Reflow soldering principle and process

1.  What is Reflow Soldering Reflow soldering is a type of soldering that achieves a mechanical and electrical connection between the solder ends or pins of a surface mount component and the printed board pads by remelting a paste-like soldering solder pre-assigned to the printed board pads. Reflow soldering is used to solder components to a PCB board, primarily for surface-mount devices. Reflow soldering relies on the hot air flow to produce an action on the solder joint, and the gelatinous fluxundergoes a physical reaction under a certain high-temperature gas flow, so as to realize the soldering of SMD (surface mount devices). It is called "reflow soldering" because the gas circulates through the welder to produce high temperatures for the purpose of welding.

The principle of reflow soldering can be described in the following aspects:

A.When the PCB enters the heating zone, the solvent and gas in the solder paste begin to evaporate, and at the same time, thefluxin the solder paste will wet the pads, component ends and pins, and the solder paste will soften, collapse and cover the pads, isolating the pads and component pins from oxygen.

B.When the PCB enters the insulation zone, the PCB and components are fully preheated, which is to prevent the PCB from suddenly entering the soldering high temperature zone and damaging the PCB and components.

C.When the PCB enters the soldering area, the temperature rises rapidly, so that the solder paste reaches a melting state, and the liquid solder wets, diffuses, diffuses, or reflows the pads, component ends, and pins of the PCB, and then forms solder contacts. D. After the PCB enters the cooling zone, the solder joints solidify, and the reflow soldering is completed.

How Dual-Rail Reflow Soldering Works Dual-track reflow ovens are capable of processing two circuit boards in parallel at the same time, which can triple the capacity of a single dual-track furnace. Currently, board manufacturers can only handle the same or similar weight boards in each track. However, a dual-lane, two-speed reflow oven with independent track speeds now makes it possible to process two boards with much more variance at the same time. First, we need to understand the main factors that affect the transfer of heat energy from the reflow oven heater to the circuit board. Typically, as shown in the diagram, the fan of the reflow oven pushes the gas (air or nitrogen) through the heating coil, which is heated and passed to the product through a series of orifices within the orifice plate.

The transfer of heat energy from the airflow to the board can be described by the following equation: q = heat energy transferred to the board; a = convective heat transfer coefficient of the board and components; t = heating time of the board; A = heat transfer surface area; ΔT = temperature difference between the convective gas and the board. Move the board-related parameters to one side of the equation and the reflow oven parameters to the other side, and the formula is obtained: q = a | t | A | | T。

Dual-track reflow soldering PCBs have become quite popular and are gradually becoming more complex. It is so popular mainly because it provides designers with a great deal of flexibility to design smaller, more compact, and low-cost products. Until now, dual-track reflow plates have been generally reflowed to solder the top (component face) and then wave soldering to the bottom (pin face). There is a trend towards dual-rail reflow soldering, but there are still some problems with this process, such as the bottom component of the large plate may fall during the second reflow soldering process, or the bottom solder joint may partially melt, causing reliability problems with the solder joint.

1.Introduction to the reflow soldering process Reflow soldering processing is a surface mount board, and its process is more complex, which can be divided into two types: single-sided placement and double-sided mounting.

A.Single-sided mounting: pre-applied solder paste → mount (divided into manual placement and machine automatic placement), →reflow soldering→ inspection and electrical testing.

B.Double-sided mounting: A side pre-coated solder paste → chip (divided into manual placement and machine automatic placement), →reflow soldering→ B-side pre-coated solder paste → chip (divided into manual placement and machine automatic placement), reflow soldering →→ inspection and electrical testing.

The simplest process of reflow soldering is "silk screen solder paste-patch-reflow soldering", the core of which lies in the accuracy of silk screen printing, the yield of the chip is determined by the PPM (parts per million) of the machine, and the reflow soldering needs to control the temperature rise, maximum temperature and falling temperature curve.

Reflow soldering process requirements Reflow soldering technology is more common in the field of electronics manufacturing, and the components on various boards in our computers are soldered to the circuit board through this process. The advantages of this process are that the temperature is easy to control, oxidation is avoided during the welding process, and the manufacturing cost is easier to control. This device has a heating circuit inside that heats nitrogen to a high enough temperature and blows it to the board where the component has been attached, so that the solder on both sides of the component melts and bonds to the motherboard.

