Reflow welding is divided into main defects, secondary defects and surface defects. Any defect that disables the function of SMA is called a major defect; Secondary defects refer to the wettability between solder joints is good, does not cause the loss of SMA function, but has the effect of product life may be defects; Surface defects are those that do not affect the function and life of the product. It is affected by many parameters, such as solder paste, paste accuracy and welding process. In our SMT process research and production, we know that reasonable surface assembly technology plays a vital role in controlling and improving the quality of SMT products.
I. Tin beads in reflow soldering
1. Mechanism of tin bead formation in reflow welding: The tin bead (or solder ball) that appears in reflow welding is often hidden between the side or the fine-spaced pins between the two ends of the rectangular chip element. In the component bonding process, the solder paste is placed between the pin of the chip component and the pad. As the printed board passes through the reflow furnace, the solder paste melts into a liquid. If the liquid solder particles are not well wetted with the pad and the device pin, etc., the liquid solder particles cannot be aggregated into a solder joint. Part of the liquid solder will flow out of the weld and form tin beads. Therefore, the poor wettability of the solder with the pad and the device pin is the root cause of the formation of tin beads. Solder paste in the printing process, due to the offset between the stencil and the pad, if the offset is too large, it will cause the solder paste to flow outside the pad, and it is easy to appear tin beads after heating. The pressure of the Z axis in the mounting process is an important reason for tin beads, which is often not paid attention to. Some attaching machines are positioned according to the thickness of the component because the Z axis head is located according to the thickness of the component, which will cause the component to be attached to the PCB and the tin bud will be extruded to the outside of the welding disc. In this case, the size of the tin bead produced is slightly larger, and the production of the tin bead can usually be prevented by simply re-adjusting the Z-axis height.
2. Cause analysis and control method: There are many reasons for poor solder wettability, the following main analysis and related process related causes and solutions: (1) improper reflux temperature curve setting. The reflux of the solder paste is related to temperature and time, and if sufficient temperature or time is not reached, the solder paste will not reflux. The temperature in the preheating zone rises too fast and the time is too short, so that the water and solvent inside the solder paste are not completely volatilized, and when they reach the reflow temperature zone, the water and solvent boil out the tin beads. Practice has proved that it is ideal to control the temperature rise rate in the preheating zone at 1 ~ 4℃/S. (2) If tin beads always appear in the same position, it is necessary to check the metal template design structure. The corrosion accuracy of the template opening size can not meet the requirements, the size of the pad is too large, and the surface material is soft (such as copper template), which will cause the external outline of the printed solder paste to be unclear and connected to each other, which mostly occurs in the pad printing of fine-pitch devices, and will inevitably cause a large number of tin beads between the pins after reflow. Therefore, suitable template materials and template making process should be selected according to the different shapes and center distances of pad graphics to ensure the printing quality of solder paste. (3) If the time from the patch to reflow soldering is too long, the oxidation of the solder particles in the solder paste will cause the solder paste to not reflow and produce tin beads. Choosing a solder paste with a longer working life (generally at least 4H) will mitigate this effect. (4) In addition, the solder paste misprinted printed board is not sufficiently cleaned, which will cause the solder paste to remain on the surface of the printed board and through the air. Deform the printed solder paste when attaching components before reflow soldering. These are also the causes of tin beads. Therefore, it should accelerate the responsibility of operators and technicians in the production process, strictly comply with the process requirements and operating procedures for production, and strengthen the quality control of the process.
two One end of the chip element is welded to the pad, and the other end is tilted up. This phenomenon is called the Manhattan phenomenon. The main reason for this phenomenon is that the two ends of the component are not heated evenly, and the solder paste is melted successively. Uneven heating at both ends of the component will be caused in the following circumstances:
(1) The component arrangement direction is not designed correctly. We imagine that there is a reflow limit line across the width of the reflow furnace, which will melt as soon as the solder paste passes through it. One end of the chip rectangular element passes through the reflow limit line first, and the solder paste melts first, and the metal surface of the end of the chip element has liquid surface tension. The other end does not reach the liquid phase temperature of 183 ° C, the solder paste is not melted, and only the bonding force of the flux is far less than the surface tension of the reflow solder paste, so that the end of the unmelted element is upright. Therefore, both ends of the component should be kept to enter the reflow limit line at the same time, so that the solder paste on the two ends of the pad is melted at the same time, forming a balanced liquid surface tension, and keeping the position of the component unchanged.
