2. Another factor limiting the reduction of reflow temperature during rework is the industry standard requirement that the temperature of surrounding components must not exceed 170°C. Therefore, the reflow temperature during rework should be compatible with both the size of the PCB assembly and the components being reflowed. Since rework essentially involves localized heating of the PCB, the process imposes a limit on the overall repair temperature of the board. The localized heating during rework must be higher than the typical temperature used during production to compensate for the heat absorption of the entire assembly.
3. In conclusion, there is insufficient justification to claim that the rework temperature for the entire board should never exceed the reflow soldering temperature used during production, as this would allow for temperatures closer to the ideal range recommended by semiconductor manufacturers.
4. Three methods for preheating PCB components before or during rework:
1. **Preheating Methods for PCB Components**
Nowadays, methods for preheating PCB components are typically categorized into three types: oven, hot plate, and hot air slot. Using an oven to preheat the substrate before rework or reflow soldering is effective for disassembling components. Additionally, the preheating oven serves to bake off internal moisture from certain integrated circuits, helping to prevent the so-called “popcorn” effect. This phenomenon refers to micro-cracking that occurs when the humidity level in the reworked SMD device is higher than normal, and it is suddenly exposed to a rapid temperature rise. The preheating time in the oven is generally long, typically around 8 hours.
2. **Disadvantages of the Preheating Oven**
One of the downsides of the preheating oven, compared to the hot plate and hot air slot, is that during the preheating phase, it is impractical for technicians to simultaneously preheat and repair the PCB. Moreover, the oven is unable to quickly cool the solder joints once the rework process is complete.
3. **Limitations of the Hot Plate**
The hot plate is considered the least effective method for preheating PCB components. This is because, in today’s mixed-technology environment, most PCBs are not single-sided. Components are typically mounted on both sides of the substrate, making it difficult for a hot plate to uniformly heat these uneven surfaces.
4. **Residual Heat from the Hot Plate**
Another drawback of the hot plate is that, once solder reflow is achieved, it continues to release residual heat to the PCB assembly. Even after unplugging the hot plate, the heat stored in the plate continues to transfer to the PCB, which slows the cooling rate of the solder joints. This delayed cooling can cause unnecessary lead precipitation, forming a lead-rich liquid pool that weakens the solder joint’s strength.
5. **Advantages of the Hot Air Slot**
The hot air slot offers a significant advantage for preheating. Unlike the hot plate, it does not rely on the shape or bottom structure of the PCB components. Hot air can easily reach all areas of the PCB, including corners and cracks. This results in more uniform heating across the entire PCB assembly and reduces the overall heating time.
6. **Secondary Cooling of Solder Joints**
As previously mentioned, one of the challenges in SMT rework is replicating the production process. It has been proven that preheating PCB components before reflow is crucial for successful PCBA production. Equally important is the rapid cooling of components immediately after reflow. Unfortunately, these two essential processes are often overlooked, even though they are particularly critical in through-hole technology and micro-welding of sensitive components.
7. **Cooling in Reflow Equipment**
In common reflow equipment, such as chain furnaces, PCB components enter the cooling zone right after passing through the reflow zone. Achieving rapid cooling is vital, and proper ventilation of the PCB components is essential to this process. Typically, rework equipment is integrated with the production equipment itself.
8. **Impact of Cooling on Solder Joint Strength**
If the cooling process is too slow after reflow, it can result in unwanted lead-rich liquid pools within the solder, weakening the solder joints. On the other hand, rapid cooling prevents lead precipitation, promoting a tighter grain structure and stronger solder joints.
9. **Advantages of Rapid Cooling**
Faster cooling also helps reduce quality issues that may arise due to accidental movement or vibration of PCB components during reflow. For both production and rework, secondary cooling minimizes the risks of misalignment or tombstone defects, particularly with small SMDs.
