PCBA Tech - Preheating of PCB components before or during rework

Nowadays, the methods of preheating PCB components are divided into three categories: oven, hot plate and hot air slot. It is effective to use an oven to preheat the substrate before rework and reflow soldering to disassemble the components.Moreover,the preheating oven uses baking to bake off internal moisture in some integrated circuits and prevent popcorn. The so-called popcorn phenomenon refers to the micro-cracking that occurs when the humidity of the reworked SMD device is higher than that of the normal device when it is suddenly subjected to a rapid temperature rise. The baking time of PCB in the preheating oven is longer, generally as long as about 8 hours.

One of the defects of the preheating oven is that it is different from the hot plate and the hot air trough.During the preheating,it is not feasible for a technician to preheat and repair at the same time. Moreover, it is impossible for the oven to quickly cool the solder joints.

Hotplates are the least effective way to preheat a PCB substrate.This is because not all electronic components to be repaired are one-sided, and in today's world of mixed technology,it is really rare for a component to be flat or have one flat surface.Preheating these uneven surfaces with a hot plate is not possible.

The second drawback of hotplates is that once solder reflow is achieved,the hotplate continues to release heat to the PCB assembly. This is because even when unplugged, the residual heat stored in the hotplate continues to transfer to the PCB,impeding the rate of solder joint cooling.This impediment to solder joint cooling can lead to unwanted lead precipitation and the 

formation of lead pools,which can reduce and deteriorate the strength of the solder joint.

The advantage of using a hot-air bath for preheating is that the hot-air bath is completely independent of the shape (and substructure) of the PCB component, and the hot air can be directed directly and quickly into the nooks and crannies of the electronic component.

Secondary cooling of solder joints in electronic components

As mentioned earlier, the challenge of SMT for PCBA (printed board assembly) rework is that the rework process should imitate the production process. It turns out:

Preheating PCB components before reflow is essential for successful PCBA production; secondly,it is also important to quickly cool the components immediately after reflow. These two simple processes have been neglected. However, preheating and secondary cooling are even more important in through-hole technology and in the microsoldering of sensitive components.

With common reflow equipment such as chain ovens, PCB components pass through the reflow zone and immediately enter the cooling zone. When electronic components enter the cooling zone, ventilation of the electronic components is very important in order to achieve rapid cooling.

Slow cooling after reflow soldering of electronic components can lead to unwanted pools of lead-rich liquid in the liquid solder, which reduces the strength of the solder joint.Rapid cooling, on the other hand, prevents lead precipitation,resulting in a tighter grain structure and stronger solder joints.

Additionally, faster solder joint cooling reduces a number of quality issues caused by accidental movement or vibration of electronic components during reflow.Reducing the misalignment and tombstoning that can occur with small SMDs is another advantage of secondary cooling electronics for production and rework.

There are many benefits of secondary cooling pcb components during correct preheating and reflow of SMT process.These two simple procedures need to be included in the repair work of technicians.In fact,when preheating the PCB,the technician can do other preparations at the same time, such as applying solder paste and flux on the printed circuit board.