Why copper is exposed in the hot air level process of rigid-flex PCB

Hot air leveling is to immerse the printed circuit board into the molten solder, and then blow off the excess solder on its surface and in the metallization hole with hot air, so as to obtain a smooth, uniform and bright solder coating with good weldability and completely free of exposed copper. Copper exposure on the surface of the pad and in the metallization hole after hot air leveling is an important defect in the finished product inspection, and is one of the common reasons for hot air leveling rework. So, what are the reasons for the hot air leveling and copper exposure of soft and hard composite plates?

1. Insufficient pretreatment and poor coarsening. The quality of the hot air leveling pretreatment process has a great impact on the quality of the hot air leveling. This process must completely remove the greasy dirt, impurities and oxide layer on the bonding pad to provide fresh and solderable copper surface for tin dipping. Now the more commonly used pretreatment process is mechanical spraying. Copper exposure caused by poor pretreatment occurs in a large number, and copper exposure points are often distributed throughout the board, especially on the edge. In case of similar situations, chemical analysis should be carried out on the micro etching solution, check the second pickling solution, adjust the concentration of the solution, replace the solution with serious pollution due to long use time, and check whether the spray system is smooth. Appropriately extending the treatment time can also improve the treatment effect.

2. The surface of the pad is dirty, and there is residual solder resist polluting the pad. Most manufacturers use full board screen printing liquid photosensitive solder resist ink, and then remove the excess solder resist through exposure and development to obtain the solder resist pattern of time. Whether there are defects on the solder mask, whether the composition and temperature of the developer are correct, and whether the development speed, that is, the development point, is correct. Any of these conditions will leave residues on the pad. In the design of soft and hard bonding board, a post should be set up to check the graphics and the inside of the metallized hole before the curing process, so as to ensure that the solder pad and metallized hole of the printed circuit board sent to the next process are clean and free of solder resist ink residue.

3. The flux is not active enough. The function of flux is to improve the wettability of copper surface, protect the laminate surface from overheating, and provide protection for solder coating. If the activity of the flux is not enough and the wettability of the copper surface is not good, the solder can not completely cover the pad, and the copper exposure phenomenon is similar to the poor pretreatment. Prolonging the pretreatment time can reduce the copper exposure phenomenon. The selection of a stable and reliable flux by process technicians has an important impact on hot air leveling, and excellent flux is the guarantee of hot air leveling quality.