furnaces adopt a heating method of 100% full hot air and hot air+infrared compensation. In the development process of reflow soldering furnaces, infrared heating has also appeared. Infrared heating has a fast speed and can timely warm up devices with high heat absorption. However, due to the different infrared absorption reflectivity of devices with different colors and the shadow effect caused by adjacent original devices, both of these situations can cause temperature differences and pose a risk of jumping out of the process window for lead-free soldering, Therefore, infrared heating technology has gradually been phased out in the multiple independent heating methods of reflow soldering furnaces, replaced by full hot air and hot air with infrared external compensation.

In lead-free welding, it is necessary to pay attention to the heat transfer effect and heat exchange efficiency. Especially for components with large heat capacity, if sufficient heat transfer and exchange cannot be achieved, it will lead to a significantly slower heating rate than small heat capacity components, resulting in a lateral temperature difference. The design of the air transport structure of the reflow soldering furnace directly affects the heat exchange rate. There are two hot air transfer methods for reflow soldering. One is called micro circulation hot air transfer method, and the other is called small circulation hot air transfer method.

The hot air in the microcirculation is blown out from the holes of the heating plate, and the flow of the hot air is limited within a small range, resulting in poor heat transfer efficiency in the surrounding area. The design of the small loop is due to the concentrated flow of hot air and its clear directionality. This kind of hot air heating increases the heat transfer effect by about 15%, and the increase in heat transfer effect will have a significant effect on reducing the lateral temperature difference of large and small heat capacity devices.

(2) Chain speed control can affect the lateral temperature difference of the circuit board. Generally speaking, reducing the chain speed will give larger thermal capacity devices more heating time, thereby reducing the lateral temperature difference. However, after all, the setting of the furnace temperature curve depends on the requirements of the solder paste, so it is not realistic to reduce the chain speed without limitation in actual production.

(3) We have conducted an experiment on the control of wind speed and air volume. Keeping the other conditions in the reflow soldering furnace unchanged and only reducing the fan speed in the reflow soldering furnace by 3%, the temperature on the circuit board will decrease by about 10 degrees. It can be seen that the control of wind speed and air volume is important for furnace temperature control.

In order to achieve control over wind speed and airflow, two points need to be noted:

A. The speed of the fan should be controlled by variable frequency to reduce the impact of voltage fluctuation on it;

B. Try to reduce the exhaust air volume of the equipment as much as possible, as the central load of the exhaust air is often unstable and can easily affect the flow of hot air in the furnace.

C. Stability of equipment

Even though we have obtained the optimal furnace temperature curve setting, achieving it still requires ensuring the stability, repeatability, and consistency of the equipment. Especially in lead-free production, if there is slight drift in the furnace temperature curve due to equipment reasons, it is easy to jump out of the process window and cause cold welding or damage to the original components. So, more and more manufacturers are beginning to demand stability testing for their equipment· The arrival of the lead-free era in the use of nitrogen has made it a hot topic of discussion whether to charge nitrogen in reflow soldering. Due to the lower fluidity, solderability, and wettability of lead-free solder compared to lead-based solder, especially when using OSP process (bare copper plate with organic protective film) for circuit board pads, the pads are prone to oxidation, often resulting in large wetting angles and exposed copper on the pads. In order to improve the quality of solder joints, we sometimes need to use nitrogen gas during reflow soldering. Nitrogen is an inert protective gas that can protect circuit board pads from oxidation during welding, significantly improving the solderability of lead-free solder. However, due to the improvement of solder quality and the cost of nitrogen, there are not many companies in the current market that use nitrogen welding! The nitrogen supply system of Lifeng is generally optional.

·Effective cooling device and flux management system

The welding temperature of lead-free production is significantly higher than that of lead-based production, which puts higher requirements on the cooling function of the equipment. In addition, a controllable faster cooling rate can make the lead-free solder joint structure denser, which helps improve the mechanical strength of the solder joint. Especially when we produce high heat capacity circuit boards such as communication backboards, if we only use air cooling, it will be difficult for the circuit board to achieve a cooling requirement of 5 degrees per second during cooling. If the cooling slope does not meet the requirements, it will cause the solder joint structure to loosen and directly affect the reliability of the solder joint.

