Setting and optimization of reflow temperature curves

Detailed explanation of reflow soldering technology and temperature curve setting optimization

1. Overview of reflow soldering technology

The process of reflow soldering is as follows: solder paste – a sticky mixture made of powdered solder mixed with flux – temporarily connects one or more electrical components to the corresponding contact pad, then heats the entire assembly to melt the solder and thus achieve a secure connection between the components and the pad. Heating can be done in a variety of ways, from passing the components through a reflow oven, to welding the joints with the help of an infrared lamp or a hot air pen.

Reflow soldering is the most common method of connecting surface-mount components to circuit boards, but it can also be applied to through-hole components by filling the holes with solder paste and allowing the component leads to pass through the paste. However, because wave soldering is easier and less costly, pure through-hole plates generally do not use reflow soldering. On hybrid boards with SMT and through-hole insertion (THT) components, through-hole reflow eliminates the wave soldering step in the assembly process, potentially reducing assembly costs.

The core goal of the reflow process is to melt the solder and heat adjacent surfaces while avoiding damage to electrical components due to overheating. The traditional reflow soldering process is typically divided into four stages, or four "zones," each with varying heat distribution, namely preheating, hot dipping (often referred to as soaking), reflow, and cooling, all of which are strictly protected from overheating and damaging electrical components.

2. Step-by-step analysis of reflow soldering process

(1) Warm-up stage

Preheating is the first stage of the reflow process, which gradually warms up the entire board assembly toward the target immersion or residence temperature. The primary purpose of this stage is to ensure that the entire assembly can safely and consistently reach the temperature before immersion or reflow, as well as to degasse volatile solvents in the solder paste.

(2) Hot immersion stage

The hot-dip stage, typically a 60 to 120-second heating process, removes volatiles from the solder paste and activates theflux, during which the flux components begin to undergo redox reactions on component leads and pads. It is important to note that high temperatures during this stage can cause a range of problems, such as solder splashing or balling, as well as oxidation of solder paste, attachment pads, and component terminals.

(3) Reflow area

The reflux zone, also known as "above the reflow time" or "above the liquid phase" (TAL), is the stage where the highest temperature is reached in the entire process. In this area, the solder melts sufficiently, allowing for a reliable connection between the component and the pad.

(4) Cooling area

The cooling zone works by gradually cooling the machined board and curing the solder joints. Proper cooling inhibits the formation of excess intermetallic compounds and avoids thermal shock to the components. The typical temperature range for the cooling zone is 30-100°C (86-212°F). Choosing a fast cooling rate helps produce fine-grained structures with optimal mechanical properties. Unlike the maximum acceleration rate, the reduced rate is often easily overlooked. In fact, the maximum allowable slope of any component should be strictly followed, regardless of whether it is heated or cooled. A cooling rate of 4°C/s is generally recommended, which is a key parameter to consider when analyzing process results.

3. The key significance of the reflow soldering temperature curve

Reflow soldering technology is the most commonly used technique in modern electronics assembly processes, and the setting of the reflow temperature curve is the most critical part of the PCB assembly reflow soldering process. The electronics industry is often considered a mature industry, and the PCB reflow soldering process is also considered a very mature technology, but new challenges are still emerging.

For example, existing components range in size from 01005 to 50mm×50mm and are distributed across double-sided PCBs with extremely high assembly density. In addition, issues such as the layout of the selected device, component size, package form, and heat capacity, as well as the maximum allowable temperature of different heat-sensitive components, and different formulations of solder and flux, all pose challenges in setting up the reflow temperature profile. If these issues are not taken into account, a set reflow temperature profile can lead to unacceptable solder joints, failed components, and reduced overall reliability. Therefore, it is necessary to conduct in-depth discussions on the setting and optimization of reflow soldering temperature curves. The following takes the most commonly used lead-free solder paste, Sn96.5Ag3.0Cu0.5 tin-silver-copper alloy, as an example, to introduce the ideal reflow temperature curve setting optimization scheme and analyze its principle.

4. Detailed explanation of the temperature curve of SAC305 alloy lead-free solder paste reflow soldering

In the typical SAC305 alloy lead-free solder paste reflow temperature curve, each point represents the temperature measured at the corresponding time when the temperature measurement point on the PCB is passed through the furnace. This curve is typically divided into 4 zones, which gives a clear picture of the time the PCB goes through each zone as it passes through reflow. Among them, the slope represents the rate at which the PCB heats up and is an important process parameter in the temperature curve. The four sections A, B, C, and D in the figure are defined as: A is the heating zone, B is the preheating and constant temperature zone (also known as the insulation zone or activation zone), C is the reflow welding zone (also known as the welding zone or Reflow zone), and D is the cooling zone.

