Reflow soldering - solutions to problems such as solder beads, chips, bridging, suction, and blistering of solder film

Reflow soldering is mainly divided into main defects, secondary defects, and surface defects. Any defect that causes the SMA function to fail is referred to as the main defect; Minor defects refer to defects where the wetting between solder joints is good and will not cause loss of SMA function, but may affect the product's lifespan; Surface defects refer to those that do not affect the functionality and lifespan of the product. It is influenced by many parameters, such as solder paste, pasting accuracy, and welding process. We are well aware that reasonable surface assembly technology plays a crucial role in controlling and improving the quality of SMT products during the research and production of SMT processes.

1、 Tin beads in reflow soldering

1. Mechanism of solder bead formation in reflow soldering: The solder beads (also known as solder balls) that appear in reflow soldering are often hidden between the sides or fine pitch pins between the ends of rectangular chip components. During the component bonding process, solder paste is placed between the pins and pads of the chip component. As the printed circuit board passes through the reflow soldering furnace, the solder paste melts into liquid. If it is poorly wetted with the pads and device pins, the liquid solder particles cannot aggregate into a solder joint. Part of the liquid solder will flow out of the weld seam, forming tin beads. Therefore, the poor wettability between solder and solder pads and device pins is the fundamental reason for the formation of solder beads. In the printing process, due to the deviation between the template and the solder pad, if the deviation is too large, it will cause the solder paste to flow out of the solder pad, and it is easy to produce solder beads after heating. The pressure on the Z-axis during the mounting process is an important cause of solder beads, which is often overlooked. Some mounting machines, due to the Z-axis head being positioned based on the thickness of the component, can cause the component to press the solder bud out of the solder pad instantly when it is attached to the PCB. This part of the tin will obviously cause solder beads. In this case, the size of the solder beads produced is slightly larger, and usually, simply adjusting the Z-axis height can prevent the generation of solder beads.

2. Cause analysis and control methods: There are many reasons for poor wettability of solder, and the following main analysis is related to relevant processes and solutions:

(1) Improper setting of reflux temperature curve. The reflow of solder paste is related to temperature and time. If sufficient temperature or time is not reached, the solder paste will not reflow. The temperature rise rate in the preheating zone is too fast and the time is too short, which causes the moisture and solvent inside the solder paste to not completely evaporate. When it reaches the reflow soldering temperature zone, it causes the moisture and solvent to boil and splash out solder beads. Practice has proven that controlling the temperature rise rate of the preheating zone at 1-4 ℃/S is ideal.

(2) If tin beads always appear in the same position, it is necessary to check the design structure of the metal template. The corrosion accuracy of the template opening size cannot meet the requirements, the solder pad size is too large, and the surface material is relatively soft (such as copper templates), which can cause the outer contour of the printed solder paste to be unclear and interconnected. This situation often occurs when printing solder pads for fine-spaced devices. After reflow, a large number of solder beads will inevitably be generated between the pins. Therefore, suitable template materials and manufacturing processes should be selected based on the different shapes and center distances of solder pad shapes to ensure the quality of solder paste printing.

(3) If the time from SMT to reflow soldering is too long, the oxidation of solder particles in the solder paste, the deterioration of the solder paste, and the reduction of its activity will cause the solder paste to not reflow and produce solder beads. Choosing a solder paste with a longer working life (usually at least 4H) will reduce this impact.

(4) In addition, insufficient cleaning of printed boards with solder paste misprints can cause solder paste to remain on the surface of the printed board and in the air. When placing components before reflow soldering, the printed solder paste may deform. These are also the reasons for the tin beads. Therefore, it is necessary to accelerate the sense of responsibility of operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process.

II The Manhattan phenomenon refers to the phenomenon where one end of a chip component is welded to a solder pad and the other end is tilted. The main reason for this phenomenon is uneven heating at both ends of the component, and the melting of the solder paste occurs sequentially. Uneven heating at both ends of the component can occur in the following situations:

(1) The design of component arrangement direction is incorrect. We imagine a reflow soldering limit line in the reflow soldering furnace that spans the width of the furnace, and once the solder paste passes through it, it will immediately melt. One end of the rectangular component is first limited by reflow soldering, and the solder paste is melted first. The metal surface of the fully wetted component end has liquid surface tension; However, the other end did not reach the liquid phase temperature of 183 ℃, and the solder paste did not melt. Only the bonding force of the solder paste was much smaller than the surface tension of the reflow solder paste, thus causing the component end of the unmelted end to stand upright upwards. Therefore, it is necessary to maintain both ends of the component entering the reflow soldering limit line at the same time, so that the solder paste on the solder pads at both ends melts simultaneously, forming a balanced liquid surface tension and maintaining the position of the component unchanged.

