Preheat PCB assembly before or during rework

Preheat PCB assembly before or during rework
Three methods of preheating PCB components before or during rework:
At present, the preheating methods of PCB components are divided into three categories: oven, hot plate and hot air trough. Before rework, it is effective to use the oven to preheat the substrate and reflow to disassemble the components. In addition, the preheating furnace uses baking to bake the internal moisture in some integrated circuits and prevent popcorn. The so-called popcorn phenomenon refers to that when the humidity of the reprocessed SMD equipment is higher than that of the normal equipment, when it is suddenly heated rapidly, microcracks will occur. The baking time of PCB in preheating furnace is long, generally about 8 hours.
One of the defects of preheating furnace is that it is different from hot plate and hot air trough. During warm-up, technicians cannot warm up and repair at the same time. In addition, it is not possible for the oven to cool the solder joints quickly.
Hot plate is the most effective way to preheat PCB. Since not all PCB components to be repaired are single-sided, it is rare for PCB components to be flat or flat on one side in today's world of mixed technology.

PCB components are usually installed on both sides of the substrate It is impossible to preheat these uneven surfaces with a hot plate
The second defect of the hot plate is that once solder reflow is achieved, the hot plate will continue to release heat to PCB components. This is because even after unplugging the power supply, the residual heat stored in the hot plate will continue to transfer to the PCB and hinder the cooling rate of solder joints. This resistance to solder joint cooling will lead to unnecessary lead precipitation, forming a lead pool, thus reducing and degrading the strength of solder joints.
The advantage of using the hot air slot to preheat is: the hot air slot does not consider the shape (and bottom structure) of the PCB component at all The whole PCB assembly is heated evenly and the heating time is shortened
Secondary cooling of solder joints in PCB components
As mentioned earlier, SMT's challenge to the rework of PCBA (printed circuit board assembly) is that the rework process should imitate the production process. The result is:
First, preheating PCB components before reflow is a necessary condition for successful production of PCBA; Secondly, it is also important to cool the components quickly after reflow soldering. These two simple processes have been ignored. However, preheating and secondary cooling are more important in through-hole technology and micro welding of sensing elements.
Common reflux devices, such as chain furnace and PCB components, enter the cooling area immediately after passing through the reflux area. When PCB components enter the cooling area, in order to achieve rapid cooling, it is very important to ventilate PCB components. Usually, rework is combined with the production equipment itself.
Slow cooling of PCB components after reflow soldering will lead to unnecessary lead rich liquid pool in liquid solder, which will reduce the strength of solder joints. However, the use of rapid cooling can prevent lead precipitation, resulting in a tighter grain structure and stronger solder joints.
In addition, cooling solder joints faster will reduce a series of quality problems caused by unexpected movement or vibration of PCB components during reflow. For production and rework, another advantage of secondary cooling PCB assembly is to reduce the possibility of dislocation and tombstone of small patches.
Summary
The secondary cooling has many advantages. The SMT chip is processed during the correct preheating and reflow of PCB components These two simple steps need to be included in the maintenance work of technicians In fact, when preheating PCB, technicians can make other preparations at the same time, such as coating solder paste and flux on PCB