Solder Ball Defects

Defect: solder balls

Solder balls are very small fines of solder that have separated from the main body that forms the joint. Their formation is promoted by excessive oxides in the solder paste that inhibit solder coalescence during reflow. Solder ball defects are probably the most common reflow solder defect, and there are many causes of solder ball defects beyond the system that will contribute to their formation.

Process and design-related causes of solder ball defects:

• Improper pad design
• Weak solder paste (for ambient con-ditions)
• Expired solder paste
• Misaligned print (overlapping solder mask)
• Misaligned solder mask (overlapping solder pad and print area)
• Flux is too weak for the oxidation level of the board and/or components
• Components have marginal solderability (a degree of contamination)
• Boards have marginal solderability (a degree of contamination)
• Components placed with too much Z-axis downforce by the pick&place machine
• Excessive solder paste slump after printing
• Sphere size of solder paste too large for pitch components being soldered
• Excessive moisture absorbed by solder paste
• Solder paste exposed beyond worklife
• Ambient humidity and temperature beyond solder paste work envelope

Reflow-related causes of solder ball defects:

• Too rapid heating, particularly in preheat resulting in splattering. Keep below 4K/sec. per any 20 sec. interval or follow specific parameters specified by solder paste manufacturer
• Profile incompatible with solder paste formulation resulting in drying out paste prematurely. Assure that the preflow soak (if used) is compatible with paste manufacturer’s specifications
• Reduction of oxidation can be achieved by reflow soldering in an inert (nitrogen) atmosphere. Some ultra-low residue, no-clean formulations specify this