A solder ball is a small solder powder that is separated from the main body of the solder joint. Too much oxide in the solder paste will promote its formation, and these oxides will inhibit solder coalescence during reflow. Solder balls are probably the most common Reflow soldering defects, and there are many reasons outside the system that will help it form.
Reasons related to process and design:
• Improper pad design
• The flux function is weakened due to environmental temperature factors
• Expired solder paste
• Printing errors (overlapping solder mask)
• Misalignment of the solder mask (the pad and the printed area overlap)
• The degree of oxidation of the flux to the circuit board and/or components is too weak
• Marginal solderability of components (degree of contamination)
• The marginal solderability of the circuit board (a certain degree of contamination)
• The placement machine places components under excessive pressure on the Z axis
• Excessive slumping of solder paste after printing
• The spherical size of the solder paste is too large to solder pitch components
• Solder paste absorbs too much water
• Solder paste is exposed to air and has exceeded its service life
• Ambient humidity and temperature are beyond the acceptable range of solder paste
Reasons for reflux:
• Heating up too fast, especially during preheating, will cause splashing. Keep it below 4K/sec. Every 20 seconds. Interval or follow specific parameters specified by the solder paste manufacturer
• The profile is not compatible with the solder paste formulation, causing the solder paste to dry prematurely. Ensure that the prepreg (if used) is compatible with the solder manufacturer’s specifications
• Reduction of oxidation can be achieved by reflow soldering in an inert (nitrogen) atmosphere. Some ultra-low residue, no-clean formulas can illustrate this
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