PCB Technical - Analysis of exposed copper in hot air leveling PCB process

Hot air leveling is to immerse the printed circuit board in molten solder (63SN/37PB), and then use hot air to blow off the excess solder on the surface of the printed circuit board and the metallized holes to obtain a smooth, uniform and bright solder coating Cladding. The lead-tin alloy coating layer on the surface of the printed circuit board after hot air leveling should be bright, uniform and complete, with good solderability, no nodules, no semi-wetting, and no exposed copper in the coating. The exposed copper on the surface of the pad and the metallized hole after hot air leveling is an important defect in the finished product inspection, and it is one of the common causes of hot air leveling and rework. There are many reasons for this problem, and the following are common.

1. The surface of the pad is dirty, and there is residual solder resist contaminating the pad.

At present, most PCB manufacturers use full-board screen printing liquid photosensitive solder resist ink, and then remove excess solder resist through exposure and development to obtain a time-based solder resist pattern. In this process, the pre-bake process is not well controlled, and the temperature is too high and the time is too long will cause development difficulties. Whether there are defects on the solder mask film, whether the composition and temperature of the developer are correct, whether the developing speed is correct, whether the nozzle is clogged, whether the nozzle pressure is normal, whether the water washing is good, any of these conditions will be on the pad Leave residual points. For example, the exposed copper formed due to the negative film is generally more regular, all at the same point. In this case, a magnifying glass can be used to find residual traces of solder resist materials at the exposed copper.

Generally, a post should be set up to inspect the graphics and the inside of the metallized hole before the curing process to ensure that the printed circuit board is soldered to the next process. The disc and metallized holes are clean and free of solder mask ink residue.

2. Insufficient pretreatment and poor coarsening.

The quality of the pre-treatment process of hot air leveling has a great influence on the quality of hot air leveling. This process must completely remove the oil, impurities and oxide layer on the pad to provide a fresh solderable copper surface for immersion tin. The more commonly used pre-treatment process is mechanical spraying. Firstly, sulfuric acid-hydrogen peroxide micro-etching, acid pickling after micro-etching, then water spray rinsing, hot air drying, spraying flux, and immediately hot air leveling. The copper exposed phenomenon caused by poor pre-processing occurs in large numbers at the same time regardless of type and batch. The exposed copper points are often distributed over the entire board surface, and are even more serious on the edges. Using a magnifying glass to observe the pre-processed circuit board will find that there are obvious residual oxidation spots and stains on the pads. If a similar situation occurs, chemical analysis of the microetching solution should be performed, the second pickling solution should be checked, the concentration of the solution should be adjusted, and the solution with serious pollution due to long time use should be replaced, and the spray system should be checked whether it is unblocked. Properly prolonging the treatment time can also improve the treatment effect, but it is necessary to pay attention to the over-corrosion phenomenon. The reworked circuit boards are flattened by hot air and then treated in a 5% hydrochloric acid solution to remove surface oxides.

3. Insufficient flux activity

The function of the flux is to improve the wettability of the copper surface, protect the laminate surface from overheating, and provide protection for the solder coating. If the flux is not active enough and the wettability of the copper surface is not good, the solder will not be able to completely cover the pad. The copper exposure is similar to the poor pretreatment. Extending the pretreatment time can reduce the copper exposure. Almost all current fluxes are acidic fluxes, which contain acidic additives. If the acidity is too high, it will cause serious copper biting, which will cause the high copper content in the solder to cause rough lead and tin; if the acidity is too low, the activity will be weak, which will cause exposure. copper. If the copper content in the lead-tin bath is large, remove the copper in time. Process technicians choose a flux with stable and reliable quality to have an important influence on hot air leveling, and good flux is the guarantee of the quality of hot air leveling.

In addition, other parameters also have an impact on hot air leveling. Uneven flux coating, low solder level, incorrect dipping time, poor adjustment of wind and air pressure, air knife position and distance, etc., may cause problems with hot air leveling and exposed copper. . This problem is more intuitive, clear and easy to find and solve. The first inspection and inspection of the products by workers during operation, timely feedback of problems, timely analysis of the causes by PCB process technicians and timely solutions can greatly reduce the rework rate, provide product quality, and minimize the copper phenomenon.