Electronic factory reflow soldering process technology analysis!

Soldering is an important process in the assembly of electronic boards, and if it is not well mastered, there will not only be many "temporary failures", but also directly affect the life of the solder joint.

Reflow soldering is well known in electronics manufacturing, and the components on various PCBA boards in our smartphones are soldered to the circuit board through this process. SMT reflow soldering is a soldering method that creates a solder joint by melting a pre-placed solder face, without adding any additional solder during the soldering process. It uses a heating circuit inside the device to heat air or nitrogen to a high enough temperature and blow it to the circuit board where the component has been attached, so that the solder paste solder on both sides of the component melts and bonds to the motherboard. The advantage of this process is that the temperature is easy to control, oxidation during the welding process is avoided, and the manufacturing cost is more controllable.

Reflow soldering has become the mainstream process of SMT, most of the components on our commonly used smart phone boards are soldered to the circuit board through this process, which relies on the effect of hot air flow on the solder joint, and the gelatinous flux undergoes a physical reaction under a certain high temperature gas flow to achieve SMD soldering; The reason why it is called "reflow soldering" is because the gas circulates in the welder to produce high temperatures for the purpose of welding.

Reflow soldering equipment is the key equipment in the SMT assembly process, and the quality of the solder joints of PCBA soldering completely depends on the performance of the reflow soldering equipment and the setting of the temperature profile.

Reflow soldering technology has undergone different development forms such as plate radiant heating, quartz infrared tube heating, infrared hot air heating, forced hot air heating, forced hot air heating and nitrogen protection.

Reflow soldering temperature curve SAC305 lead-free solder paste reflow soldering temperature curve

The increasing requirements of the reflow soldering cooling process have also promoted the development of cooling zones for reflow soldering equipment, which have evolved from room temperature natural cooling and air cooling to water cooling systems designed to accommodate lead-free soldering.

Due to the improvement of the production process requirements for temperature control accuracy, temperature uniformity in temperature zones, transmission speed, etc., reflow soldering equipment has developed different welding systems such as five temperature zones, six temperature zones, seven temperature zones, eight temperature zones, and ten temperature zones from the initial three temperature zones.

High-quality SMT reflow soldering pays more attention to energy saving and environmental protection.

Due to the increasing need for miniaturization of electronic products, chip components have emerged, and traditional soldering methods can no longer meet the demand. The reflow soldering process was first applied in the assembly of hybrid integrated circuits, and most of the components assembled and welded at that time were chip capacitors, chip inductors, mount transistors, and diodes. With the development of SMT technology, a variety of SMC components (SMC) and mounting devices (SMD) appeared, as part of the placement technology of reflow soldering technology and equipment have also been developed correspondingly, its application is increasingly widespread, almost in all electronic product fields have been applied, and reflow soldering technology around the improvement of equipment has also experienced the following stages of development.

✔ ❶ Hot plate, push plate type conduction reflow soldering This type of reflow soldering furnace relies on the heat source heating under the conveyor belt or push plate, and heats the components on the substrate through heat conduction, which is used for single-sided assembly of thick film circuits using ceramic (Al2O3) substrates, and the ceramic substrate can only obtain enough heat when placed on the conveyor belt, and its structure is simple and cheap. Some thick film circuit factories in China have introduced such equipment in the early 80s.

✔ ❷ Infrared radiation reflow soldering This kind of reflow soldering furnace is also mostly conveyor belt type, the conveyor belt only plays the role of supporting and transporting the substrate, and its heating method mainly relies on the infrared heat source to heat in a radiant manner, and the temperature in the furnace is more uniform than the previous way, and the mesh is larger, which is suitable for reflow welding heating of the substrate assembled on both sides. This kind of reflow oven can be said to be the basic type of reflow oven, which is widely used in China and the price is relatively cheap.

✔ ❸ Infrared heating air reflow soldering This type of reflow soldering furnace is based on the IR furnace with hot air to make the temperature in the furnace more uniform. When infrared radiation is used for heating alone, it is found that in the same heating environment, different materials and colors absorb different heat, that is, the Q value in (1) formula is different, and the temperature rise ΔT caused by this is also different, for example, the package of SMD such as IC is black phenolic or epoxy, and the lead is white metal, and when it is simply heated, the temperature of the lead is lower than that of its black SMD body. In addition to the hot air, the temperature can be more uniform, overcoming the difference in heat absorption and poor shading, and the reflow oven of IR + Hot air has been widely used internationally.

