Application and detailed explanation of reflow soldering process

In-depth analysis and application discussion of reflow soldering process

In semiconductor manufacturing and electronics manufacturing, the reflow soldering process is a key technology that is widely used. Inside the computers we use every day, the components on various boards are securely soldered to the circuit board through this process. The core of the reflow soldering equipment lies in its internal heating circuit, which can heat air or nitrogen to a temperature high enough for the solder to melt, and then blow the hot air flow to the circuit board where the components have been pasted, so that the solder on both sides of the component is heated and melted, and then bonded tightly to the motherboard. This process has many advantages, such as precise temperature control, effective avoidance of component oxidation during the welding process, and relatively easy to control manufacturing costs.

Reflow soldering is an important part of the three main processes of SMT (surface mount technology), because its entire soldering process is invisible, and the whole process from entering the equipment to the final outflow is like a "black box", so it is vividly called the "black box" process in the industry. Unlike the visual operation of the paste solderprinting process and the component placement process, the invisibility of the reflow soldering process allows industry insiders to speculate on the specific situation of the welding process based on the welding results obtained after the furnace, and continuously optimize the process parameters to achieve the ideal welding effect. In order to better understand the reflow soldering process, the industry has specially developed a reflow soldering simulation system that can simulate the temperature changes of the product during the production process, allowing people to visually observe the welding process.

Not only that, in order to more accurately observe every detail of welding in actual production, the industry has also developed a high-temperature photography system. This system will follow the actual product into the reflow furnace and record all the changes that occur in the furnace as important data for later observation and analysis. In addition, in order to obtain ideal solder joints, the industry has also developed a series of special equipment such as vacuum reflow soldering, vacuum vapor phase welding, vacuum furnace, pressure furnace, three-dimensional furnace, tunnel furnace, etc., let's take a closer look at it next.

1. The working principle of traditional reflow soldering

Reflow process, written as Reflow process in English, literally translates to reflow soldering process or reflow soldering process. The basic working principle is that the PCB (printed circuit board), the printed solder paste and the mounted components enter the reflow oven together, and slowly enter the furnace chamber under the smooth drive of the track. In the early days, it was mainly heated by infrared radiation, but now it mainly relies on hot air blowing into the furnace chamber to evenly heat the PCB, solder paste and components.

The hot air blowing into the furnace is generated in this way: the blower blows out the air driven by the motor, and the air passes through the heater and is heated into hot air, and the temperature of the hot air is set by the process personnel according to actual needs. The air outlet of the furnace is equipped with a temperature measurement line, which can sense the temperature of the hot air in real time and transmit the monitored results to the main control computer in time. The main control computer will automatically adjust the current power of the heating wire according to the received temperature information, so as to ensure that the hot air temperature reaches the preset ideal value.

When hot air is blown into the furnace, it transfers heat to components, PCBs, and solder paste. As the product moves forward in orbit, its temperature gradually rises, eventually reaching the melting point of the solder paste. At this point, the solder powder particles in the paste melt with heat and sufficiently wet the solder face of the component and the pad of the PCB, forming a strong solder joint. After that, the product will enter the cooling area, and cold air will be blown into the furnace to cool the product, and after the solder is cured, the product will be sent out of the furnace, and the whole welding process is completed.

It is worth mentioning that the hot air entering the furnace will reduce the temperature after transferring heat to the product, and the cooled air will be recycled, sent to the heating wire for heating, and then sent to the furnace chamber to continue to participate in the work after heating to the set temperature, forming a cycle. We record the temperature trajectory of any point on the product over time to obtain the temperature curve.

Inside the reflow oven, the track is located in the center of the upper and lower heating zones, and the upper and lower heating zones are called a warm zone. For example, a reflow oven in the ten-temperature zone means it has ten pairs of heating units. It should be noted that the number of cooling zones of the reflow oven is generally calculated separately and is not included in the number of temperature zones.

2. Key points of performance evaluation of reflow soldering

There are many factors that factories need to consider when purchasing a reflow oven, and the following elements usually need to be taken into account and used as the standard for equipment acceptance.

1. Nitrogen furnace and air furnace

The hot air used in reflow soldering can be either ordinary air or nitrogen to replace air, and the reflow oven using nitrogen as the hot air medium is called a nitrogen furnace. The nitrogen furnace is equipped with a special pipe inside the equipment to supply nitrogen to the blower in each heating area, and the nitrogen is heated by the heating wire and sent into the furnace chamber to heat the product. Since nitrogen is an inert gas, it can effectively protect components, PCBs, andpaste solder from oxidation in high-temperature environments, which is conducive to the better wetting of the soldered end face.

