Causes and solutions of reflow solder beads

 Tin bead is one of the common defects in reflow welding, which has many reasons, not only affects the appearance of solder joints but also causes bridging. Tin beads can be divided into two categories, one appears on the side of the chip component, often an independent large ball; The other type appears around the IC pin in a scattered bead shape.

 

 

 


 

The main reasons for the production of tin beads are:

 

The temperature curve is incorrect

 

The reflow curve can be divided into four stages, namely preheating, constant temperature, reflow and cooling. The purpose of preheating and constant temperature is to make the PCB surface temperature rise to 150 ° C within 60~90s, and the constant temperature is about 90s, which can not only reduce the thermal shock of PCB and components, but also ensure that the solvent of the solder paste can be partially volatilized, and avoid splashing caused by too much solvent during reflow welding, causing the solder paste to rush out of the pad and form tin beads.

 

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

 

Quality of solder paste

 

The metal content in the solder paste is usually (90±0.5) %, the metal content is too low will lead to too much flux composition, and too much flux will cause tin beads due to the preheating stage is not volatile.

 

Increased water vapor and oxygen content in solder paste can also cause tin beads. Because the solder paste is usually refrigerated, when it is removed from the refrigerator, there is no guarantee of recovery time, which will lead to the entry of water vapor, in addition, the lid of the solder paste bottle should be closed after each use, if it is not tightly covered in time, it will also lead to the entry of water vapor.

 

After the solder paste printed on the template is completed, the remaining part should be treated separately. If it is put back into the original tank, it will cause the solder paste in the tank to deteriorate and will 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 mounting process

 

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

 

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

 

2, the Z-axis pressure in the mounting process is also an important cause of tin beads, often do not attract people's attention. Part of the Z-axis head is located according to the thickness of the component, such as the improper adjustment of the height of the Z-axis will cause the solder paste to be extruding to the outside of the solder plate at the moment the component is attached to the PCB, and this part of the solder paste will form tin beads during welding, in this case the size of the tin bead is slightly larger.

 

Solution: re-adjust the Z-axis height of the placement machine.

 

3, the thickness of the template and opening size. Excessive plate thickness and opening size will lead to an increase in the amount of solder paste, and will also cause the solder paste to spill out of the plate, especially if the template is manufactured by chemical corrosion methods.

 

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

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