✔ ❹ Nitrogen reflow soldering With the increase of assembly density and the emergence of fine pitch assembly technology, the nitrogen-filled reflow soldering process and equipment have been produced, which has improved the quality and yield of reflow soldering, and has become the development direction of reflow soldering. Nitrogen reflow soldering has the following advantages: (1) Prevent and reduce oxidation; (2) Improve the welding wetting force and speed up the wetting speed; (3) Reduce the generation of solder balls, avoid bridging, and obtain better soldering quality. It is especially important to obtain better solder quality, as the use of solder pastes with lower reactive fluxes also improves solder joint performance and reduces substrate discoloration, but the disadvantage is that the cost increases significantly, and the cost increases with the amount of nitrogen used, and the nitrogen demand varies greatly when the furnace is to reach 1000 ppm oxygen content and 50 ppm oxygen content. Today's solder paste manufacturers are working on developing no-clean solder pastes that solder well in atmospheres with high oxygen content to reduce nitrogen consumption. For the introduction of nitrogen in reflow soldering, a cost-benefit analysis must be carried out, which includes product yield, quality improvement, reduced rework or repair costs, etc., and a complete and accurate analysis often shows that the introduction of nitrogen does not increase the final cost, but on the contrary, we benefit from it. Most of the furnaces currently in use are of the forced hot air circulation type, and controlling nitrogen consumption in such furnaces is not an easy task. There are several ways to reduce nitrogen consumption, reducing the opening area of the furnace inlet and outlet, and it is important to use partitions, roller blinds or similar devices to block the unused inlet and outlet space; Another way is to use the principle that the hot nitrogen layer is lighter than air and not easy to mix, and design the furnace so that the heating chamber is higher than the inlet and outlet, so that a natural nitrogen layer is formed in the heating chamber, reducing the amount of nitrogen compensation and maintaining the required purity.

✔ ❺ Double-sided reflow soldering Double-sided PCBs have become quite popular and becoming increasingly complex, and the main reason for their popularity is that they provide designers with a great deal of flexibility to design smaller, more compact, and more low-cost products. Until now, double-sided boards have been reflowed soldered to the top (component face) and then wave soldered to the bottom (pin face). At present, there is a trend of double-sided reflow soldering, but there are still some problems with this process. The bottom component of the large plate may fall off during the second reflow soldering process, or the bottom solder joint may partially melt and cause solder joint reliability issues. Several methods have been found to achieve double-sided reflow soldering: one is to paste the first-sided component with glue, so that when it is turned over and enters the reflow soldering for the second time, the component will be fixed in place without falling, this method is commonly used, but requires additional equipment and operation steps, which increases the cost; The second is to apply solder alloys with different melting points, using an alloy with a higher melting point when doing the first side and an alloy with a low melting point when doing the second side, the problem with this method is that the choice of low melting point alloy may be limited by the operating temperature of the final product, while the high melting point alloy will inevitably increase the reflow temperature, which may cause damage to the component and the PCB itself. For most components, the surface tension of the splice is sufficient to grip the bottom component to create a high-reliability solder joint, and the ratio of component weight to pin area is a measure of the success of this soldering, typically using the 30g/in² standard in the design; The third method is to blow cold air at the bottom of the furnace, which can maintain the temperature of the solder joint at the bottom of the PCB below the melting point in the second reflow soldering, but the potential problem is that the internal stress is generated due to the temperature difference between the upper and lower levels, and effective means and processes are needed to eliminate the stress and improve the reliability. These process issues are not simple, but they are being successfully solved. There is no doubt that double-sided panels will grow a lot in terms of number and complexity in the coming years.

