Temperature and time setting of reflow 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 we share the temperature and time setting of reflow soldering furnace.

Reflow preheating temperature and 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 at 80 ~ 160 ℃ is 60 ~ 120sec, which can effectively remove the volatile solvent in the solder paste, reduce the thermal impact on the components, fully activate the flux and reduce the temperature difference. • Temperature rise rate of preheating section: in terms of heating stage, the temperature range is between room temperature and melting point temperature, and the slow rise rate 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 and time setting of reflow uniform 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 area and other welding defects, such as component warping, cold welding of some large components, etc. Another important role in the soaking stage is that the flux in the solder paste begins to have an active reaction to increase 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, the 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 retain 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 and 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 cannot 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, its 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. This time limit requires a fast temperature rise rate, The rise 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 complete good welding, 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 the thickening of intermetallic compound layer and affect the long-term reliability of solder joint.

Reflow soldering cooling temperature and 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 allowable maximum 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 to make the circuit assembly close to the temperature set in this zone. The total processing time is determined by the sum of the duration of each zone. Grasp the principle of internal temperature stress change of components, that is, the change of heating temperature is less than 3 ℃ per second and the cooling temperature drop rate is less than 5 ℃.