(2) Insufficient preheating of printed circuit components during gas phase welding. The gas phase uses inert liquid vapor to condense on component pins and PCB pads, releasing heat and melting solder paste. Gas phase welding is divided into equilibrium zone and steam zone. In the saturated steam zone, the welding temperature can reach up to 217 ℃. During the production process, we found that if the welded component is not preheated sufficiently and subjected to temperature changes above 100 ℃, the gasification force of gas phase welding is easy to float up chip components smaller than 1206 packaging size, resulting in the phenomenon of chip erection. We eliminated the phenomenon of flaking by preheating the welded components in a high and low temperature box at a temperature of 145-150 ℃ for about 1-2 minutes, and finally slowly entering the saturated steam zone for welding.

(3) The impact of pad design quality. If the size of a pair of solder pads in a chip component is different or asymmetric, it can also cause inconsistent solder paste amount in printing. Small solder pads respond quickly to temperature, and the solder paste on them is easy to melt, while large solder pads are the opposite. Therefore, when the solder paste on the small solder pads melts, the component will be straightened and erected under the surface tension of the solder paste. If the width or gap of the solder pad is too large, it may also cause chip formation. Strictly following standard specifications for pad design is a prerequisite for solving this defect.

3、 Bridging is also one of the common defects in SMT production, which can cause short circuits between components and must be repaired when encountering bridging.

(1) Solder paste quality issues: The metal content in solder paste is relatively high, especially after a long printing time, which is prone to an increase in metal content; Low viscosity of solder paste, which flows out of the solder pad after preheating; The poor slump of solder paste, which flows out of the solder pad after preheating, can lead to IC pin bridging.

(2) The printing system printing machine has poor repeatability accuracy, uneven alignment, and solder paste printing outside of copper and platinum, which is commonly seen in fine pitch QFP production; Poor alignment of steel plates and PCBs, as well as incorrect design of steel plate window size/thickness and uneven alloy coating of PCB pad design, can lead to excessive solder paste, which can lead to solder joints. The solution is to adjust the printing machine and improve the coating layer of PCB pad.

(3) Excessive pressure during placement and immersion of solder paste under pressure are common causes in production, and the Z-axis height should be adjusted. If there is insufficient placement accuracy, component displacement, and IC pin deformation, improvement should be made based on the cause. (4) The preheating and heating speed is too fast, and the solvent in the solder paste cannot evaporate in time.

IV Material suction/core suction phenomenon, also known as core pulling phenomenon, is one of the common welding defects, commonly seen in vapor phase reflow welding. Core suction phenomenon refers to the phenomenon of solder detachment from the solder pad along the pins and between the chip body, which can form a serious virtual soldering phenomenon. The reason for this is usually believed to be the high thermal conductivity of the original pins and rapid heating, resulting in solder preferentially wetting the pins. The wetting force between the solder and the pins is much greater than that between the solder and the solder pad, and the upwarping of the pins will further exacerbate the occurrence of wicking. In infrared reflow soldering, the organic flux in the PCB substrate and solder is an excellent absorbing medium for infrared radiation, while the pins can partially reflect infrared radiation. In contrast, the solder preferentially melts, and its wetting force with the solder pad is greater than that between it and the pins. Therefore, the solder part will rise along the pins, and the probability of wicking phenomenon is much lower. The solution is: during vapor phase reflow welding, the SMA should be fully preheated before being placed in the vapor phase furnace; Carefully inspect and ensure the solderability of PCB pads, and PCBs with poor solderability should not be used in production; The coplanarity of components cannot be ignored, and devices with poor coplanarity should not be used in production.

V After welding, the solder mask on the printed circuit board will blister. After welding, there will be light green bubbles around individual solder joints, and in severe cases, bubbles of the size of nail caps will appear. This not only affects the appearance quality, but also affects the performance. It is one of the common problems in the welding process. The fundamental reason for the blistering of the solder mask is the presence of gas/water vapor between the solder mask and the anode substrate. Trace amounts of gas/water vapor can be entrained into different process processes, and when encountering high temperatures, gas expansion leads to delamination between the solder mask and the anode substrate. During welding, the temperature of the solder pad is relatively high, so bubbles first appear around the solder pad. Nowadays, the processing process often requires cleaning and drying before proceeding to the next process. For example, after etching, it should be dried before applying a solder mask. At this time, if the drying temperature is not enough, water vapor will be carried into the next process. The storage environment before PCB processing is not good, the humidity is too high, and there is no timely drying treatment during welding; In the wave soldering process, water-based solder resist is often used. If the PCB preheating temperature is not enough, the water vapor in the flux will enter the interior of the PCB substrate along the hole wall of the through-hole, and the water vapor will first enter around the solder pad. When encountering high welding temperatures, these situations will generate bubbles.

The solution is:

(1) Strict control should be exercised over all aspects, and purchased PCBs should be inspected and stored in the warehouse. Generally, under standard circumstances, there should be no bubbles.

(2) PCB should be stored in a ventilated and dry environment, with a storage period not exceeding 6 months;

(3) PCB should be pre baked in an oven at 105 ℃/4H~6H before welding;