Solder balls after SMT reflow soldering are almost a common defect that occurs in the printed circuit board (PCB) assembly process. Several tinny balls surrounded along with the peripheral edge of the flux residue after reflowing are known as solder balls. In this section, we will discuss how to prevent solder balls.
Solder Balls can be present in wave and reflow soldering and hand soldering as well. Solder balls can be the cause of the failure or affect adversely the reliability of PCB Assembly products.
Acceptance criteria of solder balls
The presence of solder balls on PCBA indicates that something going wrong in the fabrication process and need to take the required action to eliminate solder balls defect and need to implement inspection checkpoints so that they can be filtered.
There are global standards to define the acceptance criteria of solder balling defects in surface mount assembly. According to the IPC inspection document IPC- A 610 the solder balling can be defined as
Solder balls defect SMT reflow soldering can also consider as process indicators when they are within 0.13 mm of lands or tracks, and defective it exceeds diameter 0.13 mm, or solder balls are found in clusters of more than 5 solder balls of (<= 0.13mm) 600 mm square area.
Now question is that what causes solder ball formation in surface mount technology and how to prevent solder ball formation in the SMT process.
What solder balls causes & how to prevent solder balls during reflow
It was observed that causes of solder ball formation can be random or Non-random in various process steps. Solder ball defect can be a result of poor solder paste handling, improper paste printing or cleaning, poor reflow temperature settings, rough PCB Pad Designing, or usage of oxidized electronics components during an assembly process.
There are several factors with can be responsible for solder ball generation. So, it becomes very important to analyze each step carefully in your process to find out the main cause of the defect and solutions to eliminate it from the process.
Based on data analysis and information gathering to prevent the solder balls formation recommended some sort of actions to prevent such Solder ball defect.
Tackling with Solder Ball Problem
To Tackling with Solder Ball Problem with solder balling it is required to map the whole process and take appropriate corrective actions to avoid solder balls and inspection to detect the defect.
More focus is required on solder paste printing quality, required proper Analysis of received inspection results from solder paste inspection machine. Few sample printed boards can be inspected on a magnifier as a visual inspection periodically.
There are other several solder balls causes that must be considered to make process solder ball free. A misprinted board must be properly cleaned and dried before use. Component placement will be done accurately, and bulky components must be placed at a slow speed.
The printed board must be reflowed quickly after component placement to avoid oxidation due to delay. Reflow profile must set precisely as per reminded by the solder paste supplier. Monitoring and controlling of shop floor humidity and temperature and per electronic manufacturing requirements.