Nitrogen furnaces can be divided into two types: full nitrogen filling and partial nitrogen filling. The whole process of nitrogen filling means that all heating temperature zones in the furnace chamber are filled with nitrogen, so as to form a comprehensive and good protection of the welding end face. Local nitrogen filling is only filled with nitrogen in the solder melting area, and air is still used as the heating medium in other heating temperature zones. For example, a reflow furnace with a ten-temperature zone, the eighth, ninth and tenth zones are the melting and welding zones, filled with nitrogen as the heating medium, and the first to seventh zones use air as the heating medium, such a reflow oven is called a local nitrogen-filled reflow oven. If the first to tenth zones of the furnace all use nitrogen as the heating medium, then it is a full nitrogen-filled reflow oven.

Because the nitrogen furnace uses nitrogen as the heating medium, it can prevent oxidation of the soldering interface at high temperature, which is conducive to solder wetting, so the solder joints obtained are usually relatively bright. HDI (high-density interconnect) board products are generally welded with nitrogen furnaces to ensure the quality of soldering due to their small component size, high density, and low amount of paste solder. Ordinary products can be welded using an air furnace. However, the nitrogen consumption of the nitrogen furnace is part of the daily operating cost of the factory, and enterprises can choose a nitrogen furnace or air furnace with full nitrogen filling and local nitrogen filling according to the actual needs of the product. When evaluating and accepting equipment, it is necessary to calculate its nitrogen consumption under the same oxygen content in the control furnace. Nitrogen consumption is closely related to the height of the furnace and the tightness of the furnace, and enterprises must strictly verify this indicator during equipment acceptance. It should be pointed out that the position of the oxygen content detection head in the furnace should be installed at the air outlet, and if it is installed at the position of the nitrogen inlet, it is a tricky behavior, and such equipment is unqualified.

2. Cooling method: water cooling and air cooling

The cooling method of the reflow oven directly determines the cooling capacity of the equipment. For products such as large plates, thick plates, multi-layer plates and large heat capacity plates, it is difficult for ordinary air cooling methods to make the products reach a qualified cooling slope.

When evaluating the cooling capacity of equipment, enterprises generally use standard aluminum substrates for temperature measurement, and determine whether the cooling capacity of the equipment meets the standards through the measurement results. The industry requires a cooling slope greater than 2°C/sec. However, there are some reflow ovens on the market that are water-cooled structures but still cannot achieve the ideal cooling effect, which involves some other factors.

3. Furnace height

The furnace height of the reflow oven determines the maximum element height that the equipment can accommodate. If the height of the furnace chamber is insufficient, those ultra-high elements will get stuck in the furnace chamber and cannot pass normally, thus affecting the smooth progress of production. In today's industry, the through-hole reflow process has become a basic process, but some cartridge connectors cannot pass through the reflow oven due to their high height, which has become a production pain point for some enterprises. Products that could have been processed through pure SMT production have to add wave soldering processes due to the capacity of reflow oven equipment, which not only lengthens the production process, reduces production efficiency, increases operating costs, but also leads to a decrease in the competitiveness of enterprises.

4. Track width

The track width of the reflow oven determines the maximum width of the product that the enterprise can produce. Generally speaking, the maximum width of the reflow oven can meet the production needs of ordinary products, but for large products such as servers and 5G base stations, the equipment needs to have a large track width, and enterprises need to fully consider this factor when purchasing equipment.

5. Double track, single track and multi-track

Single-rail, double-track and multi-track equipment is one of the important means for factories to improve production efficiency. In order to increase output per unit plant area, enterprises usually choose to use dual or multi-track equipment. In order to save equipment investment, some small and micro enterprises can purchase a double-track asynchronous reflow oven, so that one reflow oven can meet the normal production needs of two production lines.

6. Maximum temperature difference in adjacent temperature zones

The temperature difference between the heating zone and the average temperature zone does not exceed 30°C, while the maximum temperature difference in the melting zone can be set to 50°C. It should be noted that the ability to set the maximum temperature difference in adjacent temperature zones and the anti-cross-temperature ability of the equipment need to be jointly evaluated.

7. Cross temperature in adjacent temperature zones

When evaluating the cross-temperature characteristics of equipment, the temperature difference between adjacent temperature zones is generally set to the limit value of the equipment. For example, the temperature difference between zone 2 and zone 3 is set to 30°C, and the temperature difference between zone 8 and zone 9 is set to 50°C, and then the temperature curve is measured with a suspension line to determine whether there is a cross-temperature phenomenon in the adjacent temperature zones by observing the temperature curve.

