Causes and solutions of tin bead formation in reflow soldering

Tin bead is one of the common defects in reflow soldering. Its causes are various, which will not only affect the appearance but also cause bridging. Tin beads can be divided into two types. One type appears on the side of chip components, often as an independent large ball; The other type appears around the IC pin in the form of scattered beads.

The causes of tin bead formation during reflow soldering are analyzed as follows:

Reflow soldering curve can be divided into 4 sections, which are respectively the purpose of preheating, insulation, reflow and cooling preheating and insulation

1. incorrect temperature curve

The purpose is to make the PCB surface rise to 150 ℃ within 60~90s and keep the temperature for about 90s, which can not only reduce the thermal shock of PCB and components, but also mainly ensure that the solvent of solder paste can be partially volatilized so that during reflow soldering, too much solvent will cause splashing due to the rapid rise of temperature, causing the solder paste to rush out of the pad and form solder beads. Therefore, it is usually necessary to pay attention to the heating rate, adopt moderate preheating, and have a good platform to volatilize most of the solvent, so as to inhibit the formation of tin beads.

2. quality of solder paste

The metal content in the solder paste is usually (90 ± 0.5)%. Too low metal content will lead to too much flux composition, so too much flux will cause flying beads because it is not easy to volatilize in the preheating stage.

The water vapor / oxygen content in the solder paste increases. Since the solder paste is usually refrigerated, the recovery time is not ensured when it is taken out of the refrigerator, so it will lead to the entry of water vapor. In addition, the cap of the solder paste bottle should be tightly closed after each use. If it is not tightly closed 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 shall be treated separately. If it is put back into the original bottle, the solder paste in the bottle will deteriorate and tin beads will be produced.

3. printing and pasting

During the printing process of solder paste, due to the alignment offset between the template and the pad, if the offset is too large, the solder paste will flow out of the pad, and solder beads are easy to appear after heating.

Therefore, the clamping of the formwork shall be carefully adjusted to avoid looseness. In addition, poor printing environment will also lead to the formation of tin beads. The ideal ambient temperature is (25 ± 3) ℃ and the relative humidity is 50%~65%.

The pressure in Z direction during the placement process is an important reason for causing solder beads, which often does not attract people's attention. Since the z-axis head of some placement machines is positioned according to the thickness of components, it will cause the components to be pasted on the PCB and squeeze the solder paste out of the solder pad at the moment. This part of solder paste will obviously cause solder beads. In this case, the size of the tin bead produced is slightly larger. Generally, it can be prevented by readjusting the height of the Z axis.

4. thickness and opening size of formwork

The excessive thickness and opening size of the template will increase the amount of solder paste and cause the solder paste to overflow out of the solder pad, especially for the template manufactured by chemical corrosion.

The solution is to select the template with appropriate thickness and the design of opening size. Generally, the opening area of the template is 90% of the pad size.