1.It is necessary to set a reasonable reflow temperature profile and conduct real-time tests of the temperature profile regularly.

2.Soldering should be performed in the direction of soldering at the time of PCB design.

3.During the welding process, the conveyor belt should be strictly prevented from vibrating.

4.The welding effect of the first printed board must be checked.

5.It is necessary to check whether the soldering is sufficient, whether the surface of the solder joint is smooth, whether the shape of the solder joint is half-moon-shaped, the condition of solder balls and residues, the situation of continuous soldering and virtual soldering, and the color change of the PCB surface, and adjust the temperature curve according to the inspection results. During the whole batch production process, the welding quality should be checked regularly.

Factors influencing the process:

1.In general, PLCC, QFP have a larger heat capacity than discrete chip components, and it is more difficult to weld large area components than small components.

2.In a reflow oven, the conveyor belt acts as a heat dissipation system while transporting the product for reflow soldering over and over again. In addition, the heat dissipation conditions at the edge and center of the heating part are different, and the edge temperature is generally low, and the temperature of the same section in the furnace is different in addition to the different temperature requirements of each temperature zone.

3.Different product loads can have an impact. The adjustment of the reflow soldering temperature profile should take into account the good repeatability of no load, load and different load factors. The load factor is defined as: LF = L / (L + S); where L = the length of the assembled substrate and S = the interval between the assembled substrates. The reflow soldering process is more difficult to achieve reproducible results, and the larger the loading factor. Typically, the maximum load factor of a reflow oven ranges from 0.5 to 0.9, depending on the product (component soldering density, different substrates) and the type of reflow oven. To get good welding results and repeatability, practical experience is very important.

Reflow soldering is the core technology of SMT (surface mount technology) process, all electronic components on the PCB are soldered at one time through overall heating, and the quality control work of the SMT production line of the electronics factory is ultimately to obtain excellent soldering quality. Setting the temperature profile is equivalent to managing the furnace, which is something that all PE (process engineers) know. Many literatures and materials mention the setting of the reflow temperature profile. For a new product, a new furnace, a new solder paste, how to quickly set the reflow temperature profile? This requires us to have a basic understanding of the concept of temperature profiles and the principles of solder paste soldering.

In this paper, the most commonly used lead-free solder paste Sn96.5Ag3.0Cu0.5 tin-silver-copper alloy is taken as an example to introduce the ideal reflow soldering temperature curve setting scheme and analyze its principle.

In a typical SAC305 alloy lead-free solderpaste reflow soldering temperature curve, six curves are yellow, orange, green, purple, blue and black. Each point that makes up the curve represents the temperature measured at the corresponding time of the temperature measurement point on the corresponding PCB at the time of furnace passing. Connecting these points to continuously record the real-time temperature over time results in a continuously varying curve, which can also be seen as the process by which the temperature of the test points on the PCB changes over time in the furnace.

We divide this curve into 4 regions to get the time that a certain area of the PCB has experienced when it passes through reflow. Here, we would like to clarify another concept, "slope (1)". The "slope" is obtained by dividing the time it takes for a PCB to reflow an area by the absolute value of the temperature change over that time period. The concept of slope was introduced to represent the rate at which the PCB heats up after it is heated, and it is an important process parameter in the temperature curve. The four segments A, B, C, and D in the figure are defined as A: warming zone; B: Preheating the constant temperature zone (holding zone or activation zone); C: Reflow soldering area (soldering area or Reflow area); D: Cooling zone.

The following further analyzes the setting and significance of each section: 1. Heating zone A The area where the PCB enters the reflow soldering chain or mesh belt, and is heated to 150°C from room temperature is called the heating zone. The time of the heating zone is set at 60-90 seconds, and the slope is controlled between 2-4.