The so-called cross-temperature phenomenon refers to the fact that there are temperatures that should not be in the adjacent temperature range and affect each other. For example, if the temperature of one zone is set to 170°C and the next zone is set to 210°C, if the furnace has poor anti-cross-temperature ability, then the actual temperature of the temperature zone of 170°C may become 190°C, and the actual temperature of the temperature zone of 210°C may become 200°C. This phenomenon is essentially caused by poor hot air uniformity of the equipment and insufficient capacity of the return air system.

8. The temperature difference between the fixed edge, the movable side and the middle

The temperature difference between the fixed, middle and movable sides is determined by the thermal insulation capacity of the equipment. The better the thermal insulation of the equipment, the smaller the temperature difference between the three; Conversely, the temperature difference is larger. This is also a manifestation of equipment design ability, excellent equipment will fully consider the temperature difference caused by the heat dissipation of the furnace wall on the fixed side and the movable side, while the general equipment may ignore this detail.

When evaluating the temperature difference between the fixed edge, the movable side and the middle side of the equipment, a standard plate is generally used, and the temperature measurement points are arranged in the same row, and the temperature difference between the three is evaluated by measuring the temperature of these measurement points, usually requiring the temperature difference to be within 1°C.

9. No-load and full-load heat compensation capacity

The no-load and full load heat compensation capacity of the equipment is one of the key indicators of the equipment, which evaluates the difference between the temperature curve measured when the furnace is filled with products and the furnace chamber is empty in actual production, and its essence reflects the heat compensation capacity of the equipment.

When the actual temperature measurement is carried out by the enterprise, it is usually carried out before the start of production or when the production is transferred, when the furnace is in a clearance state; In the actual production of products, there will be a certain amount of product in the furnace, and the heat capacity is relatively large. If there is a large difference in the measured temperature between the two states, the actual production temperature will deviate from the preset welding conditions. The measurement of no-load and full load temperature is as follows: using the same measuring plate, 10 aluminum substrates or metal or synthetic stone carriers, and thicker PCBs, on the same furnace, keeping the temperature setting constant.

When the equipment heats up to the set temperature (all temperature zones reach the set temperature, and the equipment light is displayed in green), put in the temperature measuring plate and thermometer to measure the no-load temperature curve of the equipment; After the temperature plate is cooled to room temperature, 6 aluminum substrates are put in, then the thermometer and thermometer are put in, and then 4 aluminum substrates are followed to measure the full load temperature curve. Comparing the data differences between these two curves, if the temperature difference is within 2°C, it is qualified, and if the temperature difference exceeds 5°C, it means that the equipment is no longer capable of normal production.

10. Maximum set temperature

The maximum set temperature of the equipment is another key indicator of the equipment's capabilities. For example, some automotive electronic products and industrial control electronic products require the maximum setting temperature of the equipment to reach 500°C, but ordinary reflow soldering equipment cannot meet this production demand.

11. Maximum load capacity

For overweight products such as energy storage, inverters, 5G base stations, servers, etc., the carrying capacity of the equipment must be included in the evaluation scope. The author has seen the heaviest 5G product veneer (bare board) close to 10Kg, which is a big challenge for the transmission system of ordinary furnaces, so it must be considered when evaluating equipment.

12. Flux recovery capability

In the through-hole reflow process, flux recovery is a very important indicator. For example, in notebook computer manufacturers, if theflux recovery ability of reflow soldering is poor, it will lead to a large amount of flux remaining in the cooling area of the equipment and the first and second zones, and flux dripping will often pollute the product, which will not only increase the difficulty of equipment maintenance, but also affect the product quality and workshop environment (gas overflow in the furnace), so this indicator is also indispensable in the equipment evaluation process.

13. Hot air uniformity and flow rate

There is a special equipment on the market to determine the hot air uniformity of reflow soldering equipment, which determines the uniformity of heating and the stability of air flow, which is one of the very key indicators. Enterprises can use special equipment to measure the performance of internal equipment.

14. Return air system and return air volume

The return air system and return air volume of the equipment are the key factors to ensure the hot air volume in the furnace. Insufficient return air volume will cause gas overflow in the furnace; If the return air volume is too large, the hot air will flow too fast, which can easily cause displacement of small components and light components.