(1) Warming zone A

When a PCB enters a reflow chain or belt, the area where it heats up to 150°C from room temperature is the heating zone. The time of the heating zone is set at 60-90 seconds, and the slope is controlled between 1-3. Within this region, the temperature of the components on the PCB board rises linearly relatively quickly, and the low-boiling solvent in the paste begins to partially volatilize.

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 solvent at low boiling point or the rapid boiling of water vapor, resulting in a "solder bead" defect after the furnace. Excessive slope can also cause mechanical damage such as microcracking of ceramic capacitors, deformation and warping of PCB boards, and internal damage to BGAs due to thermal stress. Another undesirable consequence of heating up too quickly is that the solder paste cannot withstand the large thermal shock and collapses, which is one of the causes of "short circuits". Generally speaking, the actual control of the slope of this area between 1.5 and 2.5 can achieve a more satisfactory effect.

(2) Preheating and constant temperature zone B

Preheating Thermostatic Zone B, also known as the insulation zone and activation zone, is where the PCB surface temperature rises gently from 150°C to 200°C with a time window between 60-120 seconds. Parts of the PCB board are slowly heated by hot air, and the temperature slowly rises over time, with a slope between 0.3 and 0.8.

At this point, the organic solvent in the solder paste continues to volatilize and the active substance is activated by temperature, removing oxides from the pad surface, part feet, and solder alloy powder. The temperature zone is designed to warm up gently to take into account the uniform heating of mounting components of different sizes on the PCB, 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 is melted, preparing for melt soldering in the next temperature zone, which is an important way to prevent "monument" defects.

The activation temperature of the active agent in alloy solder paste formulations is mostly between 150-200°C, which is one of the reasons why this temperature curve is preheated in this temperature range. The following two points need to be noted:

1. The preheating time is too short, the reaction time between the active agent and the oxide is insufficient, the oxide on the surface of the soldered object cannot be effectively removed, the water vapor in the solder paste cannot evaporate completely and slowly, and the volatilization of the solvent at low boiling point is not enough, which will lead to the solvent boiling violently and splashing during soldering to produce "solder beads", and insufficient wetting, which may produce "less tin", "virtual soldering", "empty soldering", "copper leakage" and other undesirable phenomena such as insufficient infiltration.

2.      The preheating time is too long, the active agent consumption is excessive, and there is not enough active agent to remove and isolate the oxides produced at high temperature and the residue of high-temperature carbonization of the flux in time when melting in the next temperature area, which will show undesirable phenomena such as "virtual welding", "blackening of residue" and "gray solder joints" after the furnace.

(3) Reflow soldering area C

The reflow zone is also known as the weld zone or reflow zone. Since the melting point of SAC305 alloy is between 217°C-218°C, this area is the time when the temperature is > 217°C and the peak temperature is < 245°C for 30-70 seconds.

The temperature of forming a high-quality solder joint is generally about 15-30°C above the melting point of the solder, so the minimum peak temperature in the reflow area should be set above 230°C. Considering that the melting point of Sn96.5Ag3.0Cu0.5 lead-free solder paste is already above 217°C, in order to avoid high-temperature damage to PCBs and components, the peak temperature should be controlled below 250°C, and it is understood that the actual peak temperature of most factories is below 245°C.

After the preheating zone, the temperature on the PCB board rises to the tin alloy liquid phase at a relatively fast rate, at which point the solder begins to melt, continues to heat linearly to the peak temperature, and then begins to drop to the solid phase line after a period of time. 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 that is insulate from oxygen; Surfactant is activated to reduce the surface tension between the solder and the soldered surface, and to enhance the wetting force of the liquid solder. The active agent continues to react with oxides, continuously removing oxides and carbides produced at high temperatures and providing partial fluidity until the reaction is completely over; Some additives decompose and volatilize at high temperatures without leaving residues; The high boiling point solvent volatilizes continuously over time and is completely volatilized at the end of the resolder; The stabilizer is evenly distributed on the surface of the metal neutralizing solder joint, protecting the solder joint from oxidation; The solder powder changes from solid to liquid and expands as the flux is wetted; A small number of different metals undergo chemical reactions to form intermetallic compounds, such as typical tin, silver, and copper alloys, which will have Ag3Sn, Cu6Sn5 formation.