✔ ❻ Through-hole reflow soldering Through-hole reflow, sometimes referred to as classified component reflow, is gradually emerging. It eliminates the need for wave soldering and becomes a process link in PCB mixing technology. The biggest advantage is that it can take advantage of the advantages of the surface mount manufacturing process while using through-hole inserts to achieve better mechanical connection strength. For larger PCBs, the flatness does not allow all surface-mount components to be in contact with the pads, and even if the pins and pads can be contacted, the mechanical strength they provide is often not large enough to easily become a point of failure during product use. Despite the many benefits of through-hole reflow, there are several drawbacks in practical applications, such as the high amount of solder paste, which increases the level of contamination of the machine due to flux evaporation cooling, and the need for an effective flux residue removal device. In addition, many connectors are not designed to withstand reflow temperatures, and early furnaces based on direct infrared heating are no longer suitable, lacking the ability to efficiently transfer heat to handle typical surface-mount components on the same PCB as through-hole connectors with complex geometries. Only a high-volume, high-efficiency forced convection furnace is possible to reflow through the hole, and this has been proven in practice, the remaining question is how to ensure that the solder paste in the through hole has a suitable reflow temperature profile with the component feet. With the improvement of processes and components, through-hole reflow soldering will be more and more used.

In the traditional electronic assembly process, wave soldering technology is generally used for the welding of printed board assemblies with through-hole plug-in components (THDs). However, wave soldering has many disadvantages: it is not suitable for high-density, fine-pitch component soldering; There are many bridging and missing welds; Fluxneeds to be sprayed; The printed board is warped and deformed by large thermal shock. Therefore, wave soldering cannot adapt to the development of electronic assembly technology in many aspects. In order to adapt to the development of surface assembly technology, the measure to solve the above welding difficulties is to use through-hole reflow soldering technology (THR, through-hole Reflow), also known as through-hole reflow soldering PIHR (pin-in-holeReflow). The principle of this technology is that after the printed board is mounted, a special template with many needle tubes is used, the position of the template is adjusted to align the needle tube with the via pad of the insertion component, the solder paste on the template is leaked onto the pad with a scraper, and then the insertion component is installed, and finally the insertion component and the chip component are soldered through reflow soldering at the same time. In double-sided reflow soldering, although there are some process problems at present, these problems are gradually being solved with the development of technology. For example, more efficient and lower-cost adhesives are being developed for gluing first-side components, and ways to simplify additional equipment and operating steps are being explored; For methods of applying solder alloys with different melting points, scientists are looking for more kinds of solder alloys to meet the requirements of different operating temperatures, while minimizing damage to components and PCBs; For the method of blowing cold air at the bottom of the furnace, research is being carried out on how to better control the internal stresses caused by the temperature difference between the upper and lower levels, such as improving the reliability of the solder joint by optimizing the flow rate and direction of the cold air and working in tandem with the heating system. It can be seen that the advantages of perforated reflow soldering compared with traditional processes: first of all, it reduces the process, eliminates the wave soldering process, saves costs, and at the same time reduces the required staff and improves efficiency; Second, reflow soldering is much less likely to produce bridging than wave soldering, resulting in an improved first-pass rate. Piercing reflow soldering has great advantages over traditional processes in terms of economy and advancement.

Through-hole reflow soldering technology originated from the Japanese company SONY and began to be applied in the early 90s of the 20th century, but it is mainly used in SONY's own products, such as TV tuners and CD Walkman. China introduced the technology from Japan in the mid-90s of the 20th century, when the domestic Wuxi Radio No. 6 Factory, Shanghai Jinling Radio Factory, Chengdu 8800 Factory, Chongqing Testing Instrument Factory, Shenzhen Dongguan Tuner Factory and other tuner manufacturers applied the technology to obtain good benefits, and is currently widely used in CD, DVD laser movement servo board and DVD-ROM servo board, notebook computer motherboard and other fields. (1) High reliability, good welding quality, defective ratio DPPM can be less than 20; (2) There are few welding defects such as virtual welding and bridging, and the workload of plate repair is reduced; (3) The PCB surface is clean, and the appearance is obviously better than wave soldering; (4) Simplify the process. Since the dot (or printing) SMT process, wave soldering process, and cleaning process are omitted, the operation and management are simplified. Because the less materials and equipment used in the same product, the easier it is to manage, and the operation of the reflow oven is much simpler than that of the wave soldering machine, there is no welding slag problem, and the labor intensity is low; (5) Reduce costs and increase benefits. With this process, wave soldering equipment and cleaning equipment, wave soldering and cleaning plants, wave soldering and cleaning crews, and a large number of wave soldering materials and cleaning agent materials are eliminated. Although the price of no-clean solder paste is slightly higher than that of non-no-clean solder paste, it can greatly reduce costs and increase efficiency overall.

Through-hole reflow soldering production process: The production process is very similar to the SMT process, that is, printing solder paste - inserting components - reflow soldering, whether it is a single-sided mixed board or a double-sided mixed board, the process is the same.