Lead free solder paste often contains a large amount of flux, and the residue of the flux can easily accumulate inside the furnace to improve its heat transfer performance. Sometimes, it may even fall onto the circuit board inside the furnace, causing pollution. There are two ways to discharge residues during the production process

(1) Extraction and exhaust air

Ventilation is the simplest way to remove residual flux. However, as we mentioned earlier, excessive exhaust air can affect the stability of the hot air flow inside the furnace chamber. In addition, increasing the exhaust air volume will directly lead to an increase in energy consumption (including electricity and nitrogen consumption).

(2) Flux filtration system: Due to the direct pollution of flux emissions to the environment, the Li Feng M series, LF series, MCR, and ROHs series are all equipped with flux filtration systems.

(3) Multi level flux management system (forced cooling).

The flux management system generally includes a filtering device and a condensing device. The filtering device effectively separates and filters the solid particles in the flux residue, while the condensing device condenses the gaseous flux residue into a liquid state in a heat exchanger, and finally collects it in a collection tray for centralized processing.

(3) Requirements for equipment materials and construction

·Requirements for lead-free high-temperature equipment materials

Lead free production requires equipment to withstand a series of problems such as higher temperature chamber warping, track deformation, and poor sealing performance compared to lead based production. If there are problems with the materials used in the equipment, it will result in a furnace that will seriously affect production. Therefore, the tracks used in lead-free reflow soldering furnaces should undergo special treatments such as hardening to prevent damage and leakage after prolonged use

·Effectively prevent furnace cavity warping and track deformation

Moreover, the seam of the sheet metal should be confirmed by X-ray scanning to have cracks and the furnace chamber of the lead-free reflow soldering furnace should be made of a whole sheet metal. If the furnace chamber is made of small pieces of sheet metal spliced together, it is easy for the furnace chamber to warp under lead-free high temperature.

It is necessary to test the parallelism of the track under high and low temperature conditions. If the deformation of the track occurs under high temperature conditions due to the use of materials and design, the occurrence of jamming and falling boards will be unavoidable.

·Avoiding the generation of disturbed solder joints

The previous Sn63Pb37 lead solder was a eutectic alloy with the same melting and solidification temperatures, both at 183 ℃. The lead-free solder joints of snAgcu are not eutectic alloys, with a melting point range of 217 ℃ to 221 ℃. Temperatures below 217 ℃ are considered solid, while temperatures above 221 ℃ are considered liquid. When the temperature is between 217 ℃ and 221 ℃, the alloy exhibits an unstable state. When the solder joint is in this state, the mechanical vibration of the equipment can easily change the shape of the solder joint, causing disturbance to the solder joint, which is acceptable in electronic products! The PC-A-610D standard is an unacceptable defect. Therefore, the conveying system of lead-free reflow soldering equipment should have a good vibration free structural design to avoid the generation of disturbance to the solder joints.

(4) Requirements for reducing operating costs

·Sealing of furnace chamber

The warping of the furnace chamber and the leakage of equipment can directly cause a linear increase in the amount of electricity and nitrogen used. Therefore, the sealing of equipment is crucial for controlling production costs. Practice has shown that a small leak, even a leak hole the size of a screw hole, can increase nitrogen consumption from 15 cubic meters per hour to 40 cubic meters per hour.

·The thermal insulation performance of the equipment should not feel hot when touching the surface of the reflow soldering furnace (corresponding to the reflow zone) (the surface temperature should be below 50 degrees). If you feel hot, it indicates that the thermal insulation performance of the reflow soldering furnace is poor, and a large amount of electrical energy is converted into thermal energy, causing unnecessary energy waste. If in summer, the heat energy lost in the workshop will cause the workshop temperature to rise, and we have to use air conditioning devices to discharge this heat energy outdoors, which directly leads to double the energy waste.

·Extraction and exhaust air

If the equipment does not have a good flux management system, and the discharge of flux is entirely completed by suction and exhaust, then the equipment will also discharge heat and nitrogen while extracting residual flux, directly leading to an increase in energy consumption.

