The role and influence of the four temperature zones in reflow soldering

In the reflow soldering production process, the welding temperature curve is an important variable that has a significant impact on the product yield. Those who have been exposed to reflow soldering should know that there are a total of four temperature zones for reflow soldering based on their functions: preheating zone, constant temperature zone, reflow soldering zone, and cooling zone. Below, JEENOCE will share the roles and impacts of the four temperature zones in reflow soldering.

1、 Effect and influence of reflow soldering preheating zone

In the reflow soldering preheating zone, the temperature is between 150 ℃ and 200 ℃. Component suppliers usually recommend using a heating rate of less than 2 ℃ per second to avoid thermal shock to components that are easily affected by temperature (such as ceramic sheet resistors). The purpose of this area is to heat the PCB at room temperature as soon as possible, in order to activate the solder paste and remove moisture and solvents from the solder paste, in order to prevent the solder paste from collapsing and splashing. To achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, it may cause thermal shock and damage to the circuit board and components; If it is too slow, the solvent evaporation is not sufficient, which affects the welding quality.

2、 Function and influence of reflow soldering insulation zone

The main purpose is to stabilize the temperature of the PCB board and various components in the reflow soldering furnace, and maintain consistent temperature of the components. Due to the varying sizes of components, larger components require more heat and slow heating, while smaller components heat up faster. Sufficient time should be given in the insulation area to allow the temperature of larger components to catch up with smaller components, allowing the flux to fully evaporate and avoiding bubbles during welding. At the end of the insulation section, oxides on the solder pads, solder balls, and component pins are removed by the action of the flux, and the temperature of the entire circuit board reaches equilibrium. Guangshengde reminds that all components in this temperature range should have the same temperature at the end of this section, otherwise various poor welding phenomena will occur in the reflux section due to uneven temperatures in various parts. The temperature rise must be slow (approximately 3 ° C per second) to limit boiling and splashing, prevent the formation of small tin particles, and some components are sensitive to internal stress. If the external temperature of the components rises too fast, it can cause fracture.

3、 Effect and influence of reflow soldering zone

The temperature of the heater in the reflow soldering area reaches its maximum, and the temperature of the components rapidly rises to its maximum temperature. In the reflow street section, the peak welding temperature varies with the different solder paste used. The peak temperature is generally between 210-230 ℃, and the reflow time should not be too long to prevent adverse effects on components and PCBs, which may cause circuit boards to be burnt. In this temperature range, if the temperature is too high, the circuit board may be burned or charred. If the temperature is too low, the solder joints will appear dull and granular. The peak temperature in this temperature range should be high enough to fully activate the flux and have good wetting properties. But it should not be high enough to cause damage, discoloration, or burning of components or circuit boards. For lead-free welding, the peak temperature in this temperature range should be between 230 ℃ and 245 ℃. The time above the liquidus (TAL) should be between 30 seconds and 60 seconds. If the temperature exceeds the melting point of the solder paste or the duration of the liquidus is too long, it can damage components that are susceptible to temperature effects. It can also lead to excessive bonding between metals, making solder joints brittle and reducing their fatigue resistance.

4、 Effect and influence of reflow soldering cooling zone

The cooling rate of the solder joint after reflow is also important. The faster the cooling rate, the smaller the crystal size of the solder paste, and the higher its fatigue resistance. Therefore, the faster the cooling rate, the better. Rapid cooling will help to obtain bright solder joints with a full shape and low contact angle. Slow cooling can cause more decomposition products of PAD to enter the tin, resulting in dull and rough solder joints, and even causing poor soldering and weak bonding strength.