Analyze the causes and hazards of solder bead defects

Solder beads are produced during the soldering process. When there is a defect in the solder paste printing, the solder paste collapses, or the solder paste is extruded from the pad by the chip device, it will form solder beads on the side of the component or left under the component after reflow soldering. It resembles a solder ball, but is larger in size and typically occurs on the sides of a rectangular chip component or between fine-pitch pins.

There are many reasons for the generation of solder beads, mainly including solder mask, flux, wettability, stencil opening design, chip pressure, alloy composition, solder paste oxidation, poor solder paste collapse, insufficient solder paste activity, solder paste spatter, leakage aperture shape, reflow temperature curve, reflow without nitrogen protection, etc.

In terms of solder mask, a non-smooth solder mask can reduce the spread of residues, so it tends to produce fewer solder beads; A smooth solder mask, on the other hand, is prone to more solder beads because the flux may diffuse more easily in the liquid state.

In terms of flux composition, solder pastes with a lot of flux are prone to solder beads. This is because during the brazing process, the flux melts before the solder paste. When the solder paste begins to melt, the flux extends along the gap at the bottom of both sides of the component to the middle under the action of "capillary" until the flux converges in the middle of both ends. Too much flux will migrate with some of the small particles and deposit on the surface of the PCB. When cooling, due to the effect of surface tension, the alloy close to the pad and the solder end of the component is pulled towards the pad to form a solder joint, and the alloy away from the pad gradually shrinks towards the middle of the component, so that a solder bead is formed on the side of the component. If the flux is too small, the solder paste has poor fluidity and the brazing effect is not good. Therefore, when choosing solder paste, you should pay attention to the proportion of flux, which is neither too large nor too small.

In terms of wettability During reflow soldering, if the wettability of the solder paste, pads and components is not good, part of the liquid alloy will flow out of the weld to form solder beads during the cooling and shrinkage process of the liquid solder. There are two main reasons for poor wettability: one is the poor wettability of materials such as solder paste, pads and component pins; The second is the improper brazing process, for example, the solder paste is not brazed in time after printing, the alloy particles in the solder paste are oxidized and the flux is volatilized, which makes the wettability of the solder paste worse. In addition, in lead-free soldering, if there is no nitrogen protection, it is easy to cause oxidation of the brazing metal, the wettability will deteriorate, and the unclean pad of the printing template may also lead to poor wettability.

In terms of brazing process , when the brazing temperature is too high, the brazing metal has good fluidity, and the brazing metal alloy is easy to flow out of the brazing seam and form tin beads. In addition, if the preheating temperature rises too quickly, the solvent vaporization expands rapidly, and when the expansion force is greater than the adhesion force of the solder paste, the solder paste under the component will collapse, and the solder paste will be extruded out of the brazing seam to form solder beads. If the preheating is too slow, a strong capillary phenomenon will be generated, and the molten gold will be easily sucked out of the brazing seam and form tin beads. Therefore, a reasonable design of the temperature curve can effectively reduce the solder beads. In addition, the formation of solder beads is also related to the design of the steel mesh opening. Excessive solder paste deposition is easy to produce solder beads, so the pad aperture should be the same as the width of the component or slightly smaller, and the thickness of the template should not be too thick, otherwise it is easy to cause too much solder paste deposition and produce solder beads. The formation of solder beads is also closely related to the shape of the components, the height of the main body of the components is different, the height of the bracket is different, the movement ability of the capillary action of the solder paste under the components is different, and the capillary effect is strong and easy to form solder beads after reflow soldering.

Specific effects of solder beads on circuits Solder beads can cause circuits to work abnormally, especially solder beads next to capacitors or inductors. The solder beads, which have a large impact on the circuit, are mainly located in the center of the two sides of the capacitor or inductor, and tend to be large in size. After the solder beads are generated, the results can vary greatly. Assuming that the spacing between the two ends of the pad is 0.8mm and the solder ball is 0.3mm, then it is equivalent to the spacing between the two pads is about 0.5mm, which will reduce the PCB surface insulation value, and the more residue, the greater the PCB surface insulation value will decrease, and the leakage current will increase, which will lead to abnormal work. If two or more solder paste balls are connected together, the size of the pin spacing is more than 1/2 or greater than 0.3mm(even if it is less than 1/2 foot spacing), it is not qualified. However, not all solder beads will cause the circuit to work abnormally, depending on the location and size of the solder beads, and the solder beads next to the resistor have little effect on the circuit. While not all solder beads cause circuits to work abnormally, their presence does increase the risk of circuit failure. Therefore, in the process of electronic assembly, we should attach great importance to the problem of solder beads, and take a series of preventive measures to reduce the possibility of its occurrence and impact on the circuit.