·Maintenance costs

Reflow soldering furnaces have extremely high production efficiency in large-scale continuous production, producing hundreds of mobile phone circuit boards per hour. If the maintenance interval of the furnace is short, the maintenance workload is large, and the maintenance time is long, it will inevitably occupy a lot of production time, causing waste of production efficiency. In order to reduce maintenance costs, lead-free reflow soldering equipment should adopt modular design as much as possible, providing convenience for equipment maintenance and repair

At present, many advanced electronic product manufacturers at home and abroad have proposed a new equipment maintenance concept called "synchronous maintenance" to further reduce the impact of maintenance on production efficiency. When the reflow soldering furnace is operating at full load, the automatic maintenance and switching system of the equipment is utilized to ensure that the maintenance and upkeep of the reflow soldering furnace can be completely synchronized with production. This design completely abandons the original concept of "shutdown maintenance" and further improves the production efficiency of the entire SMT line.

(5) Requirements for process implementation

High quality equipment can only generate benefits through professional use. At present, many manufacturers encounter many problems in the production process of lead-free welding, which are not only from the equipment itself, but also need to be solved through process adjustments.

The future development of reflow soldering technology

The requirements for reflow soldering are different between mobile phone products and military products, and the requirements for reflow soldering are also different between circuit board production and semiconductor production. The production of small varieties and large quantities is gradually decreasing, and the differences in equipment requirements for different products are becoming increasingly apparent. The difference in future reflow soldering will not only be reflected in the number of temperature zones and the selection of nitrogen. The reflow soldering market will continue to be segmented according to different product requirements, which is the foreseeable development direction of reflow soldering technology in the future. Welding technology plays an extremely important role in the assembly of electronic products. Generally, welding can be divided into two categories: one is mainly suitable for the welding of through-hole plug-in electronic components and printed circuit boards - wave soldering. Wave soldering refers to the process of melting soft solder material, which is sprayed by an electric pump and electromagnetic pump to form the required solder wave peak, allowing the printed circuit board pre installed with electronic components to pass through the solder wave peak, achieving mechanical and electrical connection between the soldering end or pin of the component and the printed circuit board pad through soft soldering; Another type is mainly suitable for reflow soldering between surface mounted components and printed circuit boards. Reflow soldering, also known as reflow soldering, refers to the process of remelting the pre distributed paste like soft zinc solder on the printed circuit board pads to achieve mechanical and electrical connections between the solder ends or pins of surface mounted components and the printed circuit pads, thereby achieving circuit functions with certain reliability. With the widespread use of surface mount components in electronic products, reflow soldering technology has become the main process technology in surface mount technology. Its main process feature is to use flux to purify the surface of the metal to be welded (remove oxides), making it have good wettability to the solder; Supply molten solder to wet the metal surface; Forming intermetallic compounds between solder and welding metal; Additionally, micro welding can be achieved.

Reflow soldering is a group or point by point soldering process in which an appropriate amount and form of solder is applied to the PCB soldering area (pad) in advance, and then surface mounted components are placed. External heat sources are used to reflow the solder to meet the soldering requirements. Compared with wave soldering, reflow soldering has the following characteristics:

(1) Reflow soldering does not require direct immersion of components in molten solder like peak soldering, so the components are less susceptible to thermal shock;

(2) Reflow soldering only applies solder to the required areas, greatly saving the use of solder;

(3) Reflow soldering can control the amount of solder applied and avoid defects such as bridging;

(4) When there is a certain deviation in the placement position of the component, due to the surface tension of the molten solder, as long as the solder is placed in the correct position

Correctly set, reflow soldering can automatically correct this small deviation during soldering, fixing the components in the correct position;

(5) Local heating sources can be used, so that different reflow soldering processes can be used for welding on the same basis;

(6) Impurities are generally not mixed in the solder, and the composition of the solder can be correctly maintained when using solder paste for reflow soldering.

6. Reflow soldering techniques are classified according to heating methods, including gas-phase reflow soldering, infrared reflow soldering, infrared hot air reflow soldering, laser reflow soldering, hot air reflow soldering, and tool heating reflow soldering.

Reflow soldering principle and temperature curve:

Analyzing the principle of reflow soldering from the temperature curve (see Figure 1): When the PCB enters the heating zone (drying zone), the solvent and gas in the solder paste evaporate, while the flux in the solder paste wets the solder pads, component terminals, and pins. The solder bends, collapses, and covers the solder pads, isolating them from oxygen; When the PCB enters the insulation area, sufficient preheating is provided for the PCB and components to prevent the PCB from suddenly entering the welding area and heating up too quickly, which may damage the PCB and components; When the PCB enters the soldering area, the temperature rapidly rises, causing the solder paste to reach a molten state. Liquid solder wets, diffuses, diffuses, or mixes with the solder pads, component terminals, and pins of the PCB to form solder joints; The PCB enters the cooling zone to fix the solder joints and complete the entire reflow soldering.

