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Reflow soldering is one of the key processes of SMT, and the quality of surface assembly is directly reflected in the results of reflow soldering. However, the soldering quality problems that appear in reflow soldering are not entirely caused by the reflow soldering process, because the quality of reflow soldering is not only directly related to the temperature curve, but also related to the production line equipment conditions, the productivity design of the PCB pads, and the solderability of the components. Performance, solder paste quality, PCB processing quality and SMT process parameters of each process are closely related to the operating habits of operators.
(1) The influence of production materials on the quality of reflow soldering.
1. The influence of components. When the solder ends or pins of components are oxidized or contaminated, soldering defects such as poor wetting, false soldering, and voids will occur during reflow soldering. The poor coplanarity of components can also lead to soldering defects such as virtual soldering during soldering.
2. The influence of PCB. The assembly quality of SMT has a direct and very important relationship with PCB pad design. If the PCB pad design is correct, a small amount of skew during mounting can be corrected during reflow soldering due to the surface tension of the molten solder; on the contrary, if the PCB pad design is incorrect, even if the mounting position is very accurate, after reflow soldering On the contrary, welding defects such as component position deviation and tombstone formation may occur. PCBA assembly quality is also related to the quality of PCB pads. When PCB pads are oxidized, contaminated or damp, soldering defects such as poor wetting, false soldering, solder balls, and voids will occur during reflow soldering.
3. The influence of solder paste. The metal powder content in the solder paste, the oxygen content of the metal powder, viscosity, thixotropy, and printability all have certain requirements. If the metal powder content of the solder paste is high, the metal powder will spatter as the solvent evaporates when the reflow temperature rises. If the oxygen content of the metal powder is high, it will aggravate the spatter, forming solder balls, and cause defects such as non-wetting. In addition, if the viscosity of the solder paste is too low or the thixotropy of the solder paste is not good, the solder paste pattern will collapse after printing, or even cause adhesion, and solder balls, bridges and other soldering defects will be formed during reflow soldering. If the printability of the solder paste is not good, the solder paste will only slide on the template during printing, and the solder paste will not be printed at all. If the solder paste is taken out of the refrigerator and used directly, it will cause condensation. When the reflow temperature rises, the water vapor will evaporate and bring out the metal powder. At high temperatures, the water vapor will oxidize the metal powder and spatter to form solder balls, which will also cause problems such as poor wetting. .
SMT patch processing012
(2) The influence of production equipment on the quality of reflow soldering. The quality of reflow soldering has a very close relationship with the production equipment. The main factors affecting the quality of reflow soldering are as follows:
1. Printing equipment. The printing accuracy and repeatability of the printer will play a certain role in the printing result, and ultimately affect the quality of reflow soldering; the quality of the template will eventually also affect the printing result, that is, the quality of soldering. The thickness of the stencil and the size of the opening determine the amount of solder paste printed. Too much solder paste will cause bridging, and too little solder paste will cause insufficient solder or false soldering. The shape of the opening of the template and whether the opening is smooth will also affect the printing quality. The opening of the template must be flared downward, otherwise solder paste will remain on the chamfer of the flared when demolding.
2. Reflow soldering equipment. The temperature control accuracy of the reflow oven should reach ±(0.1 ~0.2)Y; the lateral temperature difference of the conveyor belt of the reflow oven should be below ±5 Y, otherwise it is difficult to guarantee the soldering quality; the width of the conveyor belt of the reflow oven should meet the maximum PCB size requirements; the heating zone in the reflow oven The longer the length and the greater the number of heating zones, the easier it is to adjust the temperature curve. For medium and small batch production, choose 4 to 5 temperature zones, and the length of the heating zone is about 1.8 m, which can meet the requirements. The upper and lower heaters should be independently temperature-controlled for easy adjustment and control of the temperature curve; the maximum heating temperature of the reflow oven is generally 300-350. Considering lead-free solder or metal substrates, you should choose 350 dragons or more; the conveyor belt of the reflow oven should run smoothly, and the conveyor belt will vibrate. Cause welding defects such as displacement, tombstone, cold welding, etc.
