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. SMT 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, water vapor will condense. 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. .
(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 stencil opening and whether the opening is smooth will also affect the printing quality. The stencil opening must be flared downwards, otherwise solder paste will remain on the chamfers of the flares during 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 to facilitate 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.
(3) The impact of production on the quality of reflow soldering.
① The influence of printing process. Printing process parameters, such as squeegee speed, squeegee pressure, angle of squeegee and template, and viscosity of solder paste are all restricted. Therefore, only by correctly controlling these parameters can the printing quality of the solder paste be guaranteed and the soldering effect can be ensured. The use and management of recycled solder paste, ambient temperature, humidity, and environmental sanitation all have an impact on the quality of solder joints. Store the recycled solder paste and new solder paste separately. Too high ambient temperature will reduce the viscosity of the solder paste; when the humidity is too high, the solder paste will absorb moisture in the air, and low humidity will accelerate the volatilization of the solvent in the solder paste. Dust mixed in the solder paste in the environment can cause pinholes in the solder joints.
② The influence of the placement process. The mounting components should be correct, otherwise the product will not pass the test after soldering. The component mounting position must meet the process requirements, and the solder ends or pins of the components and the land pattern should be aligned and centered as much as possible. For chip components, when one of the solder ends is not connected to the pad during mounting, displacement or tombstones will occur during reflow soldering. For IC devices, the self-positioning effect during reflow soldering is small, and the placement offset cannot be corrected by reflow soldering. Therefore, during mounting, if the mounting position exceeds the allowable deviation range, it must be manually corrected before entering the reflow oven for soldering. The patch pressure should be appropriate. If the pressure is insufficient, the solder ends or pins of the components will float on the surface of the solder paste, and the solder paste will not stick to the components, which will easily cause position shifts during transfer and reflow soldering. In addition, because the z-axis height is too high, the components are dropped from a high place during placement, which will cause the placement of the placement of the placement; excessive placement pressure will cause too much solder paste to be squeezed out, which will easily cause solder paste adhesion and reflow. Bridging occurs, and components may be damaged in severe cases.
③ The influence of reflow process. The reflow temperature curve is the key to ensuring the quality of reflow soldering. The heating rate and peak temperature of the actual temperature curve and the solder paste temperature curve should be basically the same. If the heating rate is too fast, on the one hand, the components and the PCB will be heated too fast, which will easily damage the components and cause the PCB to deform; on the other hand, the solvent in the solder paste will evaporate faster, and it will easily splash out the metal components and produce solder balls. The peak temperature should generally be set at 30-40% higher than the melting point of the solder paste metal, and the reflow time is 30-60so. The peak temperature is low or the reflow time is short, which will cause insufficient soldering, and in severe cases, the solder paste will not melt. If the peak temperature is too high or the reflow time is long, it will cause oxidation of the metal powder, increase the formation of intermetallic compounds, make the solder joints brittle, affect the strength of the solder joints, and even damage the components and PCB.
In short, from the above analysis, it can be seen that the quality of reflow soldering is related to PCB pad design, component solderability, solder paste quality, PCB processing quality, production line equipment and process parameters of each process of SMT, and even the operation of operators. Habits are closely related. At the same time, it can be seen that PCB design, PCB processing quality, components and solder paste quality are the basis for ensuring the quality of reflow soldering, because these problems are difficult or even impossible to solve in the production process. Therefore, as long as the PCB design is correct and the components and solder paste are qualified, the quality of reflow soldering can be controlled by the process of printing, mounting and each process.