In this region, the temperature of the components on the PCB rises linearly relatively quickly, and the low-boiling solvent in the solder paste begins to partially evaporate. If the slope is too large and the heating rate is too fast, the solder paste will splash due to the rapid volatilization of the low-boiling solvent or the rapid boiling of water vapor, resulting in "solder bead" defects behind the furnace. Excessive slope can also cause mechanical damage such as micro-cracking of ceramic capacitors, deformation and warping of PCB boards, and internal damage of BGA due to thermal stress.

Another undesirable consequence of heating up too quickly is that the solder paste cannot withstand a large thermal shock and collapses, which is one of the causes of "short circuits". Through the long-term service tracking of the manufacturing plant, it is found that the slope of the SMT line of many manufacturers in this area can be controlled between 1.5 and 2.5 to achieve satisfactory results. Due to the different sizes and qualities of the components mounted on each board, the temperature difference between the large and small components at the end of the heating zone is relatively large.

2. Preheating constant temperature zone B This area is also known as the insulation area and activation area in many literature and supplier materials.

The PCB surface temperature in this area rises gently from 150°C to 200°C with a time window of 60 - 120 seconds. The various parts of the PCB board are slowly heated by hot air, and the temperature rises slowly over time, with a slope between 0.3 - 0.8.

At this point, the organic solvent in the solder paste continues to volatilize, and the active material is activated by the temperature to come into play, removing oxides from the pad surface, part feet, and tin powder alloy powder. The constant temperature zone is designed to heat up gently, so that the temperature difference between components of different sizes and materials on the PCB can be evenly heated, so that the temperature difference between components of different sizes and materials is gradually reduced, and the minimum temperature difference is reached before the solder paste melts, so as to prepare for melt soldering in the next temperature zone, which is an important way to prevent "tombstone" defects. The activation temperature of the active agent in the SAC305 alloy solder paste formulations of many lead-free solder paste manufacturers is mostly between 150 and 200°C, which is one of the reasons why this temperature curve is preheated in this temperature range.

It should be noted that:1. The preheating time is too short, the reaction time between the active agent (3) and the oxide is not enough, the oxide on the surface of the solder cannot be effectively removed, the water vapor in the solder paste cannot be completely and slowly evaporated, and the volatilization of the low boiling solvent is insufficient, which will lead to the violent boiling of the solvent during soldering and spatter, resulting in "tin beads", and there will also be insufficient wetting, which may produce "less tin", "virtual soldering", "empty welding" and "copper leakage" and other undesirable situations such as insufficient wetting. 2. The preheating time is too long, the active agent is consumed excessively, and there is not enough active agent to immediately remove and isolate the oxide generated by high temperature and the residue of flux high temperature carbonization when melting in the next temperature area (soldering zone), which will show undesirable phenomena such as "virtual soldering", "residue blackening" and "gray solder joints" after the furnace.

3. Reflow soldering area C The reflow area is also called the soldering area or Refelow area.

The melting point of SAC305 alloy is between 217°C - 218°C (4), so this region is > 217°C time, peak temperature < 245°C for 30 - 70 seconds. The temperature at which a good solder joint is formed is generally around 15 - 30°C above the melting point of the solder, so the minimum peak temperature in the reflow zone should be set above 230°C. Considering that the melting point of Sn96.5Ag3.0Cu0.5 lead-free solder pasteis above 217 °C, in order to take care of the PCB and components from high temperature damage, the peak temperature should be controlled below 250 °C, and the actual peak temperature of most factories seen by the author is above 245 °C.

After the band is over, the temperature on the PCB rises at a relatively fast rate to the tin powder alloy liquidus, at which point the solder begins to melt, continues linearly to the peak temperature, holds for a period of time, and then begins to descend to the solidus.

At this point, the various components in the solder paste come into full play: rosin or resin softens and forms a protective film around the solder to isolate it from oxygen; The surfactant is activated to reduce the surface tension between the solder and the surface to be welded, and to enhance the wetting force of the liquid solder; The active agent continues to react with the oxide, continuously removing the oxide produced by the high temperature and the carbide and providing partial fluidity until the reaction is completely over; Some additives decompose and volatilize at high temperatures, leaving no residue; High boiling point solvents continue to volatilize over time and are completely volatile at the end of resoldering; The stabilizer is evenly distributed in the metal and on the surface of the solder joint, protecting the solder joint from oxidation; The solder powder transitions from solid to liquid and expands as the flux wets; A small number of different metals undergo chemical reactions to form intermetallic compounds, such as Ag3Sn and Cu6Sn5 in typical tin-silver-copper alloys.