The reflow zone is the most central section of the temperature curve. The peak temperature is too low, the time is too short, and the liquid solder does not have enough time to flow and wet, which will cause defects such as "cold soldering", "virtual soldering", "poor infiltration (copper leakage)", "not bright solder joints" and "many residues"; Excessive peak temperature or too long time can cause defects such as "PCB board deformation", "thermal damage to components", and "blackening of residue". It requires a balance between peak temperature, the upper temperature that the PCB board and components can withstand and the time it takes, and the melt time to create the best soldering results to achieve the ideal solder joint.

(4) Cooling area D

The section where the solder joint temperature decreases downwards from the liquid phase is called the cooling zone. Generally, the cooling zone of SAC305 alloy solder paste is generally considered to be the time period between 217°C and 170°C. Because the liquid solder is cooled below the liquid phase line to form a solid solder joint, the quality of the 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 solder joints is related to the long-term reliability of solder joints and must be taken seriously.

The main control points of the cooling area are the cooling rate. After many solder laboratories have concluded that rapid cooling is conducive to stable and reliable solder joints. It is often intuitively thought that it should be cooled slowly to counteract the thermal shock of individual components and solder joints. However, slow cooling of reflow solder paste brazing will form more coarse grains, resulting in larger intermetallic compound particles such as Ag3Sn and Cu6Sn5 in and inside the solder joint interface, reducing the mechanical strength and thermal cycle life of the solder joint, and may cause the solder joint to be gray, low glossy or even dull.

Rapid cooling can form smooth, uniform and thin intermetallic materials, forming fine tin-rich dendrites and fine grains diffused in the tin matrix, which significantly improves the mechanical properties and reliability of solder joints. In production applications, the larger the cooling rate, the better, to be considered in combination with the cooling capacity of the reflow soldering equipment, the thermal shock that the board, components and solder joints can withstand, and the balance between the board and components should be sought without damaging the quality of the solder joints. 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 the components and PCBs can withstand, the maximum cooling rate should be controlled at 6-10°C.

5. Summary of key points at each stage of reflow soldering

From the perspective of reflow soldering technology itself, its temperature control, time setting, and the reaction of various substances in each stage are very critical.

In the preheating stage, ensure that the components are safely heated up and reach the appropriate temperature to degas the volatile solvent in the solder paste. During the hot dipping stage, attention should be paid to temperature control to prevent solder splashing, oxidation and other problems; The reflow zone is the core part, and the setting of peak temperature and time needs to be balanced between a variety of factors, otherwise there will be various defects such as "cold soldering", "virtual soldering", "PCB board deformation" and so on. Although the cooling zone is often overlooked, the cooling rate has a great impact on the long-term reliability of the solder joint, and it is necessary to determine the appropriate cooling rate based on the equipment capacity and the ability of the board and components to withstand thermal shock.

6. The impact of PCB quality on reflow soldering

The quality of the PCB will greatly affect the effectiveness of reflow soldering, which is manifested as follows:

Insufficient coating thickness of the pad can lead to poor soldering. Insufficient coating thickness on the surface of the pad where the component is assembled can cause soldering problems, for example, insufficient tin thickness can lead to insufficient amount of tin when melting at high temperatures, resulting in a weak weld between the component and the pad.

Dirty pad surfaces can also cause bad soldering. If the PCB is not cleaned properly, impurities can remain on the surface of the pad, affecting the quality of the solder.

Wet film deviation can also lead to poor welding. Defects in the pads can cause parts to be difficult to weld or fall off.

BGA pads develop uncleanly and have impurities, which can lead to improper soldering or missoldering. The protruding jack on the BGA can cause insufficient contact between the BGA component and the pad, making it easy to break.

If the distance between the locating hole and the circuit diagram is not up to standard, it can cause solder paste deviation or a short circuit.

During the reflow soldering process, several factors can affect the quality of the weld. In terms of PCB quality, such as insufficient coating thickness, dirty surface, wet film deviation, defects, unclean BGA pad development, protruding jacks, and non-compliance with the distance between the positioning hole and the circuit diagram, etc., will lead to poor soldering. These factors indicate that in the reflow soldering process, it is necessary to pay attention not only to the process parameters of the reflow soldering technology itself, but also to the quality of the PCB.