Analysis of copper exposure in hot air leveling process

- Nov 03, 2018-

Hot air leveling is to dip the printed circuit board into the molten solder (63SN/37PB), and then use hot air to blow away the excess solder on the surface of the printed circuit board and in the metallized hole to obtain a smooth, uniform and shining solder coating.The surface of printed circuit board should be bright, uniform and complete, with good weldability, no nodules, no semi-wetting, and no exposed copper coating.The exposed copper on the surface of the welding plate and the metallized hole in the hot air leveling is an important defect in the inspection of finished products, and is one of the common causes of hot air leveling and rework.

1. Insufficient pretreatment and poor coarsening.

The quality of PCB board before hot air leveling has a great influence on the quality of hot air leveling. The process must thoroughly remove oil, impurities and oxide layers on the pad to provide fresh solvable copper surface for tin soaking.Now more commonly used before the process is mechanical spray, first sulfuric acid - hydrogen peroxide micro-etching, micro-corrosion after immersion acid, and then water spray rinsing, hot air dry, spray flux, immediately hot air leveling.The copper exposure caused by improper pretreatment is not divided into different types and lots of occurrence at the same time. The copper exposure point is usually distributed throughout the board surface, and it is even more serious on the edge.Using a magnifying glass to observe the pre - treated circuit board will find significant residual oxidation spots and stains on the pad.In case of similar situation, chemical analysis should be carried out for the microetching solution, check the second pickling solution, change the concentration of the solution to replace the solution seriously polluted due to too long time, and check whether the spray system is smooth.Proper extension of treatment time can also improve treatment effect, but it is necessary to pay attention to the phenomenon of over-corrosion. The reworked circuit board is treated in 5% hydrochloric acid solution after hot air leveling to remove the oxide on the surface.

2. The surface of the welding pad is not clean, and there is residual flux contamination on the welding pad.

At present, most of the manufacturers use full-plate screen printing liquid photoresist soldering ink, and then through exposure, development to remove the excess resistance flux, time resistance welding graphics.In this process, the pre-baking process is not well controlled, the temperature is too high and the time is too long will cause the difficulty of developing.Defects on the solder block film, correct composition and temperature of the developer, correct velocity of the development point, nozzle blockage and pressure of the nozzle, good washing, any of which will leave a residue on the pad.As a result of the film of the cause of exposure copper is generally more regular, all at the same point.In this case, a magnifying glass can be used to find traces of solderable substances in the exposed copper. PCB design generally should set up a post to check the inside of the graph and metallization hole before curing, so as to ensure the clean solderable ink remains in the welding pad of printed circuit board and metallization hole of the next process. 

3. Insufficient flux activity

The function of the flux is to improve the wettability of the copper surface, to protect the laminate surface from overheating, and to provide protection for the solder coating.If the flux is not active enough and the surface of the copper is not wettable, the solder can not fully cover the pad. The copper exposure is similar to the poor pretreatment, and prolonged pretreatment time can reduce the copper exposure.The current flux is almost entirely acidic and contains acidic additives, such as too acidic will lead to serious copper bite, resulting in high copper content in the solder lead tin rough;Acidity is too low, then activity is weak, can bring about dew copper.If the copper content in the lead tin tank should be removed in time.Process technicians choose a stable and reliable flux to have an important effect on hot air leveling.