Conclusion Solder beads are a common defect in the soldering process, and its causes are complex. Solder bead formation can be effectively reduced by optimizing solder mask properties, precisely controlling solder paste composition, improving wettability, designing the brazing process and stencil hole openings, and paying attention to the influence of component shape and height. At the same time, the formed tin beads are detected and cleaned in time to ensure the performance and reliability of electronic products. And take corresponding countermeasures to reduce the generation of solder beads, so as to improve the soldering quality and reliability of electronic products. Solder beads are a problem that cannot be ignored in the welding process, and its generation involves the interaction of multiple links and factors. From the point of view of the source of solder beads, various conditions in the solder paste printing process, such as printing defects, solder paste collapse and solder pads extruded by the chip device, etc., are easy to promote the formation of solder beads after the reflow soldering process, and the position of the solder beads is mostly on the side of the component or below the component.

Many factors have different effects on solder beads. Depending on the characteristics of the solder mask, the number of solder beads produced is also different, and the residue of the non-smooth solder mask spreads less, and the amount of solder beads produced is smaller, while the smooth solder mask layer is the opposite. The proportion of flux in the solder paste has a significant effect, and the flux is easy to produce solder beads, because the flux melts first and migrates and deposits under the action of "capillary" during brazing, and the lack of flux will lead to poor fluidity of the solder paste and affect the brazing effect. In terms of wettability, the poor wettability of solder paste, pads and components will promote the formation of solder balls, which is not only related to the wettability of the material itself, but also related to whether the brazing process is appropriate. The parameters of the brazing process are also crucial, the brazing temperature is too high, the preheating temperature is too fast or too slow, the solder ball will be generated, the design of the steel mesh hole opening will affect the amount of solder paste deposition and then related to the generation of solder balls, and the movement ability of the solder paste capillary action under the different shapes of the components will also affect the formation of solder balls.

The influence of the solder ball on the circuit is not generalized, the solder ball next to the capacitance or inductor, especially in the center and the larger size of the solder bead has a greater impact on the circuit, which will reduce the insulation value of the PCB surface and increase the leakage current, resulting in abnormal work, and the connection of multiple solder paste balls and the size to a certain extent is also unqualified, but the solder ball next to the resistor has basically no effect on the circuit. However, even if some of the solder beads do not cause the circuit to work abnormally for the time being, it increases the risk of circuit failure.

In the process of electronic assembly, it is necessary to comprehensively consider the various factors of solder beads, from optimizing the solder mask layer, controlling the composition of solder paste, improving wettability, designing the brazing process and template hole opening, paying attention to the shape and height of components, etc., and timely detection and cleaning of solder beads, so as to reduce the formation of solder beads and improve the welding quality and reliability of electronic products.

Solder beads are a problem that cannot be ignored in the welding process, and its generation involves the interaction of multiple links and factors. From the point of view of the source of solder beads, various conditions in the solder paste printing process, such as printing defects, solder paste collapse and solder pads extruded by the chip device, etc., are easy to promote the formation of solder beads after the reflow soldering process, and the position of the solder beads is mostly on the side of the component or below the component.

Many factors have different effects on solder beads. Depending on the characteristics of the solder mask, the number of solder beads produced is also different, and the residue of the non-smooth solder mask spreads less, and the amount of solder beads produced is smaller, while the smooth solder mask layer is the opposite. The proportion of flux in the solder paste has a significant effect, and the flux is more prone to solder beads, because the flux melts first and is "capillary" during brazing migration and deposition under the action, and the lack of flux will lead to poor fluidity of the solder paste and affect the brazing effect. In terms of wettability, the poor wettability of solder paste, pads and components will promote the formation of solder balls, which is not only related to the wettability of the material itself, but also related to whether the brazing process is appropriate. The parameters of the brazing process are also crucial, the brazing temperature is too high, the preheating temperature is too fast or too slow, the solder ball will be generated, the design of the steel mesh hole opening will affect the amount of solder paste deposition and then related to the generation of solder balls, and the movement ability of the solder paste capillary action under the different shapes of the components will also affect the formation of solder balls.

The influence of the solder ball on the circuit is not generalized, the solder ball next to the capacitance or inductor, especially in the center and the larger size of the solder bead has a greater impact on the circuit, which will reduce the insulation value of the PCB surface and increase the leakage current, resulting in abnormal work, and the connection of multiple solder paste balls and the size to a certain extent is also unqualified, but the solder ball next to the resistor has basically no effect on the circuit. However, even if some of the solder beads do not cause the circuit to work abnormally for the time being, it increases the risk of circuit failure.

In the process of electronic assembly, it is necessary to comprehensively consider the various factors of solder beads, from optimizing the solder mask layer, controlling the composition of solder paste, improving wettability, designing the brazing process and template hole opening, paying attention to the shape and height of components, etc., and timely detection and cleaning of solder beads, so as to reduce the formation of solder beads and improve the welding quality and reliability of electronic products.