Causes and Countermeasures of the hottest solder b

2022-08-01
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Causes and Countermeasures of solder beads

Abstract: the phenomenon of solder beads is the main defect in the process of surface mount (SMT), which mainly occurs around the chip resistance capacitance element (chip), and is caused by many factors. This paper analyzes the causes of solder beads and puts forward corresponding solutions

abstract: soldier ball phenomenon is the main defect ID SMT process, it appears slightly beside the chips, made by many is article analyze the cause & Countermeasure of soldier ball generating

key words: solder bead solder paste reflow temperature profile slump stencil pcb

solder bead phenomenon is one of the main defects in the surface mounting process. Its generation is a complex process and the most annoying problem. It is very difficult to completely eliminate it

the diameter of solder beads is about 0.2mm ~ 0.4mm, and some are beyond this range, mainly concentrated around the chip resistance and capacitance elements. The existence of solder beads not only affects the appearance of electronic products, but also affects the quality of products. At present, the medium and high voltage cable samples containing graphene composite high molecular semiconductive shielding are being trial produced, which has buried hidden dangers. The reason is that modern printed board components have high density and small spacing, and solder beads may fall off during use, resulting in short circuit of components and affecting the quality of electronic products. Therefore, it is necessary to find out the causes of it and control it effectively, which is particularly important. Generally speaking, the causes of solder beads are various and comprehensive. The printing thickness of the solder paste, the composition and oxidation degree of the solder paste, the making and opening of the template, whether the solder paste absorbs water, the component mounting pressure, the solderability of the components and pads, the setting of reflow soldering temperature, and the impact of the external environment may all be the reasons why the raw materials for solder beads come from renewable biological materials such as agricultural crop straw

next, I will analyze the causes and solutions of solder beads from various aspects

the selection of solder paste directly affects the welding quality. The content of metal in the solder paste, the oxidation degree of the solder paste, the particle size of alloy solder powder in the solder paste and the thickness of the solder paste printed on the printed board can all affect the production of solder beads

a. metal content of solder paste. The metal content in the solder paste is about 88% ~ 92% by mass and about 50% by volume. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during preheating. In addition, with the increase of metal content, the metal powders are closely arranged so that they can be more easily combined during melting without being blown away. In addition, the increase of metal content may also reduce the "collapse" after solder paste printing, so it is not easy to produce solder beads

b. metal oxidation degree of solder paste. In the solder paste, the higher the metal oxidation degree, the greater the metal powder bonding resistance during welding, and the less wetting between the solder paste and the pad and components, resulting in the reduction of weldability. The results show that the occurrence rate of solder beads is directly proportional to the oxidation degree of metal powder. Generally, the oxidation degree of solder in the solder paste shall be controlled below 0.05%, and the maximum limit shall be 0.15%

c. particle size of metal powder in solder paste. The smaller the particle size of the powder in the solder paste, the larger the overall surface area of the solder paste, resulting in a higher degree of oxidation of the finer powder, thus aggravating the phenomenon of solder beads. Our experiments show that the solder powder is easier to be produced when the solder paste with finer particle size is selected

d. printing thickness of solder paste on printed board. The thickness of solder paste after printing is an important parameter for leakage plate printing, usually between 0.12mm and 0.20mm. Too thick solder paste will cause "collapse" of solder paste and promote the production of solder beads

e. amount of flux in solder paste and activity of flux. Too much flux will cause local collapse of the solder paste, which makes solder beads easy to produce. In addition, when the activity of the flux is small, the deoxidization ability of the flux is weak, so it is easy to produce tin beads. The activity of non cleaning solder paste is lower than that of rosin and water-soluble solder paste, so it is more likely to produce solder beads

f. in addition, the solder paste is generally refrigerated in the refrigerator before use. After it is taken out, it should be restored to room temperature and then opened for use. Otherwise, the solder paste is easy to absorb moisture and generate solder beads when the solder splashes again

2. Fabrication and opening of formwork. We usually make the template according to the pad on the printed board, so the opening of the template is the size of the pad. When printing solder paste, it is easy to print the solder paste on the solder resist layer, thus producing solder beads during reflow soldering. Therefore, we can make the template by reducing the opening of the template by 10% compared with the actual size of the pad. In addition, we can change the shape of the opening to achieve the desired effect. The following are several recommended pad designs:

the thickness of the template determines the printing thickness of the solder paste, so appropriately reducing the thickness of the template can also significantly improve the solder bead phenomenon. We have conducted such experiments: at first, we used 0.1formlabs to launch two templates of 8mm thick new resin material for dental 3D printing. After flow welding, we found that the solder beads next to the resistance and capacitance elements were serious. Later, we made a new template with a thickness of 0.15mm. The opening form was the previous design in the above figure, and then flow welding basically eliminated the solder beads

mounting pressure of components and weldability of components. If the pressure is too high during mounting, the solder paste is easy to be squeezed onto the solder resist layer under the component, and the solder melts and runs around the component to form solder beads during reflow soldering. The solution can reduce the pressure during mounting, and adopt the template opening recommended above to prevent the solder paste from being squeezed to the outside of the pad. In addition, the solderability of components and pads also has a direct impact. If the oxidation degree of components and pads is serious, solder beads will also be produced. After solder paste printing, the hot-air leveled pad changes the ratio of solder to flux, reducing the ratio of flux. The smaller the pad, the more serious the ratio imbalance. This is also a reason for solder beads

setting of reflow welding temperature. Solder beads are produced during reflow soldering of printed boards. Reflow soldering can be divided into four stages: preheating, insulation, reflow and cooling. In the preheating stage, the temperature of the solder paste, components and pads will rise to 1200c-1500c to reduce the thermal shock of components during reflow. At this stage, the solder in the solder paste begins to vaporize, which may cause small particles of metal to separate and run under the components, and then run around the components during reflow to form solder beads. At this stage, the temperature shall not rise too fast, generally less than 1.50c/s. too fast is likely to cause solder spatter and form solder beads. Therefore, the temperature curve of reflow soldering should be adjusted, and the moderate preheating temperature and preheating speed should be adopted to control the production of solder beads

influence of external factors. Generally, the best temperature for solder paste printing is 250C + 30C, and the humidity is 60% of the relative humidity. Too high a temperature will reduce the viscosity of solder paste, which is prone to "collapse". The humidity is too high, and the solder paste is easy to absorb moisture, which is easy to splash. All these are the reasons for solder beads. In addition, the printed board will absorb moisture after being exposed to the air for a long time, and the pad oxidation will occur, resulting in poor solderability. It can be baked in a 1200c-1500c drying oven for 12-14h to remove water vapor

to sum up, the production of solder beads is a very complex process. We should consider comprehensively when adjusting parameters and explore experience in production to achieve the best control of solder beads. (end)

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