In SMT circuit board assembly process, two key crafts for PCB board repair(2)

5.Three methods for preheating PCB components before rework or during repair:

Today, preheating PCB components methods fall into three categories: ovens, hot plates, and hot air grooves. It is effective to use an oven to preheat the substrate before reworking and reflow soldering to remove components. Moreover, the use of baking is an advantageous method in preheating ovens to bake out internal moisture in some integrated circuits and to prevent popcorn. The so-called popcorn phenomenon refers to the micro-cracking that occurs when the reworked SMD compoent is higher in humidity than that of the normal device when it is suddenly subjected to rapid temperature rise. The baking time of the PCB in the preheating oven is long, generally up to 8 hours in smt circuit board assembly process.

The hot plate is the most ineffective way to preheat the PCB. Because the PCB components to be repaired are not all single-sided, today is the world of hybrid technology, and it is indeed rare for all-round flat or planar PCB components. The PCB components are usually mounted on both sides of the substrate. It is impossible to preheat these uneven surfaces with hot boards in smt circuit board assembly process.

The second drawback of the hot board is that once the solder reflow is achieved, the hot board will continue to release heat to the PCB components. This is because, even after the power is removed, there is still stored residual heat in the hot plate that continues to be conducted to the PCB, which hinders the cooling rate of the solder joint. This hindrance of solder joint cooling causes unnecessary lead precipitation to form a lead liquid pool, which causes the solder joint strength to decrease and deteriorate in smt circuit board assembly process.

The advantage of using hot air tank preheating is that the hot air tank does not consider the shape of the PCB components (and the bottom structure), and the hot air can enter the corners and cracks of the PCB components directly and quickly. The entire PCB components are heated evenly and the heating time is shortened.

6. Secondary cooling of solder joints in PCB components

As mentioned earlier, the challenge for SMT to rework PCBA (printed board assembly) is that the rework craft should mimic the manufacturing craft. It turns out that: First, preheating the PCB assembly before reflow is necessary for the successful production of PCBA; second, it is also important to quickly cool the component immediately after reflow. These two simple processes have been ignored. However, pre-heating and secondary cooling are more important in through-hole technology and micro-welding of sensitive components in smt circuit board assembly process.

Common reflow equipment such as chain furnaces, the PCB components enter the cooling zone immediately after passing through the reflow zone. As the PCB components enter the cooling zone, it is important to ventilate the PCB components for rapid cooling. The general rework is integrated with the production equipment itself.

Slowing down the cooling of the PCB components after reflow can cause unneeded lead-rich baths in the liquid solder to cause a decrease in solder joint strength. However, the use of rapid cooling prevents the precipitation of lead, making the grain structure tighter and the solder joints stronger in smt circuit board assembly process.

In addition, cooling the solder joints faster will reduce the number of quality issues associated with accidental movement or vibration of the PCB components during reflow. For production and rework, reducing the potential for misalignment and tombstones phenomenon in small SMD is another advantage of secondary cooling PCB components in smt circuit board assembly process.