The temperature curve is the key to ensuring welding quality, and the actual temperature curve and the solder paste temperature curve should have similar heating slopes and peak temperatures. The heating rate before 160 ℃ should be controlled between 1 ℃/s and 2 ℃/s. If the heating slope rate is too fast, on the one hand, it will heat the components and PCB too quickly, which is easy to damage the components and cause PCB deformation; On the other hand, the solvent in solder paste evaporates too quickly, which can easily splash out metal components and produce solder balls. The peak temperature is generally set at around 20 ℃ to 40 ℃ higher than the melting temperature of the solder paste (for example, the melting point of Sn63/Pb37 solder paste is 183 ° C, and the peak temperature should be set at around 205 ℃ to 230 ℃), with a reflow time of 10 seconds to 60 seconds. If the peak temperature is low or the reflow time is short, it will cause insufficient welding, and in severe cases, it will cause the solder paste to not melt; Excessive peak temperature or long reflow time can cause metal powder oxidation, affect welding quality, and even damage components and PCBs.

According to the temperature curve and reflow principle of reflow soldering, the current reflow soldering machines on the market are generally simple four temperature zone reflow soldering machines, and there are also large six, eight, or even twelve temperature zone reflow soldering machines. The QHL320A reflow soldering machine adopts 20 programmable temperature control, which is equivalent to a 20 temperature zone reflow soldering machine. This subdivides the reflow temperature curve, thereby controlling the temperature more accurately and fitting the ideal reflow temperature curve, achieving perfect soldering.

How to ensure good welding quality? The QHL320A reflow soldering machine not only fully conforms to the temperature curve of reflow soldering in control, but also allows users to truly understand the principle of reflow soldering. The QHL320A reflow welding machine has the function of a large-sized transparent window, which allows users to control the entire welding process through the transparent window. At the same time, they can observe the changes in the solder paste throughout the welding process, making it easy to identify problems that may occur during the welding process. By adjusting the parameters to improve them, good welding quality can be achieved. At the same time, QHL320A reflow welding machine is a small desktop reflow welding machine that adopts fully static welding, effectively preventing the small vibration generated by the tracked transmission of large multi temperature zone reflow welding machines. This vibration may have an impact on the welding of small spacing ICs (such as spacing<0.5mm) and components (such as 0603, 0402, and 0201) under the dynamic state of melting solder paste in the welding area, leading to component drift, solder beads, solder bridges, and other welding defects, However, fully static welding completely avoids the possible defects mentioned above.

Basis for setting the temperature curve of reflow soldering:

(1) Design according to the temperature curve of the solder paste used. Solder paste with different metal contents has different temperature curves, and the specific product's reflow soldering temperature curve should be set according to the temperature curve provided by the solder paste manufacturer;

(2) Based on the material, thickness, multi-layer board, size, etc. of the PCB;

(3) Set according to the density of components carried on the surface assembly board, the size of components, and the presence or absence of special components such as BGA and CSP.

(4) Set according to the specific situation of the equipment, such as the length of the heating zone, the material of the heating source, the structure of the reflow soldering furnace, and the heat conduction mode.

7. There is a significant difference between hot air reflow soldering machines and infrared reflow soldering machines: infrared reflow soldering machines mainly rely on radiation conduction, with the advantages of high thermal efficiency, large temperature gradients, and easy control of the upper and lower temperatures of the PCB during double-sided soldering; Its disadvantage is uneven temperature. Due to differences in color, material, and size of the devices on the same PCB, the temperature varies. In order to achieve the welding temperature of large volume components with solder joints around dark colored devices, it is necessary to increase the welding temperature. The hot air reflow welding machine mainly relies on convective conduction, and its advantages are uniform temperature and good welding quality; The disadvantage is that the temperature difference between the upper and lower parts of the PCB, as well as the temperature gradient along the length direction of the welding furnace, are difficult to control.