The reflow zone is the most central segment of the temperature curve. If the peak temperature is too low and the time is too short, the liquid solder does not have enough time to flow and wet, which will cause defects such as "cold soldering", "virtual soldering", "poor wetting (copper leakage)", "solder joints are not bright" and "many residues". If the peak temperature is too high or too long, it will cause defects such as "PCB board deformation", "component thermal damage", and "residue blackening". It requires a balance between peak temperatures, upper and lower temperature limits and time that PCBs and components can withstand, and melting time to achieve optimal soldering results to achieve the ideal solder joint.

Fourth, the cooling zone D The section where the temperature of the solder joint decreases downward from the liquidus line is called the cooling zone. In general, the cooling zone of SAC305 alloy solder paste is generally considered to be between 217°C - 170°C (some literature suggests a minimum of 150°C).

Since the liquid solder is cooled below the liquidphase to form a solid solder joint, the quality of the formed solder joint cannot be judged by the naked eye in the short term, so many factories often do not pay much attention to the setting of the cooling zone. However, the cooling rate of the solder joint is related to the long-term reliability of the solder joint and must be taken seriously.

The main control point of the cooling zone is the cooling rate. After many soldering laboratory studies, it has been concluded that rapid cooling is beneficial for stable and reliable solder joints.

It is often intuitively assumed that the cooling should be done slowly to compensate for the thermal shock of the individual components and solder joints. However, the slow cooling of reflow solder paste brazing will form more coarse grains, and generate larger intermetallic particles such as Ag3Sn and Cu6Sn5 in the interface layer and inside the solder joint, which will reduce the mechanical strength and thermal cycle life of the solder joint, and may cause the solder joint to be gray, low gloss or even dull.

Rapid cooling can form smooth, uniform and thin intermetallic compounds, form fine tin-rich dendritic crystals and fine grains dispersed in tin matrix, so that the mechanical properties and reliability of solder joints can be significantly improved and improved.

In production applications, the higher the cooling rate, the better, but the balance between the board and the components should be sought by combining the cooling capacity of the reflow soldering equipment and the thermal shock that the board, components and solder joints can withstand. The minimum cooling rate should be above 2.5°C and the optimal cooling rate should be above 3°C. Considering the thermal shock that components and PCBs can withstand, the maximum cooling rate should be controlled at 6 - 10°C. When selecting equipment for factories, it is best to choose reflow soldering with water cooling to obtain a strong reserve of cooling capacity.

1.What are the advantages of reflow soldering technology? 1) When soldering with reflow soldering technology, the printed circuit board does not need to be immersed in molten solder, but uses local heating to complete the soldering task, so the soldered components are subject to less thermal shock and will not be damaged due to overheating. 2) In soldering technology, soldering defects such as bridging are avoided because only solder is applied at the solder site and soldered locally heated. 3) In the reflow soldering technology, the solder is used once and there is no reuse, so the solder is pure, there are no impurities, and the quality of the solder joint is guaranteed.

2.Precautions for reflow soldering

3.Solder collapse can also occur during the heating process of bridging reflow soldering, which occurs in both preheating and main heating. When the preheating temperature is in the range of tens to 100 degrees, the solvent, which is one of the components in the solder, will reduce the viscosity and flow out, and if the tendency of the outflow is very strong, the solder particles will be extruded at the same time to the gold-containing particles outside the solder zone, and if they cannot return to the solder zone during melting, a retained solder ball will be formed. In addition to the above factors, whether the end electrode of SMD components is smooth and good, whether the circuit board wiring design and solder zone spacing are standardized, the selection of solder mask coating method and its coating accuracy will all be the reasons for bridging.

