Hot air leveling is to dip the printed wiring board into molten solder (63SN/37PB) and then use hot air to blow off the excess solder on the surface of the printed wiring board and in the metallization holes to get a smooth, even and shiny solder coating layer. After the hot air leveling of the surface of the printed wiring board lead-tin alloy coating layer should be bright and uniform and complete, with good solderability, no knots and no semi-wetting, coating completely without copper exposure. After hot air leveling pad surface and metalization hole copper exposure is an important defect in the finished product inspection, is one of the common causes of hot air leveling rework, causing the problem for many reasons, commonly have the following kinds.
1. Insufficient pre-treatment, poor roughening PCB board hot air leveling pre-treatment process is good or bad on the quality of hot air leveling has a great impact on the process must be thoroughly removed from the pads of oil, impurities and oxidation layer, to provide fresh solderable copper surface for the dip tin. Now the more commonly used pretreatment process is mechanical spraying, first sulfuric acid – hydrogen peroxide micro-etching, micro-etching after dipping acid, followed by water spraying rinse, hot air blowing dry, spray flux, immediately hot air leveling. Poor pre-treatment caused by the copper dew phenomenon is not the type of batch at the same time a large number of dew copper point is often distributed throughout the board, more serious on the edge. Use a magnifying glass to observe the pre-treatment circuit board will be found on the pad with obvious residual oxidation points and stains. A similar situation should be micro-etching solution for chemical analysis, check the second pickling solution, adjust the concentration of the solution to replace the solution due to the use of too much time to pollute the serious solution, check whether the spray system is smooth. Properly extend the processing time can also improve the treatment effect, but need to pay attention to the phenomenon of overcorrosion that will occur, reworked circuit board by hot air leveling after processing line again in 5% hydrochloric acid solution to remove the surface of the oxide.
2. The surface of the pad is unclean with residual solder resist contaminating the pad Most manufacturers currently use full board screen printing of liquid photoresist ink, followed by exposure and development to remove excess solder resist and obtain a time definitive solder resist pattern. In the process, the pre-baking process is not well controlled, the temperature is too high for too long will cause difficulties in the development of the shadow. Whether there are defects on the solder resist negative, whether the composition and temperature of the developing solution is correct, the speed of the development that the development point is correct, whether the nozzle is blocked and the nozzle pressure is normal, whether the water wash is good, any of these conditions will leave residual spots on the pad. Such as the formation of copper due to the reason of the negative is generally more regular, are in the same point. This situation can be found using a magnifying glass at the dew copper residual traces of solder-resisting substances, PCB design should generally be set up before the curing process to check the graphics and metallization holes inside, to ensure that the pads and metallization holes sent to the next process of printed wiring boards clean without solder-resisting ink residue.
3. Insufficient flux activity The role of flux is to improve the wettability of the copper surface, protect the surface of the laminate from overheating, and provide protection for the solder coating. Such as flux activity is not enough, the copper surface wettability is not good, the solder can not completely cover the pad, the copper phenomenon and pre-treatment is similar to poor, extend the pre-treatment time to reduce the copper phenomenon. Now almost all fluxes are acid fluxes, containing acid additives, such as high acidity will produce a serious phenomenon of copper bite, resulting in high copper content in the solder caused by lead tin rough; acidity is too low, the activity is weak, which will lead to copper exposure. If the copper content in the lead-tin bath is large to remove copper in a timely manner. Process technicians choose a stable and reliable quality of flux on the hot air leveling has an important impact on the excellent flux is a guarantee of the quality of hot air leveling.