(2) Insufficient preheating of printed circuit components during gas phase welding. The gas phase is the use of inert liquid vapor condensation on the component pin and PCB pad, release heat and melt the solder paste. The gas phase welding is divided into the balance zone and the steam zone, and the welding temperature in the saturated steam zone is as high as 217 ° C. In the production process, we found that if the welding component is not sufficiently preheated, and the temperature change above 100 ° C, the gasification force of the gas phase welding is easy to float the chip component of the package size of less than 1206, resulting in the vertical sheet phenomenon. By preheating the welded component in a high and low temperature box at 145 ~ 150℃ for about 1 ~ 2min, and finally slowly entering the saturated steam area for welding, the phenomenon of sheet standing was eliminated.
(3) The impact of pad design quality. If a pair of pad size of the chip element is different or asymmetrical, it will also cause the amount of printed solder paste is inconsistent, the small pad responds quickly to the temperature, and the solder paste on it is easy to melt, the large pad is the opposite, so when the solder paste on the small pad is melted, the component is straightened under the action of the surface tension of the solder paste. The width or gap of the pad is too large, and the sheet standing phenomenon may also occur. The design of pad in strict accordance with the standard specification is the prerequisite to solve the defect.
Three. Bridging Bridging is also one of the common defects in SMT production, which can cause short circuits between components and must be repaired when the bridge is encountered.
(1) The solder paste quality problem is that the metal content in the solder paste is high, especially after the printing time is too long, the metal content is easy to increase; The viscosity of the solder paste is low, and it flows out of the pad after preheating. Poor slump of solder paste, after preheating to the outside of the pad, will lead to IC pin bridge.
(2) The printing system printing press has poor repeat accuracy, uneven alignment, and solder paste printing to copper platinum, which is mostly seen in fine-pitch QFP production; The steel plate alignment is not good and PCB alignment is not good and the steel plate window size/thickness design is not uniform with the PCB pad design alloy coating, resulting in a large amount of solder paste, which will cause bonding. The solution is to adjust the printing press and improve the PCB pad coating layer.
(3) The sticking pressure is too large, and the soaking of the solder paste after pressure is a common reason in production, and the Z-axis height should be adjusted. If the accuracy of the patch is not enough, the component is shifted and the IC pin is deformed, it should be improved for the reason. (4) The preheating speed is too fast, and the solvent in the solder paste is too late to volatilize.
The core-pulling phenomenon, also known as the core-pulling phenomenon, is one of the common welding defects, which is more common in vapor phase reflow welding. The core suction phenomenon is that the solder is separated from the pad along the pin and the chip body, which will form a serious virtual welding phenomenon. The reason is usually considered to be the large thermal conductivity of the original pin, the rapid temperature rise, so that the solder is preferred to wet the pin, the wetting force between the solder and the pin is much greater than the wetting force between the solder and the pad, and the upwarping of the pin will aggravate the occurrence of the core suction phenomenon. In infrared reflow welding, PCB substrate and solder in the organic flux is an excellent infrared absorption medium, and the pin can partially reflect infrared, in contrast, the solder is preferentially melted, its wetting force with the pad is greater than the wetting between it and the pin, so the solder will rise along the pin, the probability of core suction phenomenon is much smaller. The solution is: in the vapor phase reflow welding, the SMA should be fully preheated first and then put into the vapor phase furnace; The weldability of PCB pad should be carefully checked and guaranteed, and PCB with poor weldability should not be applied and produced; The coplanarity of components cannot be ignored, and devices with poor coplanarity should not be used in production.
Five. After welding, there will be light green bubbles around the individual solder joints, and in serious cases, there will be a bubble the size of a nail, which not only affects the appearance quality, but also affects the performance in serious cases, which is one of the problems that often occur in the welding process. The root cause of the welding resistance film foaming is the presence of gas/water vapor between the welding resistance film and the positive substrate. Trace amounts of gas/water vapor are carried to different processes, and when high temperatures are encountered, gas expansion leads to the delamination of the solder resistance film and the positive substrate. During welding, the temperature of the pad is relatively high, so the bubbles first appear around the pad. Now the processing process often needs to be cleaned, dry and then do the next process, such as after etching, should be dried and then stick the solder resistance film, at this time if the drying temperature is not enough will carry water vapor into the next process. The PCB storage environment is not good before processing, the humidity is too high, and the welding is not dried in time; In the wave soldering process, often use a water-containing flux resistance, if the PCB preheating temperature is not enough, the water vapor in the flux will enter the inside of the PCB substrate along the hole wall of the through hole, and the water vapor around the pad will first enter, and these situations will produce bubbles after encountering high welding temperature.
The solution is: (1) all aspects should be strictly controlled, the purchased PCB should be inspected after storage, usually under standard circumstances, there should be no bubble phenomenon.
(2) PCB should be stored in a ventilated and dry environment, the storage period is not more than 6 months; (3) PCB should be pre-baked in the oven before welding 105℃/4H ~ 6H;
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