4.Floating height of the monument component (Manhattan phenomenon) Warping occurs when the chip component is subjected to rapid reflow soldering, because the rapid heat causes a temperature difference between the two ends of the element, and the solder on one side of the electrode is completely melted to obtain good wetting, while the solder on the other side is not completely melted and causes poor wetting, which causes the component to warp. Therefore, reflow soldering should be considered from the perspective of time factor, so that the horizontal heating forms a balanced temperature distribution and avoids the rapid heat of reflow soldering. The main factors to prevent the components from warping are as follows: (1) Select solder with strong adhesion to improve the printing accuracy of the solder and the placement accuracy of the component; (2) The external electrodes of the element need to have good wettability and wetting stability. Recommended: The temperature is below 40°C, the humidity is below 70%RH, and the service life of the incoming components should not exceed 6 months; (3) A small solder zone width is used to reduce the surface tension on the component ends when the solder is melted. In addition, the printing thickness of the solder can be appropriately reduced, such as 100μm; (4) The setting of welding temperature management conditions is also a factor in component warping. The usual goal is to heat evenly, especially before the weld fillet is formed at both ends of the component, and the uniform heating must not fluctuate.

5.Poor wetting Poor wetting refers to the fact that during the reflow soldering process, the solder and the solder area (copper foil) of the circuit board or the external electrode of the SMD do not form a reaction layer with each other after wetting, resulting in missed soldering or less soldering failure. Most of the reasons for this are contamination of the surface of the solder zone, solder mask, or the formation of a metal compound layer on the surface of the bond. For example, the presence of sulfides on the surface of silver and oxides on the surface of tin will cause poor wetting. In addition, if the residue of aluminum, zinc, cadmium, etc. in the solder exceeds 0.005%, the degree of activation is reduced due to the hygroscopic effect of the flux, and poor wetting may occur. Therefore, it is necessary to take anti-fouling measures on the surface of the soldering substrate and the surface of the components, select appropriate solder, and set a reasonable soldering temperature curve for reflow soldering.

5. Five steps of lead-free soldering:

 1. Choosing the appropriate material and method In the lead-free soldering process, the selection of soldering materials is the most challenging. Because for the lead-free soldering process, the selection of lead-free solder, solder paste, flux and other materials is the most critical and the most difficult. These materials are also selected taking into account the type of soldered component, the type of circuit board, and their surface coating. The materials selected should be proven in their own research, recommended by authoritative institutions or literature, or have been used in practice. These materials are tabulated for trial in process tests to provide an in-depth study of their impact on all aspects of the process.   For the welding method, it is necessary to choose according to your actual situation, such as component type: surface mount components, through-hole insertion components; the condition of the circuit board; The number and distribution of components on the board, etc. For the welding of surface-mounted components, the method of reflow soldering is required; For through-hole reflow cartridge components, wave, dip, or spray soldering can be selected as the case may be. Wave soldering is more suitable for the welding of through-hole insertion components on the whole board (large); Dip soldering is more suitable for reflow soldering of through-hole insertion components on a whole board (small) or in a local area of the board; Local spray fluxes are more suitable for reflow soldering of individual components on the board or a small number of through-hole insertion components. In addition, it should be noted that the whole process of lead-free reflow soldering is longer than that of leaded solder, and the required soldering temperature is higher, due to the fact that lead-free solder has a higher melting point than leaded solder, and its wettability is poorer.   After the welding method is selected, the type of welding process is determined. At this time, it is necessary to select equipment and related process control and process inspection instruments according to the requirements of the welding process, or upgrade. The selection of welding equipment and related instruments is as critical as the selection of welding materials.

 2. Determine the process route and process conditions After the first step is completed, the welding process test can be carried out on the selected welding material. The route and process conditions are determined through experiments. In the test, the selected welding materials from the list need to be fully tested to understand their properties and their impact on the process. The purpose of this step is to develop a sample of lead-free soldering.

 3. Developing a sound welding process This step is a continuation of the second step. It is the analysis of the test data collected in the second step of the process test to improve the material, equipment or change the process in order to obtain a sound process under laboratory conditions. In this step, it is necessary to understand the contamination that may occur in the soldering process of lead-free alloys, know how to prevent it, determine the process capability (CPK) value of each soldering characteristic, and compare it with the original tin/lead process. Through these studies, it is possible to develop procedures for the inspection and testing of the welding process, as well as to identify ways to deal with process runaways.  