Solder paste printing process: The selection of solder paste, the solder paste used for through-hole reflow has low viscosity and good fluidity, which is convenient to flow into the through-hole. If the solder paste alloy composition used in SMT is 63Sn37Pb, then in order to ensure that the SMT component will not melt again and fall off when the through hole is reflowed, the solder alloy composition in the solderpaste can be 46Sn46Pb8Bi (178°C) with a slightly lower melting point, and the solder particle size is less than 25μm< 10%, 25 - 50μm >89%, and above 50μm <1%.

Due to the increasing emphasis on miniaturization and multi-functionality of electronic products, the density of components on circuit boards is increasing, and many single-sided and double-sided boards are dominated by surface-mount components. However, due to factors such as connection strength, reliability, and suitability, some through-hole components are still not available in chips, especially peripheral connectors. In the traditional SMT mixed assembly process, most of the through-hole insertion components use wave soldering, selective wave soldering, soldering robots, and manual soldering, and the quality of these traditional methods, especially wave soldering and manual soldering, is far inferior to the quality of reflow soldering; At present, the proportion of through-hole components in many electronic products only accounts for 10% - 5% or even less of the total number of components, and the assembly cost of wave soldering, selective wave soldering, automatic soldering robots, manual soldering and crimping methods far exceeds this proportion, and the cost of a single solder joint is very high. Therefore, the reflow soldering technology of through-hole components is becoming more and more popular, and the reflow soldering of through-hole insertion components instead of wave soldering (i.e., pure reflow soldering process) has become one of the current development trends of SMT process technology.

With the continuous development of electronic technology, reflow soldering technology is also constantly innovating and improving. For example, in lead-free reflow soldering, despite the challenges of oxidation and PCB damage due to higher melting point temperatures, it has also led to the development of ancillary technologies, such as nitrogen protection technology to better cope with the problems caused by increased temperatures. At the same time, furnace design improvements are not limited to increasing the operating temperature, but also include protective measures for heat-sensitive components, such as the use of special thermal insulation materials or the improvement of the heat dissipation structure of the components.

✔ ❼Lead-free reflow soldering For environmental reasons, lead will be strictly restricted in the 21st century. Although the use of lead in the electronics industry is extremely small, less than 1% of the total use, it is also banned and will be phased out in the coming years. Reliable and economical lead-free solders are now being developed. Many of the alternatives that have been developed today generally have a melting point temperature of around 40°C higher than that of tin-lead alloys, which means that reflow soldering must be performed at higher temperatures. Nitrogen protection can partially eliminate the increased oxidation due to increased temperature and damage to the PCB itself. However, the industry may have to go through a painful learning period to solve the problems encountered, and the time is running out to apply the process as soon as possible, many of the furnaces used today are designed to operate at temperatures up to 3000°C, and for lead-free solder or non-soluble spot solder (for BGA, double-sided boards, etc.), higher furnace temperatures are required, these new processes usually require temperatures of 3500°C - 4000°C in the reflow zone, and the furnace design must be changed to meet the requirements. Heat-sensitive parts in the machine must be modified, or measures must be taken to prevent heat transfer to these parts. FOR VERTICAL VACUUM REFLOW SOLDERING, IT IS EXPECTED TO BE USED IN MORE HIGH-END ELECTRONIC PRODUCTS IN THE FUTURE DUE TO ITS ADVANTAGES IN HANDLING CSPS SUCH AS FLIP CHIP. In addition to its applications in chip packaging, researchers are also exploring its potential for soldering other miniaturized electronic components, such as tiny sensors or high-frequency RF components, where vertical vacuum reflow soldering may be an ideal soldering technique due to its small size and high requirements for soldering quality and reliability. In addition, with the continuous improvement of vertical oven technology, how to further improve its curing efficiency and reduce energy consumption has also become the focus of research.

✔ ❽ Continuous Flex Board Reflow Soldering Special furnaces have been developed to handle continuous flex boards with SMT components attached. The biggest difference from ordinary reflow ovens is that this type of furnace requires a special track to transfer the flexible plates, and of course, this kind of furnace also needs to be able to handle the problem of continuous plates. For the separated PCB board, the flow rate in the furnace has no dependence on the condition of the first few stations, but for the continuous flexible board in rolls, the flexible board is continuous in the whole production line, and the pause caused by any special problem on the line means that the whole line must be stopped, so that a special problem arises, the part that is stopped in the furnace will be damaged due to overheating, therefore, such a furnace must have the ability to deal with random stops, continue to process the flexible board, and return to normal working state when the whole line resumes continuous operation.

