Hot air leveling is to immerse PCB into molten solder (63Sn / 37Pb), and then blow off the excess solder on PCB surface and metallized hole with hot air to obtain a smooth, uniform and bright solder coating layer. After hot air leveling, the lead tin alloy coating on the surface of printed circuit board shall be bright, uniform and complete, with good weldability, no nodulation, no semi wetting, and completely free of exposed copper. Copper exposed on the surface of welding pad and in the metallized hole after hot air leveling is an important defect in the inspection of finished products, which is one of the common causes of hot air leveling rework. There are many reasons for this problem, including the following.
1. The surface of the pad is not clean, and there is residual solder resist to pollute the pad.
At present, most manufacturers use full screen printing liquid photosensitive solder resist ink, and then remove the excess solder resist through exposure and development to get the solder resist pattern of time. In this process, the pre drying process is not well controlled, the temperature is too high and the time is too long will cause the development difficulty. Whether there are defects on the solder mask, whether the composition and temperature of the developer are correct, whether the development speed, i.e. whether 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 which will leave residual points on the pad. For example, the exposed copper formed due to the reason of negative film is generally more regular, all at the same point. In this case, a magnifying glass can be used to find traces of solder blocking materials at the exposed copper. Generally, a post shall be set up to check the inside of the figure and metallized hole before the curing process, so as to ensure that the solder pad and metallized hole of the printed circuit board delivered to the next process are clean and free of solder blocking oil and ink residues.
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. In this process, the greasy dirt, impurities and oxide layer on the welding pad must be completely removed to provide a fresh and solderable copper surface for tin dipping. At present, the most commonly used pretreatment process is mechanical spray. The first is sulfuric acid hydrogen peroxide micro etching, acid pickling after micro etching, followed by water spray washing, hot air drying, spraying flux, and hot air leveling immediately. The phenomenon of copper exposure caused by poor pretreatment occurs in large quantities at the same time, regardless of the type of batch. The copper exposed points are often distributed throughout the board, especially on the edge. Using a magnifying glass to observe the circuit board after pretreatment, it will be found that there are obvious oxidation spots and stains on the pad. In case of similar situation, chemical analysis shall be carried out for the micro etching solution, check the second pickling solution, adjust the concentration of the solution, replace the solution with serious pollution due to too long use, and check whether the spray system is unobstructed. Proper prolongation of treatment time can also improve the treatment effect, but it is necessary to pay attention to the over corrosion phenomenon. The reworked circuit board is treated in 5% hydrochloric acid solution after hot air leveling by the treatment line to remove the oxide on the surface.
3. Insufficient flux activity
The function of flux is to improve the wettability of copper surface, protect the surface of laminate from overheating, and provide protection for solder coating. If the activity of flux is not enough and the wettability of copper surface is not good, the solder can not completely cover the pad. The phenomenon of copper exposure is similar to that of poor pretreatment. Prolonging the pretreatment time can reduce the phenomenon of copper exposure. The current fluxes are almost all acid fluxes, which contain acid additives. If the acidity is too high, the phenomenon of copper biting will be serious, resulting in high copper content in the solder, resulting in rough lead tin; if the acidity is too low, the activity will be weak, resulting in exposed copper. If the copper content in the lead tin bath is large, remove the copper in time. The selection of a flux with stable and reliable quality by process technicians has an important influence on hot air leveling. The quality assurance of hot air leveling is the key to excellent flux.
In addition, other parameters also have influence on hot air leveling, such as uneven flux coating, low solder level, incorrect soaking time, bad adjustment of wind force and wind pressure, etc., which may cause copper exposure. This problem is easy to find and solve. The first inspection and inspection of products at any time during the operation of workers, timely feedback of problems, and timely analysis and solution of causes by process technicians can greatly reduce the rework rate, provide product quality, and minimize the copper phenomenon.
Sep 29, 2019