 4. It is also necessary to conduct a reliability test on the welding sample to identify whether the quality of the product meets the requirements. If the requirements are not met, the cause needs to be identified and resolved until the requirements are met. Once the reliability of the soldered product has been achieved, the development of a lead-free soldering process has been successful, and the process is ready for large-scale production, ready for operation, and now ready to move from prototype to industrial production. At this point, the process still needs to be carried out to keep it under control.

 5. Controlling and Improving the Process The lead-free soldering process is a dynamic and changing arena. Factories must be alert to possible problems to avoid process runaways, and they also need to continuously improve the process to ensure that the quality and good crystal rate of the product are continuously improved. As with any lead-free soldering process, improvements in soldering consumables and newer equipment can improve the soldering performance of the product.

6. Reflow soldering occupies an extremely important position in the field of electronic manufacturing, and its knowledge of the principles, processes, process requirements, advantages, precautions, and lead-free soldering is essential to ensure the quality and reliability of electronic products.

1. **Principle Aspect**

   - The reflow soldering process is divided into zones, including a heating zone, a holding zone, a soldering zone, and a cooling zone. In the heating zone, the solvent in the solder paste begins to evaporate, the flux plays a role, and the temperature of the components rises linearly. The insulation zone is to allow the PCB and components to fully preheat, gently heat up, and reduce the temperature difference between different components; The solder zone is the core area where the solder melts and forms solder joints, and the various components come into full play; The cooling rate of the cooling zone has an impact on the long-term reliability of the solder joint, and rapid cooling is conducive to improving the mechanical properties of the solder joint.

   - In the principle of dual-track reflow soldering, the production capacity can be increased by two times, the transfer of heat energy from the reflow oven heater to the circuit board is affected by a variety of factors, and the design and application of dual-track reflow soldering has certain characteristics and problems, such as the ability to handle different circuit boards and the problems that may occur when the bottom component is reflowed twice.

2. **Process Aspects**

   - There are two types of reflow soldering processes for surface mount boards: single-sided placement and double-sided mounting. Single-sided placement includes pre-applied solder paste, mounting, reflow soldering and inspection and electrical testing; The double-sided placement process is more complex, involving both A-side and B-side operations. At the heart of the simplest process is screen printing accuracy, placement yield, and temperature control.

3. **Process Requirements**

   - Set a reasonable reflow soldering temperature curve and test it in real time, weld according to the welding direction to prevent the conveyor belt from vibrating, check the welding effect of the first printed board and adjust the temperature curve according to the inspection results, and regularly check the welding quality of the whole batch production. At the same time, factors such as the heat capacity of different components, heat dissipation of the conveyor belt, and product loading capacity can all affect the process. For the setting of the reflow soldering temperature curve, taking lead-free solder paste Sn96.5Ag3.0Cu0.5 as an example, different areas have different temperature, time and slope requirements, and the setting of each zone is very important to prevent various soldering defects.

4. **Advantages**

   - Reflow soldering adopts local heating method, which has low thermal shock of the soldered components, which can avoid welding defects such as bridging, and the solder is pure to ensure the quality of solder joints.

5. **Precautions**

   - Bridging problems may be caused by a variety of factors, such as solvent outflow during preheating, electrodes at the end of components, wiring of circuit boards, and application of solder mask. The floating height of the monument component is related to the rapid heat of reflow soldering, which can be prevented by selecting suitable solder, improving the placement accuracy, and controlling the wettability of the external electrode of the component. Poor wetting is mainly due to surface contamination in the solder area, the formation of a metal compound layer on the surface of the bonded object, or excessive residual impurities in the solder, etc., and it is necessary to take anti-fouling measures and set a reasonable temperature curve.

6. **Lead-free soldering**

   - There are five steps in lead-free soldering, including selecting appropriate materials and methods (considering factors such as soldering components, circuit board type, etc.), determining the process route and process conditions (determined by testing), developing and improving the soldering process (analyzing test data to improve the process), conducting reliability testing (ensuring that the product quality meets the requirements), and controlling and improving the process (keeping the process under control and continuously improving product quality).