✔ ❾ Vertical Vacuum Reflow Soldering The need for smaller volumes has led to more applications for CSPs such as FLIP CHIP, which result in a smaller footprint and higher signal transfer rate after component placement. Filler or glue is used to strengthen the structure of the solder joint so that it can withstand the stress caused by the inconsistent coefficient of thermal expansion between the silicon wafer and the PCB material, and the wafer is usually glued up by the drip or enclosure method. Many of these encapsulants require long curing times, which is not practical for in-line furnaces, and batch ovens are often used, but vertical ovens have been shown to be successful in the curing process and have a simpler temperature profile than regular reflow ovens, which use a PCB transport system to act as a buffer/accumulation zone, thus extending the residence time of the PCB board in the oven with a small footprint.

The market's demand for product miniaturization has led to the increasing application of reverse solder lugs and DCA (flip chip technology). In this technology, the chip is upside down and soldered directly to the substrate with a solder ball, which increases the signal transmission speed and reduces the size. The other is the underfill process, in which the filler material is poured into the gap between the chip and the substrate because the coefficient of expansion between the chip and the substrate material is inconsistent, and the filler material protects the solder joint from this stress. There are also spherical capping and dam filling, both of which are processes in which soldered bare dies are encapsulated with covering material. Underfill encapsulation materials were originally used to improve the reliability of flip chips on early alumina (Al2O3) substrates. The solder joints at the outermost part of the chip are prone to fatigue and cause the chip to fail, and the relatively small difference in thermal expansion between the silicon wafer and the substrate is the root cause of the problem when the chip is subjected to thermal cycling, and the temperature range and number of cycles of the thermal cycle determine the service life of the chip. Filling the chip and substrate with curable encapsulating material is a good way to distribute the stresses that are concentrated around the solder joints due to the difference in thermal expansion to the entire die coverage

scope.

Almost all of these encapsulation materials require a long curing time, so it is not practical to use an online continuous production curing oven, usually we often use a "batch oven", but the technology of vertical oven also tends to be perfect, especially when the heating curve is simpler than that of a reflow oven, the vertical oven is fully competent. The vertical oven uses a vertically raised conveyor system as a "buffer and accumulator" through which each PCB must be cycled. The result is a sufficiently long curing time while reducing the footprint.

In the field of continuous flex board reflow soldering, in addition to special orbital transfer flex boards, researchers are exploring more intelligent monitoring and adaptive control technologies to prevent pause and overheating damage caused by special problems on the line. By setting up multiple sensors in the furnace, the operating status, temperature distribution and other parameters of the flexible board are monitored in real time, and once an abnormality is found, the control system can quickly adjust the heating power, transmission speed and other parameters to avoid damage to the flexible board. Moreover, in order to improve the efficiency and reliability of the entire production process, we are also studying how to optimize the production process of flexible boards and make them better connected with other production links.

In terms of through-hole reflow soldering, as its application becomes more and more extensive, the requirements for the solder paste printing process are also increasing. In addition to the existing solder paste selection criteria, new solder pastes may be developed in the future that are more suitable for through-hole reflow soldering, which not only have good flow and low viscosity, but also guarantee excellent soldering results in different through-hole structures and component types. In addition, in order to ensure that the solder paste in the through-hole has a more suitable reflow temperature profile with the component pins, more precise temperature control techniques and devices may be developed. For example, the intelligent temperature control system can automatically adjust the temperature profile according to different PCB board types, component layouts and soldering requirements, so as to improve the consistency of soldering quality.

In response to the strong demand for miniaturization of products, the processes related to DCA (flip chip technology) will continue to evolve in the market. For example, in the underfill process, the properties of the filler material are continuously optimized, not only to have good stress dispersion, but also to have a faster curing speed to increase production efficiency. At the same time, the spherical capping and dam filling technology will continue to improve in the selection of materials and packaging processes to meet higher packaging quality requirements and improve the reliability and service life of chips. The continuous development and improvement of these technologies will provide solid welding technology support for the miniaturization and high performance of electronic products in the future.