How to set the reflow temperature and test furnace temperature curve!
Date: 2015/1/17 14:19
Correct solder paste process setting and optimization of reflow temperature curve
The matching of reflow soldering temperature curve and process is the guarantee of high through rate after the furnace
Selection of real test points
Reflow soldering is the core technology of the SMT process. All the electronic components on the PCB are completed by one-time soldering. The quality control of the SMT production line in the electronics factory is an absolute part of the work to obtain excellent soldering quality. After setting the temperature curve, the furnace is managed, which is what all PE knows. Many literatures and materials mention the setting of the reflow temperature curve. For a new product, new furnace, new solder paste, how to quickly set the reflow temperature curve? This requires us to have a basic understanding of the concept of temperature curve and the principle of solder paste soldering.
This article 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 soldering temperature curve setting scheme and analyze its principle. Figure one:
Figure 1 SAC305 lead-free solder reflow temperature curve
Figure 1 shows a typical SAC305 alloy lead-free solder reflow temperature curve. In the figure, the six curves of yellow, orange, green, purple, blue and black are the temperature curves. Each point constituting the curve represents the temperature measured at the corresponding time when the temperature measurement point on the corresponding PCB passes through the furnace. The real-time temperature is continuously recorded over time, and these points are connected to obtain a continuously changing curve. It can also be seen as the process of the temperature of the test point on the PCB in the furnace over time.
Then, we divide this curve into 4 areas, and get the time that a certain area of the PCB passes through reflow soldering. Here, we need to clarify another concept, "Slope ①". The time taken by reflowing a certain area of the PCB by the PCB is divided by the absolute value of the temperature change during this period, and the resulting value is the "slope". The concept of slope is introduced to indicate the rate of temperature rise after PCB is heated. It is an important process parameter in the temperature curve. The four sections A, B, C, and D in the picture are defined as A: heating zone, B: preheating constant temperature zone (heat preservation zone or activation zone), C: reflow soldering zone (welding zone or Reflow zone) D: cooling zone.
Continue to analyze the settings and significance of each section:
One. Heating zone A
The PCB enters the reflow soldering chain or mesh belt, and the area heated from room temperature to 150 ° C is called the heating zone. The time in 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 board rises relatively quickly and linearly, and the low-boiling-point solvents in the solder paste begin to partially evaporate. If the slope is too large and the heating rate is too fast, the solder paste will splatter due to the rapid volatilization of low-boiling solvents or the rapid boiling of water vapor, which will cause "tin beads" defects after the furnace. Excessive slope can also cause mechanical damage such as ceramic capacitor micro-cracks, PCB board deformation and warping, and internal damage to the BGA due to thermal stress.
Another bad consequence of too fast heating is that the solder paste can not withstand large thermal shocks and collapses, which is one of the causes of "short circuit". For long-term service tracking of manufacturers, the slope of the SMT line of many manufacturers in this area can be controlled to be between 1.5 and 2.5, and satisfactory results can be obtained. Due to the different size and quality of the components mounted on each board, the temperature difference between the large and small components is relatively large at the end of the heating zone.
two. Preheating constant temperature zone B
This area is also called insulation zone and activation zone in many literatures and supplier materials.
The surface temperature of the PCB in this area gradually rises from 150 ° C to 200 ° C, and the time window is between 60-120 seconds. Each part of the PCB is slowly heated by hot air, and the temperature rises slowly with time. The slope is between 0.3-0.8.
At this time, the organic solvent in the solder paste continues to volatilize. The active material is activated by temperature and starts to play a role in removing oxides from the pad surface, part feet and tin powder alloy powder. The purpose of the constant temperature zone is to increase the temperature gradually, in order to take into account that the components of different sizes mounted on the PCB can evenly heat up. 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 the fusion welding in the next temperature zone. This is an important way to prevent "tombstone" defects. The activation temperature of the active agent in the formulation of SAC305 alloy solder paste of many lead-free solder paste manufacturers is mostly between 150-200 ° C, which is one of the reasons for the preheating of this temperature curve in this temperature range.
