Analysis of exposed copper in hot air leveling process
Summary of content: 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. The lead-tin alloy coating on the surface of the printed circuit board after hot-air leveling should be bright, uniform and complete, with good solderability, no nodules and no semi-wetting, and no copper exposure. Copper exposed on the pad surface and metallized holes after hot air leveling is an important defect in finished product inspection. It is one of the common causes of hot air leveling 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 to contaminate the pad.
At present, most of the manufacturers use full-screen screen printing of liquid photosensitive solder resist ink, and then remove excess solder resist through exposure and development to obtain a time-resistant solder resist pattern. In this process, the pre-baking process is not well controlled. If the temperature is too high, it will cause development difficulties. Are there any defects on the solder mask, the composition and temperature of the developer, the speed of development, that is, whether the development point is correct, whether the nozzle is clogged and the pressure of the nozzle is normal, and whether the washing is good, any of which will be given to the pad Leave a residue. For example, the exposed copper formed due to the negative film is generally more regular and is at the same point. In this case, using a magnifying glass, you can find residual traces of solder resist in the exposed copper. Generally, a post should be set up to inspect the interior of the pattern and metallized hole before the curing process to ensure the soldering of the printed circuit board to the next process. Clean solder mask-free ink residue in the plate and metallized holes.
2. Insufficient pre-treatment and poor roughening.
The quality of the hot air leveling pretreatment process has a great impact on the quality of hot air leveling. This process must completely remove the oil, impurities and oxide layers on the pads to provide a fresh solderable copper surface for immersion tin. Currently, the more commonly used pre-treatment process is mechanical spraying. First, sulfuric acid-hydrogen peroxide micro-etching, acid etching after micro-etching, and then water spray washing, hot air drying, and fluxing, immediately hot air leveling. The phenomenon of exposed copper caused by poor pretreatment occurs in large numbers at the same time regardless of the type of batch. The exposed copper points are often distributed throughout the board surface, and are even more serious on the edges. Observing the pre-processed circuit board with a magnifying glass will find obvious oxidation spots and stains on the pads. In a similar situation, a chemical analysis of the micro-etching solution should be performed, the second pickling solution should be checked, the concentration of the solution should be adjusted, and the solution that has been contaminated due to excessive use of time should be checked. Properly extending the treatment time can also improve the treatment effect, but it should be noted that over-corrosion may occur. The reworked circuit boards are leveled by hot air and the treatment line is then treated in a 5% hydrochloric acid solution to remove surface oxides.
3. The flux is not sufficiently active
The role 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 sufficiently active and the copper surface has poor wettability, the solder cannot completely cover the pads. The copper exposure phenomenon is similar to the poor pretreatment. Extending the pretreatment time can reduce the copper exposure phenomenon. Almost all current fluxes are acidic fluxes, which contain acidic additives. If the acidity is too high, it will cause copper bite, which will cause the copper content in the solder to be high and lead to lead and tin rough. If the acidity is too low, the activity will be weak and it 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 that has an important impact on hot air leveling. The best flux is to ensure the quality of hot air leveling.
In addition, other parameters also affect hot air leveling, uneven flux coating, low solder level, incorrect dipping time, poor wind and wind pressure adjustment, and the position of the blade can cause hot air leveling and copper exposure. . This problem is more intuitive and clear to find and solve. Workers' first inspection and inspection at any time during the operation, timely feedback of problems, and timely analysis of the causes by process technicians can greatly reduce the rework rate, provide product quality, and minimize the phenomenon of copper roads.