After completing the PCBA inspection, the defective PCBA needs to be repaired. The company has two methods to repair the PCBA of SMT. One is to repair with constant temperature soldering iron (manual welding) and the other is to repair with repair workbench (hot air welding). No matter which method is adopted, it is required to form good welding points in the shortest time. Therefore, when using soldering iron, it is required to complete the welding point in less than 5 seconds, preferably about 3 seconds.
5.1 Ferrochrome Repair Method Manual Welding
Treatment of new soldering iron before use: the new soldering iron can be used normally only after a layer of solder is coated on the soldering iron head before use. After the soldering iron is used for a period of time, an oxide layer is generated on the edge and around the soldering iron head, which makes it difficult to “eat tin”. At this time, the oxide layer can be filed off and the solder can be plated again.
How to hold the soldering iron:
- Reverse grip: Use five fingers to hold the handle of the soldering iron in your palm. This method is suitable for high-power electric soldering irons, welding parts with large heat dissipation.
- Positive grip: In addition to the thumb, four fingers hold the electric iron handle, and the thumb compresses along the direction of the electric iron. The electric iron used in this method is also relatively large, and most of them are curved iron heads.
- Pen holding method: Holding electric soldering iron like holding pen, suitable for low-power electric soldering iron and welding small weldments. Our company adopts the pen holding method.
Precautions for Welding:
- The temperature of the soldering iron tip should be appropriate. Putting the soldering iron tip of different temperature on the rosin block will produce different phenomena. Generally speaking, the temperature when the rosin melts faster and does not emit smoke is more suitable.
- The welding time shall be appropriate. Generally, it shall be completed within a few seconds from heating the welding point to melting and filling the welding point with solder. If the welding time is too long, the flux on the welding point will completely volatilize and lose the role of welding aid. If the welding time is too short, the temperature of the welding point cannot reach the welding temperature, and the solder cannot be fully melted, which is easy to cause false welding.
- The use of solder and flux should be appropriate. Generally, too much or too little solder and flux on the welding point will have a great impact on the welding quality.
- Prevent the solder from flowing freely on the welding point. The ideal welding should be that the solder is only welded where it needs to be welded. In the welding operation, the solder shall be less at the beginning. When the welding point reaches the welding temperature and the solder flows into the gap of the welding point, the solder shall be supplemented to complete the welding quickly.
- Do not touch the welding point during welding. When the solder on the welding point has not fully solidified, do not move the welded devices and wires on the welding point, otherwise the welding point will be deformed and false welding will occur.
- Do not scald the surrounding components and wires. When welding, pay attention not to make the electric soldering iron scald the plastic insulation layer of the surrounding wires and the surface of components, especially the products with compact welding structure and complex shape.
- Timely clean up after welding. After welding, the cut wire head and tin slag falling during welding shall be removed in time to prevent hidden dangers from falling into the product.
- Repair of worktable
The repair workbench is mainly used when the defects of QFP, BGA, PLCC and other components cannot be repaired manually. It usually uses the hot air heating method to heat the welding feet of components, but it must cooperate with the corresponding nozzles. The heating zone of the higher-level repair worktable can make a temperature curve similar to that of the reflux furnace, such as the company’s smd-1000 repair worktable. For the operation of the repair workbench, please refer to the manual.