Detailed explanation of the solder paste process in the dropped pieces and solder ball problem

In the electronic assembly process, dropped parts and solder balls are common quality problems, they have a direct impact on the reliability and performance of the product.

The following is a detailed analysis of the problem of dropped parts and solder balls:

1. Dropped parts Influencing factors: Solder adhesion: the adhesion between the solder and the component pin is a key factor in preventing dropped parts. The stronger the bond, the more stable the component is during the soldering process, and the lower the probability of dropping.

Component type: different types of components due to their size, weight and shape differences, the welding stability requirements are different. For example, 0201 capacitors and resistors, compared to the capacitor drop probability is high. This is because the average height of the capacitor is 0.305mm, the quality of 0.00028g, while the average height of the resistor head 0.229 ~ 0.254mm, the quality of 0.00014g. In the placement process, if the resistor has 2/3 of the volume of the solder paste into the capacitance of only 1/2 of the volume of the volume of the solder paste into the capacitance when the circuit board is moving or air blowing, capacitance is more likely to be dislodged from its position. When the board moves or air blows, the capacitor is more likely to fall out of its position.

Pad Shape: The shape and size of the pad has a significant impact on the quality of the soldering, and H-shaped pads can provide stronger adhesion due to their larger area and higher volume transfer rate, thus preventing the occurrence of dropouts more effectively.

 Solution: Optimise solder formulation: Adjust the composition and proportion of the solder to improve its adhesion to component pins.

Improve the design of components: for the type of components that are prone to falling off, you can consider increasing their weight or changing their shape to improve their stability in the soldering process.

Optimise pad design: Use H-shaped pads or pads of other shapes and sizes to improve soldering quality and prevent dropped parts.

 2. Solder balls Solder balls are a kind of defects formed during the reflow process and are usually classified into three categories: (1) Solder balls around the solder ends of chip components These solder balls are usually caused by moisture absorption of solder paste or improper setting of temperature profile. In the preheating stage, if the temperature rises too quickly or the preheating time is not enough, the moisture and solvent inside the solder paste can not be fully evaporated, in the reflux will cause boiling and spattering of molten metal, the formation of solder balls. Solution: Adjust the temperature profile to reduce the rate of climb in the preheat zone and increase the preheat time. Ensure that the solder paste has sufficiently returned to room temperature before use and avoid using it immediately after removing it from the refrigerator.

(2) Stencil Residue Stencil residue is usually caused by a poorly functioning automatic cleaning system in the solder paste printer. Solder paste that has not been cleaned will remain on the bottom surface of the stencil and adhere to the board surface when it is reprinted, forming tiny solder balls. Solution: Regularly check and maintain the automatic cleaning system of the solder paste printer to ensure its effectiveness. Thoroughly clean the stencil before printing to avoid solder paste residue.

(3) Solder balls caused by poor printing When there is poor contact between the stencil and the circuit board, or the aperture of the stencil is too large, or when non-contact printing is used, the solder paste may be squeezed out from underneath the stencil and smeared on the circuit board, resulting in large solder balls. Solution: Ensure good contact between the stencil and the board to avoid leakage and extrusion during printing. Regularly check the aperture and shape of the stencil to ensure that it meets the printing requirements. For non-contact printing, optimise the printing parameters and process flow to reduce solder paste extrusion and smearing.

By optimising the solder formula, improving the design of components and pads, adjusting the temperature profile, and strengthening the maintenance and cleaning of equipment, the occurrence of soldering defects such as dropped parts and solder balls can be effectively reduced, and the quality and reliability of electronic products can be improved.