PCBA Tech - Preheating of PCB components before or during rework

Three methods for preheating 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.

The hot plate is the most ineffective way to preheat the PCB. Because the PCB components to be repaired are not all single-sided, in today's world of mixed technology, it is indeed rare for PCB components to be flat or flat on one side.

PCB components are generally installed on both sides of the substrate. It is impossible to preheat these uneven surfaces with hot plates.

The second defect of the hot plate is that once the solder reflow is achieved, the hot plate will continue to release heat to the PCB assembly. This is because even after the power is unplugged, the residual heat stored in the hot plate will continue to be transferred to the PCB and hinder the cooling rate of the solder joints. This obstruction to the cooling of the solder joint will cause unnecessary precipitation of lead to form a lead liquid pool, which reduces and deteriorates the strength of the solder joint.

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, and the hot air can directly and quickly enter all corners and cracks of the PCB component. The entire PCB assembly is heated evenly, and the heating time is shortened.

Secondary cooling of solder joints in PCB 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:

First, preheating the PCB components before reflow is necessary for the successful production of PCBA; second, it is also very important to quickly cool the components immediately after reflow. And these two simple processes have been ignored by people. However, preheating and secondary cooling are more important in through-hole technology and micro-welding of sensitive components.

Common reflow equipment such as chain furnace, PCB components enter the cooling zone immediately after passing through the reflow zone. As the PCB components enter the cooling zone, in order to achieve rapid cooling, it is very important to ventilate the PCB components. Generally, rework is integrated with the production equipment itself.

The slow cooling of PCB components after reflow will cause unwanted lead-rich liquid pools in the liquid solder to be produced, which will reduce the strength of the solder joints. However, the use of rapid cooling can prevent the precipitation of lead, make the grain structure tighter and the solder joints stronger.

In addition, faster cooling of solder joints will reduce a series of quality problems caused by accidental movement or vibration of PCB components during reflow. For production and rework, reducing the possible misalignment and tombstone phenomena of small SMDs is another advantage of secondary cooling PCB components.

Summarize

There are many benefits of secondary cooling PCB components during correct preheating and reflow of SMT chip processing. 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 PCB.