What is the ideal temperature curve, do you know
Before starting the temperature setting, you must have a basic understanding of the ideal temperature curve. Theoretically an ideal curve consists of four parts or sections. The first three sections are heated and the last section is cooled. The more temperature areas of the reflow soldering furnace, the more accurate and close the profile of the temperature curve can be. Most solder pastes can be successfully reflowed with four basic temperature zones.
The preheating zone (also called the ramp zone) of the reflow soldering furnace generally occupies 25%~33% of the entire heating channel length. It is used to increase the temperature of the PCB from the ambient temperature to the required active (holding) temperature. In this area, the temperature of the product continuously rises at a rate of no more than 2C~51 per second. If the temperature rises too fast, it will cause some defects, such as micro-cracks in ceramic capacitors. If the temperature rises too slowly, the solder paste will over-sensitize. There is not enough time for the PCB to reach the active temperature.
The active area (also called heat preservation area, dry or soaking area) generally accounts for 33% to 50% of the heating channel. It has two functions. The first is to sense the temperature of the PCB at a fairly stable temperature, allowing components of different quality Homogeneous in temperature, reducing their considerable temperature difference. The second is to allow the flux to be activated, and volatile substances volatilize from the solder paste. The general active temperature range is 120r~150t. If the temperature of the active zone is set too high, the flux will not have enough time to activate, and the slope of the temperature curve is an upwardly increasing slope. Although some solder paste manufacturers allow some temperature increase during activation, the ideal curve requires a fairly stable temperature so that the temperature of the PCB is equal at the beginning and the end of the active zone. Some reflow soldering furnaces cannot maintain a flat activation temperature curve. Choosing a reflow soldering furnace that can maintain a flat activation temperature curve will improve the weldability and users have a larger processing window.
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The function of the reflow zone (also called the peak zone or the last heating zone) is to increase the temperature of the PCB assembly from the active temperature to the recommended peak temperature. The activation temperature is always a little lower than the melting point of the alloy, and the peak temperature is always at the melting point. The typical peak temperature range is 205V~230C. Setting the temperature in this zone too high will cause the temperature rise slope to exceed 2C~51 per second, or the peak reflux temperature reached is higher than the recommended. This situation may cause excessive curling, delamination or burnout of the PCB, and damage the integrity of the components.
At present, the most commonly used alloy is Sn63/Pb37. This ratio of tin and lead makes the alloy eutectic. Eutectic alloys are alloys that melt at a specific temperature. Non-eutectic alloys have a melting range instead of a melting point, sometimes called a plastic state. All the examples described here refer to eutectic tin/lead, which has a melting point of 183X3.
The ideal cooling zone curve should be a mirror image relationship with the reflux zone curve. The closer to this mirror image relationship, the tighter the solder joints reach the solid structure, the higher the quality of the solder joints and the better the integrity of the joints.
