Standard lead-free reflow soldering temperature curve
Release time：2024-04-09Publisher:Jeenoce
The standard lead-free reflow soldering temperature curve reflects the temperature change of a certain point on the PCB during the entire reflow soldering process of the reflow soldering solder paste alloy over time. It intuitively reflects the temperature change of that point throughout the entire soldering process, providing a scientific basis for achieving the best soldering effect. This curve consists of five temperature ranges, namely heating zone, preheating zone, rapid heating zone, reflow zone, and cooling zone. Most solder pastes can successfully achieve reflow soldering using these five temperature ranges.
A detailed understanding of the temperature in each temperature zone of reflow soldering, the heating time of the solder paste alloy, and the changes in the solder paste in each temperature zone can help to gain a deeper understanding of the significance of the ideal temperature curve. Below, we will explain it in detail.
1. Heating zone
The heating zone usually refers to the area where the temperature rises from room temperature (25 ℃) to around 160 ℃. In this heating zone, SMA is steadily heated up, and the solvent in the solder paste slowly evaporates. Various components, especially IC components, slowly heat up to meet the requirements of welding temperature later on. However, the size of components on PCBs varies, and the temperature rise rate of various components is also not exactly the same. Therefore, the temperature rise rate in the heating zone needs to be controlled between 0.5-2.0 ℃/s, and the recommended speed is between 1.0-1.5 ℃/s.
If the heating rate is too fast, due to thermal stress, it may cause fine cracks in ceramic capacitors, PCB deformation, and IC chip damage. At the same time, the solvent in the solder paste evaporates too quickly, leading to the occurrence of poor solder beads.
If the heating rate is too slow, the temperature of SMA and various components is insufficient, resulting in solder paste not being able to wet the components and causing virtual soldering. At the same time, the solvent in the solder paste cannot be completely evaporated, causing tin beads to explode in the reflow zone.
2. Preheating zone
The preheating zone, also known as the insulation zone, refers to the area where the temperature rises from 160 ℃ to around 180 ℃. The residual solvent in the solder paste evaporates completely, and the activity of the flux in the solder paste gradually increases with the increase of temperature, removing oxides and dirt from the surface of PCB pads and component terminals. SMA slowly heats up, and components of different sizes and materials maintain the same temperature rise rate.
The heating time of PCB in the preheating zone is about 40-70 seconds, and the temperature rise rate is below 0.5 ℃/s.
If the heating time is too short, the solvent in the solder paste will not completely evaporate, and during reflow soldering, it will boil and produce tin beads. If the heating time is too long and the activity of the flux disappears before reflow soldering, it is easy to produce poor wetting during welding, and the metal particles of the solder paste are prone to oxidation, resulting in tin beads.
3. Rapid heating zone
The rapid heating zone refers to the area where the temperature rises from 180 ℃ to the melting point of solder paste (217 ℃). In this area, the solder paste alloy rapidly rises to the welding temperature within 10-20 seconds, and the temperature rise rate is required to be greater than 2 ℃/s. The solder paste also rapidly heats up and approaches the melting state.
4. Reflux zone
The reflux zone refers to the area where the temperature rises from 217 ℃ to 240 ℃ and then gradually decreases to 217 ℃. In the reflow zone, the solder paste melts into a liquid state and quickly wets the solder pad. As the temperature further increases, the surface tension of the solder decreases, and the solder climbs along the component pins, forming a crescent surface. At this point, tin in the solder and copper on the PCB pad form intermetallic compounds, and tin atoms and copper atoms penetrate each other at their interfaces. The initial structure of Cu Sn alloy is Cu6Sn5, with a thickness of about 1-3um. If the reflow time is too long and the temperature is too high, copper atoms further penetrate into Cu6Sn5, and its local structure will change from Cu6Sn5 to Cu3Sn. The former alloy has high welding strength and good conductivity, while the latter is brittle, with low welding strength and poor conductivity, so the production of Cu3Sn needs to be suppressed.
If the solder paste alloy is left in the reflow zone for too long or at a high temperature, it can cause the PCB board surface to burn and blister, resulting in damage to the components. SMA reflow at ideal temperature, PCB color can remain unchanged, and solder joints are bright. During reflow, the surface tension generated by the melting of solder paste can be moderately calibrated due to the offset of component pins caused by the SMT process. At the same time, it can also cause various welding defects due to unreasonable pad design, such as "standing monument" and "bridging".
The maximum temperature in the reflux zone is 240 ± 5 ℃, and the SMT stays in the reflux zone for 50-60 seconds.
If the solder paste alloy is left in the reflow zone for too long or at a high temperature, it can cause the PCB board surface to burn and blister, resulting in damage to the components. The solder paste alloy refluxes at ideal temperatures, maintaining the original color of the PCB and making the solder joints bright. During reflow, the surface tension generated by the melting of solder paste can be moderately calibrated due to the offset of component pins caused by the SMT process. At the same time, it can also cause various welding defects due to unreasonable pad design, such as "standing monument" and "bridging".
The maximum temperature in the reflux zone is 240 ± 5 ℃, and the SMT stays in the reflux zone for 50-60 seconds.
5. Cooling zone
After the heating program reaches the cooling zone, the solder joints quickly cool down and the solder solidifies. Rapid cooling of solder joints can refine the lattice of the solder, improve the bonding strength, make the surface of the solder joints bright, and form a continuous crescent surface. The cooling rate of the cooling zone is required to be greater than 4 ℃/s. QHL360 small lead-free reflow soldering adopts forced heat dissipation cooling, and the cooling rate can reach 8 ℃/s.