Note: 1. The warm-up time is too short. The reaction time between the active agent ③ and the oxide is insufficient, and the oxide on the surface of the solder cannot be effectively removed. The water vapor in the solder paste does not completely evaporate slowly, and the low-boiling point solvent has insufficient volatilization, which will cause the solvent to boil violently during soldering and generate "tin beads". Inadequate wetting may cause defects such as "less tin", "empty soldering", "empty soldering" and "copper leakage" that are insufficiently wetted. 2. The warm-up time is too long. The active agent is consumed too much. When the welding zone melts in the next temperature zone, there is not enough active agent to immediately remove and isolate the oxides and flux carbonization residues caused by the high temperature. This situation will also show "bad welding", "black residue", and "dark solder joints" after the furnace.
three. Reflow soldering area C
The reflow area is also called the solder area or the Reflow area.
The melting point of SAC305 alloy is between 217 ° C and 218 ° C. ④ Therefore, the time in this region is> 217 ° C, the peak temperature is <245 ° C, and the time is 30-70 seconds. The temperature for 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 zone 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 protect the PCB and components from high temperature damage, the peak temperature should be controlled below 250 ° C. Most factories I have seen are actually The peak temperature is below 245 ° C.
After the preheating zone is over, the temperature of the PCB board rises to the liquid phase of the tin powder alloy at a relatively fast rate. At this time, the solder begins to melt, continues to linearly rise to the peak temperature, and then continues to fall for a period of time before beginning to fall to the solid phase line.
At this point, the various components in the solder paste are fully functional: the rosin or resin softens and forms a protective film around the solder to isolate it from oxygen. Surfactants are activated to reduce the surface tension between the solder and the surface being soldered, and enhance the wetting force of the liquid solder. The active agent continues to react with the oxide, continuously removing oxides and carbides generated at high temperature and providing partial fluidity until the reaction is completely completed. Some additives decompose at high temperatures and volatilize without leaving a residue. High-boiling solvents evaporate over time and completely evaporate at the end of reflow. The stabilizer is evenly distributed in the metal and protects the solder joint surface from oxidation. The solder powder changes from solid to liquid and expands as the flux wets. A small number of different metals undergo chemical reactions to produce intermetallic compounds. For example, a typical tin-silver-copper alloy will have Ag3Sn and Cu6Sn5 formed.
The reflow zone is the most central section of the temperature profile. The peak temperature is too low and the time is too short. The liquid solder does not have enough time to flow and wet, resulting in "cold soldering", "virtual soldering", "poor wetting (copper leakage)", "not bright solder joints" and "many residue "; The peak temperature is too high or the time is too long, causing defects such as" PCB board deformation "," component thermal damage "," black residue "and so on. It needs to find a balance between the peak temperature, the upper temperature limit and time that the PCB board and components can withstand, and the melting time to form the best soldering effect, in order to obtain the ideal solder joint.
four. Cooling zone D
The section where the solder joint temperature decreases from the liquidus line is called the cooling zone. Generally, the cooling zone of SAC305 alloy solder paste is generally considered to be a time period between 217 ° C and 170 ° C (some literatures have proposed a minimum of 150 ° C).
Since the temperature of the liquid solder drops below the liquidus level, a solid solder joint is formed, and the quality of the solder joint cannot be judged by the naked eye in a short period of time. Therefore, many factories do not attach much importance 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 cannot be taken seriously.
The main control points of the cooling zone are the cooling rate. After a lot of soldering laboratory research, it is concluded that rapid temperature reduction is beneficial to obtain stable and reliable solder joints.
Usually people's intuition thinks that the temperature should be slowly reduced to offset the thermal shock of each component and solder joint. However, slower cooling of reflow solder paste will form more coarse grains, and larger intermetallic compound particles such as Ag3Sn, Cu6Sn5 will be generated in the solder joint interface layer and inside. Reduce the mechanical strength and thermal cycle life of the solder joints, and may cause the solder joints to have low dull gloss and even dullness.
Rapid cooling can form smooth, uniform and thin intermetallic compounds, and form small tin-rich dendrites and fine grains dispersed in the tin matrix, which significantly improves and improves the mechanical properties and reliability of the solder joint.
