Solder bead is one of the common defects of reflow soldering, the cause is multi-faceted, not only affects the appearance of the solder joints and will cause bridging. Tin beads can be divided into two categories, one appears on the side of the chip components, often a large independent ball; the other appears in the IC pins around, scattered small beads.

The main reasons for the generation of solder beads are as follows:

Incorrect temperature profile

Reflow profile can be divided into four stages, respectively, preheating, constant temperature, reflow and cooling. Preheating, constant temperature is designed to make the PCB surface temperature in 60 ~ 90s to 150 ℃, and constant temperature of about 90s, which not only reduces the PCB and components of the thermal shock, but also to ensure that the solvent of the solder paste can be partially volatile, to avoid reflow soldering due to too much solvent caused by the splash, resulting in the solder paste rushed out of the pad and the formation of tin beads.

Solution: pay attention to the rate of heating, and take moderate preheating, so that there is a good platform to make most of the solvent volatile.

Quality of solder paste

Solder paste in the metal content is usually (90 ± 0.5)%, the metal content is too low will lead to too much flux components, and too much flux will not be easy to evaporate due to the preheating stage caused by tin beads.

Increased water vapor and oxygen content in the solder paste can also cause beads. Since solder paste is usually refrigerated, when it is taken out of the refrigerator, the recovery time is not ensured, so it can lead to the entry of water vapor. In addition, the lid of the solder paste bottle should be tightened after each use, and if it is not tightened in time, it can also lead to the entry of water vapor.

Put on the template printed solder paste in the completion of the remaining part should be handled separately, if then put back into the original tank, will cause the deterioration of the solder paste in the tank, but also produce tin beads.

Solution: Choose high quality solder paste; pay attention to the storage and use requirements of solder paste.

Problems in the printing and patching process

(1) in the solder paste printing process, due to the template and pad alignment will be offset, if the offset is too large will lead to the solder paste dip flow to the pad outside, easy to appear after heating beads. In addition, the printing environment is not good will also lead to the generation of tin beads, the ideal printing environment temperature (25 ± 3) ℃, relative humidity of 50% ~ 65%.

Solution: carefully adjust the clamping of the template to prevent loosening; improve the printing environment.

(2) the patch process Z-axis pressure is also an important cause of tin beads, often do not attract attention. Part of the mounter Z-axis head is based on the thickness of the component to locate, such as z-axis height adjustment is not appropriate, will cause the component pasted to the PCB on the moment the solder paste will be extruded to the pad outside the phenomenon, this part of the solder paste will be formed in the welding of the tin bead, in this case, the size of the tin bead is slightly larger.

Solution: Re-adjust the height of the Z-axis of the mounter.

(3) The thickness of the template and the opening size. Stencil thickness and opening size is too large, will lead to an increase in the amount of solder paste, but also cause the solder paste flow to the pad outside, especially with chemical corrosion method of manufacturing templates.

Solution: Select the appropriate thickness of the template and the opening size of the design, the general template opening area of 90% of the size of the pad, it is recommended to use the template opening shape as shown in the figure. After the right side of the plate size improvement, no longer appear tin ball.