Analysis of the causes and hazards of tin bead defects

Tin beads are formed during the soldering process, when there are solder paste printing defects, solder paste collapses or is pressed out of the pad by the SMD device, after re-flow, in the side of the component or stay under the component to form a tin bead. It is similar to the solder ball, but the size is very large, usually in the huge chip components between the two guillotine or fine-pitch pins. Tin beads produced by a number of reasons, mainly including soldermask, flux, wettability, stencil aperture design, patch pressure, alloy composition, paste oxidation, paste slumping poor paste, paste activity is not enough, paste splash, leakage plate aperture shape, reflow temperature profile, reflow without nitrogen protection, and so on.

Reason for the Rising of tin bead

1. Solder Resist Layer

An unsmooth soldermask reduces the spreading of residue and thus tends to produce fewer beads: whereas a smooth soldermask tends to produce more beads because the flux may spread more readily when it is in the liquid state.

2. flux composition

Flux more solder paste, easy to produce tin beads, this is because in the brazing process, the flux before the solder paste melting, when the solder paste began to melt, the flux in the "capillary" action along the bottom of the component on both sides of the gap to the middle, until the two ends of the flux in the middle of the convergence. The excess flux migrates with some of the smaller particles and is deposited on the surface of the PCB at the same time. When cooled, due to surface tension, the alloy near the pad and the solder end of the component is pulled towards the pad to form a solder joint, while the alloy away from the pad gradually shrinks towards the middle of the component to form a tin bead on the side of the component. If the flux is too little, the fluidity of the solder paste is poor and the brazing effect is poor. Therefore, when selecting solder paste, it should be noted that the proportion of flux is neither too large nor too small.

3、Wetting quality

Reflow soldering, if the solder paste and pad and component wetting is poor, liquid solder in the cooling contraction process, part of the liquid alloy will flow out of the solder seam to form tin beads. Caused by poor hole wetting mainly due to two aspects; First, the solder paste, pads and component pins and other materials such as poor wettability: Second, improper brazing process, such as solder paste after printing, not timely brazing, solder paste in the alloy particles of oxidation, flux evaporation, so that the solder paste wettability becomes poor. In addition, lead-free soldering if there is no nitrogen protection brazing process is likely to cause oxidation of the brazing material, poor wettability. Printing template pad is not clean may also cause poor wettability.

4、Brazing process

Brazing temperature is too high, brazing material fluidity, brazing alloy is easy to flow out of the brazing seam, the formation of tin beads. In addition, the preheating temperature is too fast, the solvent gasification expansion rapidly, when the expansion force is greater than the adhesive force of the solder paste, it will make the solder paste under the component collapse, the solder paste is extruded out of the brazing seam and the formation of tin beads. Preheating too slowly, will produce a strong capillary phenomenon, the molten alloy is easy to be sucked out of the brazing seam, the formation of tin beads, so the reasonable design of the temperature curve can effectively reduce the tin beads. (See Figure 1.1 below).

Figure 1.1 Tin beads

In addition, the formation of tin beads is also related to the stencil aperture opening design. Too much solder paste deposition is prone to produce tin beads, so the pad aperture should be the same as the width of the component or slightly smaller, the stencil thickness should not be too thick, otherwise it is easy to cause too much solder paste deposition, resulting in tin beads. Tin bead formation is also closely related to the shape of the components, components of the main body of different heights, different heights of the bracket, components under the capillary action of the solder paste mobile capacity is also different, capillary action is strong in the reflow soldering is easy to form tin beads.

5. Specific effects of tin beads on circuits

Tin beads cause circuits to operate abnormally, especially next to capacitors or inductors. The beads that have a greater impact on the circuit are mainly located close to the centre of the sides of the capacitor or inductor and tend to be larger in size, but after the beads are generated, the results change dramatically. Assuming that the pad spacing between the two ends of the 0.8m, tin beads for 0.3mm, then the equivalent of two pad spacing of 0.5mm or so, so that the PCB surface insulation value is reduced, and the more residual, the greater the decline in the insulation value of the PCB surface, the greater the increase in leakage current, resulting in abnormal operation. If more than two solder paste balls together, the size of more than 1/2 of the pin spacing or greater than 0.3mm (even if less than 1/2 of the pin spacing), are unqualified. However, not all solder balls can cause the circuit to work abnormally, it depends on the location and size of the solder balls, and the solder balls next to the resistor have little effect on the circuit. Although not all tin beads cause the circuit to work abnormally, their presence does increase the risk of circuit failure. Therefore, tin beads should be given high priority in the electronics assembly process, and their likelihood of occurrence and impact on circuits should be reduced by taking a series of preventative measures.

summaries

Tin beads, as a common defect in the soldering process, have a variety of complex causes. The formation of tin beads can be effectively reduced by optimising the characteristics of the soldermask, precisely controlling the composition of the solder paste, improving wettability, reasonably designing the brazing process and the opening of the stencil holes, as well as paying attention to the influence of the shape and height of the components. At the same time, timely detection and cleaning of the formed solder beads can ensure the performance and reliability of electronic products. And take corresponding countermeasures to reduce its generation, so as to improve the soldering quality and reliability of electronic products.