Temperature and time setting of reflow soldering furnace

Reflow time and temperature are the main factors that determine the reflow quality. If the reflow temperature is constant, the speed of reflow time determines the main factor of reflow quality. If the time is too fast or too slow, a large number of bad reflow products will be produced. Here, I would like to share with you the temperature and time settings of the reflow soldering furnace.

Reflow soldering preheating temperature time setting:

It depends on the part with the largest heat capacity on the PCB, PCB area, PCB thickness and the performance of the solder paste used. Generally, the preheating period is 60 ~ 120sec at 80 ~ 160 ℃, so as to effectively remove the volatile solvent in the solder paste, reduce the thermal shock to the components, fully activate the flux and reduce the temperature difference Temperature rise rate of preheating section: for the heating stage, the slow rise rate between room temperature and melting point temperature is expected to reduce most defects. For the best curve, a slow rise rate of 0.5 ~ 1 ℃ /sec is recommended. For the traditional curve, it is better to raise the temperature below 3 ~ 4 ℃ /sec.

Temperature time setting of reflow soldering average temperature:

The purpose of reflow soldering is to make the whole PCB reach a uniform temperature (about 175 ℃). The purpose of soaking is to reduce the thermal stress impact entering the reflow zone and other welding defects, such as component warping, cold welding of some large components, etc. Another important role of soaking stage is that the flux in the solder paste begins to react actively, increasing the wettability (and surface energy) of the weldment surface, so that the melted solder can well wet the weldment surface. Due to the importance of soaking section, soaking time and temperature must be well controlled to ensure that the flux can clean the welding surface well and that the flux is not completely consumed before it reaches reflow. It is necessary to keep the flux until the reflow stage. It can promote the solder wetting process and prevent the reoxidation of the welding surface. In particular, with the increasing use of low residue, no clean solder paste technology, the activity of solder paste is not very strong, and the reflow soldering is mostly air reflow soldering. It should be noted that the flux should not be consumed in the soaking stage.

Temperature and time settings for reflow soldering:

Due to factors such as the formation rate of common boundary metal compounds and the decomposition rate of base metals in solder, their generation and filtration are not only proportional to the temperature, but also proportional to the time above the melting point temperature of solder. In order to reduce the generation and filtration of common boundary metal compounds, the time above the melting point temperature must be reduced, which is generally set between 45 ~ 90 seconds. A fast temperature rise rate is required for this time limit, The rising rate must be between 2.5 ~ 3.5 ℃ /see, and the maximum change rate shall not exceed 4 ℃ /sec.

The time in the reflow zone should be as short as possible on the premise of ensuring that the components are well welded, generally 30-60 seconds is the best. Too long reflow time and high temperature, such as reflow time greater than 90 seconds and maximum temperature greater than 230 degrees, will cause thickening of the intermetallic compound layer and affect the long-term reliability of the solder joint.

Reflow soldering cooling temperature time setting:

Rapid cooling of reflow soldering will lead to too high temperature gradient between components and substrate, resulting in mismatch of thermal expansion, resulting in splitting of solder joint and pad and deformation of substrate. Generally, the maximum allowable cooling rate is determined by the tolerance of components to thermal shock. The cooling rate in the cooling area is generally about 4 ℃ /s, and it can be cooled to 75 ℃.

The belt speed determines the duration of the board exposed to the temperature set in each zone. Increasing the duration allows more time for the circuit assembly to approach the temperature set in that zone. The total processing time is determined by the total duration of each zone. Grasp the principle of internal temperature stress change of components, that is, the heating temperature change is less than 3 ℃ per second and the cooling temperature drop rate is less than 5 ℃.