Solder beading is one of the common defects in reflow soldering, it not only affects the appearance but also causes bridging. Tin beads can be divided into two categories, one appears on the side of chip components, often in the shape of an independent large ball; the other appears around the IC pins, in the shape of scattered beads. There are many reasons for the generation of tin beads, and the analysis is as follows：

2.1、The temperature curve is incorrect. The reflow soldering curve can be divided into 4 sections, namely preheating, heat preservation, reflow and cooling. The purpose of preheating and heat preservation is to increase the surface temperature of the PCB to 150°C within 60-90s, and keep it for about 90s , This can not only reduce the thermal shock of the PCB and components, but also ensure that the solvent of the solder paste can be partially volatilized to avoid splashing due to too much solvent during reflow soldering, causing the solder paste to punch out of the pad and form tin beads.

Solution: Pay attention to the heating rate and take a moderate preheating so that it has a good platform to make most of the solvent evaporate。

2.2quality of solder paste

2.2.1 The metal content in the solder paste is usually (90±0.5)℅. If the metal content is too low, the composition of the flux will be too much. Therefore, too much flux will cause flying beads because it is not easy to volatilize during the preheating stage.

2.2.2 Increased water vapor and oxygen content in solder paste can also cause fly beading. Since solder paste is usually refrigerated, when it is taken out of the refrigerator, if the recovery time is not ensured, water vapor will enter; in addition, the lid of the solder paste bottle should be tightly closed after each use. entry of steam.

The solder paste printed on the stencil after completion. The remaining part should be treated separately. If it is put back into the original bottle, it will cause the solder paste in the bottle to deteriorate and also produce tin beads.

Solution: choose high-quality solder paste, pay attention to the storage and use requirements of solder paste.

2.3, printing and patching

2.3.1 In the printing process of solder paste, due to the deviation between the template and the pad, if the offset is too large, the solder paste will flow out of the pad, and solder balls are likely to appear after heating. In addition, poor printing working environment will also lead to the formation of tin beads. The ideal printing environment temperature is 25±3℃, and the relative humidity is 50℅～65℅

Solution: carefully adjust the clamping of the template to prevent loosening. Improve the printing work environment.

2.3.2 The pressure of the Z-axis during the placement process is also an important cause of solder balls, but it often does not attract people's attention. The Z-axis head of some placement machines is positioned according to the thickness of the component. If the height of the Z-axis is not adjusted properly, it will cause the solder paste to be squeezed out of the pad at the moment when the component is attached to the PCB. This part of the solder paste will Tin balls are formed during soldering. The resulting tin bead size is slightly larger in this case.

Solution: Readjust the Z-axis height of the placement machine。

2.3.3 Thickness and opening size of the template. Excessive thickness of the stencil and opening size will lead to an increase in the amount of solder paste, and it will also cause the solder paste to flow out of the pad, especially for the template manufactured by chemical corrosion methods.

Solution: Choose a template with an appropriate thickness and the design of the opening size. Generally, the opening area of the template is 90℅ of the pad size.