Factor 1: The impact of solder paste selection on soldering quality
a. Metal content of solder paste: The metal content in solder paste is typically 88% to 92% by mass and 50% by volume. Increasing the metal content can increase viscosity, resistance to preheating forces, and reduce the likelihood of producing solder beads.
b. Oxidation degree of metal powder in solder paste: Higher oxidation degrees can lead to decreased solderability and increased likelihood of tin bead occurrence. Typically, solder oxidation should be controlled below 0.05% with a maximum limit of 0.15%.
c. Size of metal powder in solder paste: Finer particle sizes can lead to increased oxidation and intensify solder bead formation.
d. Flux amount and activity: Excessive flux can lead to local solder paste collapse and increased likelihood of producing solder beads. Weak flux activity can also contribute to tin bead generation.
e. Other precautions: Various factors such as moisture absorption, damp PCBs, heavy indoor humidity, wind blowing against solder paste, excessive diluent, and stirring time can all promote tin bead formation.
Factor 2: Production and opening of steel mesh for solder paste
a. Opening of steel mesh: Opening the stencil 10% smaller than the actual pad size and adjusting opening shape can prevent solder paste from spilling onto the solder mask layer and reduce the occurrence of solder beads.
b. Thickness of steel mesh: The stencil thickness should be between 0.12 and 0.17mm to prevent solder paste collapse and tin bead formation.
Factor 3: Placement pressure of the placement machine
High placement pressure can cause solder paste to be squeezed onto the solder mask, leading to solder bead formation. Effective solution is to reduce mounting pressure and use suitable stencil opening forms to prevent solder paste spillage.
Factor 4: Setting of furnace temperature curve
In the preheating stage, controlling the temperature rise to less than 2.5°C/s can prevent solder splash and reduce the occurrence of tin beads during reflow soldering. Adjusting preheating temperature and speed is crucial in controlling tin bead generation.