In production applications, the larger the cooling rate, the better. Consider the cooling capacity of the reflow soldering equipment, and the thermal shock that the board, components and solder joints can withstand. A balance should be sought between the board and the components while ensuring 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 components and PCB can withstand, the maximum cooling rate should be controlled at 6-10 ° C. When choosing equipment in the factory, it is best to choose reflow soldering with water cooling function to obtain a strong cooling capacity reserve.
Figure 2 SAC305 lead-free solder paste process window
As shown in Figure 2. The minimum activation temperature and the maximum activation temperature of the solder paste, the shortest and longest effective activation time, the melting point and the optimal soldering temperature of the solder paste, the highest temperature and time that the PCB board and components can carry, the interval between these parameters is formed Up the craft window. In actual production, the process parameters of each brand of solder paste are different, and the PCB material, component density, and component heating capacity of the product are also different. After designing the reflow temperature curve, it needs to be adjusted and optimized in the process window in order to obtain the most optimized Reflow Profile.
Characteristics of temperature curve Lead-based solder Lead-free solder, average heating rate (Tsmax to Tp) up to 3 ° C / s up to 3 ° C / s, preheating: minimum temperature (Tsmin) 100 ° C 150 ° C, preheating: highest Temperature (Tsmax) 150 ° C 200 ° C, preheating: time (tsmin to tsmax) 60-120 seconds 60-180 seconds, time to maintain above temperature: temperature (TL) 183 ° C 217 ° C, maintain above Time for temperature: time (tL) 60-150 seconds 60-150 seconds, peak / class temperature (Tp) 215 ° C 260 ° C.
The micro-circulation air transport system and the principle of pressurized multi-point jet ensure uniform temperature in the furnace, good anisotropy in each temperature zone, and the board surface does not generate heated empty areas due to refraction when heated, no shadows, and PCB surface Horizontal △ T <± 2 ℃, which fundamentally improves the heating efficiency, fast and efficient thermal compensation performance, the difference between the actual temperature of the soldering zone pcb and the set temperature is less than 3 ℃, especially suitable for bga, csp, qfp and other components and multilayer Perfect soldering of circuit boards. The temperature setting of the two adjacent temperature zones can reach 100 ° C, and the temperature difference between the upper and lower sides of the pcb can reach 60 ° C, which can fully meet the reliable welding of the two panels. The patented guide rail does not deform at high temperature, which effectively guarantees the guide rails to be parallel, prevents the board from falling off, and prevents the occurrence of jamming, free cleaning, and easy adjustment. Automatic and manual width adjustment. <>
Lateral temperature deviation of different series of Rio Tinto reflow soldering:
： 裸板FR4板材测试： PCB板面横向△T<±3℃<> S series (mesh belt type) : bare board FR4 board test: PCB board surface transverse △ T <± 3 ℃ <>
： 裸板FR4板材测试： PCB板面横向△T<±2℃<> ES series (mesh belt type) : bare board FR4 plate test: PCB board surface transverse △ T <± 2 ℃ <>
： 裸板FR4板材测试： PCB板面横向△T<±1.5℃ 适用BGA焊接 M series (mesh belt type) : bare board FR4 plate test: PCB board surface transverse △ T <± 1.5 ℃ suitable for BGA welding
系列（导轨+网带型）： 裸板FR4板材测试： PCB板面横向△T<±1.5℃适用BGA焊接 MS series (rail + mesh belt type): bare board FR4 board test: PCB board surface transverse △ T <± 1.5 ℃ suitable for BGA welding
系列（导轨+网带型）： 裸板FR4板材测试： PCB板面横向△T<±1℃ 适用大容量BGA焊接 RS series (rail + mesh belt type): bare board FR4 plate test: PCB board surface transverse △ T <± 1 ℃ suitable for large capacity BGA welding
系列研发型（导轨+网带型）： 裸板FR4板材测试： PCB板面横向△T<±0.5℃ 适用大容量 BGA焊接 RS series research and development type (rail + mesh belt type): bare board FR4 plate test: PCB board surface transverse △ T <± 0.5 ℃ suitable for large capacity BGA welding