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Discussion on Copper Exposed in FPC Hot Air Leveling Process

After FPC hot air leveling, the exposed copper on the pad surface and in the metallized hole is an important defect in the finished product inspection, which is one of the common reasons for hot air leveling rework. There are many reasons for this problem, including the following.

  1. Insufficient flux activity

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. FPC circuit board process technicians choose a stable and reliable flux which has an important impact on the hot air leveling. The excellent flux is the guarantee of the hot air leveling quality. If the activity of the flux is not enough and the wettability of the copper surface is not good, the solder cannot completely cover the pad, and the copper exposure is similar to the poor pre-treatment. Prolonging the pre-treatment time can reduce the copper exposure. The current fluxes are almost all acidic fluxes. If the acidity is too high, it will cause serious copper biting, which will lead to high copper content in the solder and lead tin roughness; If the acidity is too low, the activity will be weak, which will lead to copper exposure. If the copper content in the lead tin bath is large, remove the copper in time.

  1. Insufficient pre-treatment and poor coarsening.

The quality of hot air leveling before FPC plate making has a great impact on the quality of hot air leveling. In this process, the oil, impurities and oxide layer on the bonding pad must be completely removed to provide a fresh solderable copper surface for tin dipping. At present, the commonly used pre-treatment process is mechanical spraying. First, sulfuric acid hydrogen peroxide micro etching, pickling after micro etching, and then water spraying washing, hot air drying, spraying flux, and immediate hot air leveling. Copper exposure caused by poor pre-treatment occurs in large numbers at the same time regardless of type and batch. Copper exposure points are often distributed on the whole board, especially on the edge. Using a magnifying glass to observe the pretreated PCB, it will be found that there are obviously residual oxidation spots and stains on the pad. In case of similar situations, chemical analysis shall be carried out for the micro etching solution, the second pickling solution shall be checked, the concentration of the solution shall be adjusted, and the solution that is seriously polluted due to too long use time shall be replaced, and the spraying system shall be checked for smoothness. Proper extension of the treatment time can also improve the treatment effect, but attention should be paid to the over corrosion phenomenon that may occur. After the reworked circuit board is leveled by hot air, the treatment line should be treated in 5% hydrochloric acid solution to remove the oxides on the surface.

  1. The pad surface is not clean, and there is residual solder resistance polluting the pad.

At present, most FPC manufacturers use full screen printing liquid photosensitive solder mask ink, and then remove the excess solder mask through exposure and development to obtain the solder mask pattern of time. In this process, poor control of the pre baking process and excessive temperature for too long will cause difficulties in development. Whether there are defects on the solder mask, whether the composition and temperature of the developer are correct, whether the speed of development, i.e., the development point, is correct, whether the nozzle is blocked, whether the pressure of the nozzle is normal, and whether the water washing is good. Any of these conditions will leave residue on the bonding pad.



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