15. Heating accuracy

Heating accuracy refers to the amount of drift between the set temperature of the equipment and the actual temperature. For example, if the temperature of a certain temperature zone is set to 210°C, if the temperature of the equipment drifts within the range of 210°C±1°C, it means that the temperature control of the equipment is accurate. If it drifts within the range of 210°C±3°C, it means that the equipment capacity is normal and at the average level of the industry; If the drift range reaches 210°C±5°C, it indicates that the equipment is weak and enterprises need to be extra cautious when using it.

16. Furnace surface temperature

The surface temperature of the equipment is a direct reflection of the furnace's thermal insulation capabilities and one of the key indicators of equipment energy consumption. SMT workshops usually use air conditioners to consume electrical energy to maintain a constant temperature in the workshop, if the surface temperature of the equipment is too high, it means that the reflow soldering equipment is heating the workshop while consuming electrical energy, this internal friction will increase the operating costs of the enterprise, the most intuitive manifestation is the increase in electricity bills, and enterprises often may not be aware of this, this problem cannot be ignored. The basic control standard of the industry is that when the equipment is working normally, personnel should not feel obviously hot when touching the surface of the equipment (the highest temperature zone) with their hands.

17. Real-time monitoring system

The real-time monitoring system of reflow soldering equipment is the basic guarantee of the information factory and the standard configuration of automotive electronic product production. The system can record the working status of equipment every minute and second, provide a basis for quality tracking, and can also proactively notify production technicians to avoid production problems caused by drift in equipment performance indicators.

18. Orbital vibration

Track vibration directly affects the probability of double-sided drops. When measuring, the industry generally selects samples with a ratio of element weight to welding area of 0.8mN/mm², and allows them to pass through the furnace 10 times to observe whether there is a phenomenon of dropping parts, so as to evaluate the vibration of the equipment's tracks, and of course, professional tools can also be used to measure.

19. Chain speed uniformity

There are mature chain speed uniformity testers on the market, and the uniformity of the equipment chain rotation can also be monitored through a real-time monitoring system.

20. The self-cleaning ability of the chain

Evaluating the self-lubrication and self-cleaning capabilities of the equipment itself is an important indicator of concern in the production of high-demand electronic products such as automotive electronics. Many automotive electronic products require zero repair, the appearance of defective products will bring huge cost pressure to enterprises, and in the bad factors of automotive electronic products, foreign objects are the primary bad factors, so the self-cleaning and lubrication ability of the equipment chain is one of the key indicators to consider.

21. Number of temperature zones and furnace length

The more temperature zones the equipment, the more delicate the temperature curve can be adjusted, which is conducive to PCBA companies to optimize the temperature curve. The larger the furnace length, the faster the chain speed during production and the higher the production efficiency.

22. Orbital flow direction

The track flow needs to meet the overall flow planning of the production line, such as from left to right or from right to left.

23. Track deformation

The amount of track deformation is an important indicator for equipment evaluation. When the equipment works in a high-temperature environment, there are often bells and bellies during the track widening process. The bell-shaped track will increase the probability of dropping parts and plates during production, while the shape of the belly will cause abnormal plate drops. The track deformation of the equipment generally needs to be verified during acceptance, and the track deformation is usually required to be within 0.5mm, which can be measured by the one-plate method.

3. Development status and application of advanced technology of reflow soldering

Traditional nitrogen furnace and air furnace technology has been quite mature and widely used in the industry. However, with the increasing requirements for high reliability of products, it is becoming more and more difficult for traditional reflow ovens to meet the production needs of products, so the industry has developed other types of welding equipment to adapt to the development of the industry.

1. Vacuum reflow soldering and vacuum vapor phase welding

It is known that during the soldering process, when the solder melts, the gas that fails to escape inside the solder joint is encased in the solder and bubbles form. The presence of these bubbles can affect the strength of the solder joint, interfere with the transmission of high-frequency signals, and thus affect the reliability of the product. In order to obtain low-bubble solder joints, vacuum extraction is an effective means when the solder melts, and vacuum reflow soldering has come into being.

The working mechanism of vacuum reflow soldering is as follows: the equipment track is cut into three sections from one piece, taking the reflow oven in the ten temperature zone as an example, zones 1 to 3 are the heating zone, zones 4 to 7 are the uniform temperature zone, zones 8 and 9 are the solder melting zone, and zone 10 is the vacuum zone, which is used to extract the air bubbles in the solder joint, and then the product enters the cooling zone. Among them, the 1st to 9th temperature zone is the first track, the 10th zone is a separate track, and the cooling zone and the furnace part are the third track.