"Solder beads" is one of the common defects in reflow soldering. It not only affects the appearance but also causes bridging (Will be discussed below). "Solder beads" can be divided into two types:
One type appears on one side of the chip component, often in the shape of an independent large ball (as shown in below picture);
The other type appears around the IC pins and is in the shape of scattered small beads.
The main reason why "solder beads" happen are as follows:
Factor A: The temperature curve is incorrect:
The reflow soldering curve can be divided into 4 sections: preheating, heat preservation, reflow and cooling. The purpose of preheating and heat preservation is to make the surface temperature of the PCB rise to 150°C within 60-90s and keep it warm for about 90s. This not only reduces the thermal shock of the PCB and components, but also ensures that the solvent of the solder paste can be partially volatilized. Avoid splashing caused by too much solvent during reflow soldering, causing the solder paste to rush out of the pad and form solder beads.
★Solution: The factory needs to pay attention to the heating speed and adopt suitable preheating to fully volatilize the solvent.
Factor B: the quality of solder paste 1. The metal content in the solder paste is usually (90±0.5)℅. If the metal content is too low, it will cause too much flux. Therefore, too much flux will cause flying beads due to the fact that the preheating stage is not easy to volatilize; 2. The increase of water vapor and oxygen content in the solder paste will also cause flying beads. Since solder paste is usually stocked in refrigerator, when it is taken out of the refrigerator, if it is not fully warmed and stirred evenly, the water vapor would enter. In addition, the lid of the solder paste bottle must be tightly closed after use. If it is not tightly closed in time, It will also cause the entry of water vapor; 3. After the solder paste printed on the stencil is completed, the remaining part should be treated separately. If it is put back in the original bottle, it would cause the solder paste in the bottle to deteriorate and this would also produce solder beads;
★Solution: require the factory to choose high-quality solder paste, pay attention to the storage and use requirements of the solder paste.
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