Solder balling and bridging are some of the most common defects that occur in the SMT assembly process carried out using solder reflow. In most cases, these issues are caused as a result of flaws in the PCB assembly operations or poor understanding of materials involved in the manufacturing process. This blog post explains some of the reasons for solder ball creation and bridging defects, and explores different corrective actions that can be taken to resolve these issues.
Solder Balls
Solder balls are usually created due to air or water vapor that may be left trapped in the solder paste. When this air/vapor escapes too quickly, it may take a small amount of liquid solder from the joint to create a ball like structure that violates the minimum electrical clearance principle.
Corrective Action for preventing solder balls:
• Manage the solder reflow profile by increasing the pre-heating temperature of the PCB so that the flux evaporates properly before solder paste application.
• Tweak the pad size for the PCB in accordance with your operational recommendations to prevent space that may trap air inside.
• Manage solder flux application to prevent excessive amounts being plastered onto the board.
• Bake the PCB before beginning the SMT printing and reflow process.
• Ensure that the thickness of the copper plating holes on the PCB is greater than 25μm to prevent water from getting trapped inside.
Solder Bridging
Solder paste may form unwanted connections on the circuit surface if it flows abnormally and unexpectedly on the board. These connections usually connect adjacent surfaces, thereby making a conductive path which may eventually ruin the circuitry. This defect is known as solder bridging.
Corrective Action for preventing solder balls:
• Add a solder mask between adjacent pads and surfaces to prevent abnormal flow of solder paste.
• Tweak the design of the PCB stencil and change hole-aperture to better control solder paste application.
• Avoid old and new flux being used together in a single application
• Adjust the pressure of solder paste printing as well as the pick-and-place nozzle to prevent splashing.
• Ensure there is a zero print gap between the PCB and the stencil.
Hope these inputs will help you in getting better results from your SMT reflow processes. If you are still experiencing solder balls or bridging defects in your PCBs, make sure you consult an expert for corrective action.
Solder Balls
Solder balls are usually created due to air or water vapor that may be left trapped in the solder paste. When this air/vapor escapes too quickly, it may take a small amount of liquid solder from the joint to create a ball like structure that violates the minimum electrical clearance principle.
Corrective Action for preventing solder balls:
• Manage the solder reflow profile by increasing the pre-heating temperature of the PCB so that the flux evaporates properly before solder paste application.
• Tweak the pad size for the PCB in accordance with your operational recommendations to prevent space that may trap air inside.
• Manage solder flux application to prevent excessive amounts being plastered onto the board.
• Bake the PCB before beginning the SMT printing and reflow process.
• Ensure that the thickness of the copper plating holes on the PCB is greater than 25μm to prevent water from getting trapped inside.
Solder Bridging
Solder paste may form unwanted connections on the circuit surface if it flows abnormally and unexpectedly on the board. These connections usually connect adjacent surfaces, thereby making a conductive path which may eventually ruin the circuitry. This defect is known as solder bridging.
Corrective Action for preventing solder balls:
• Add a solder mask between adjacent pads and surfaces to prevent abnormal flow of solder paste.
• Tweak the design of the PCB stencil and change hole-aperture to better control solder paste application.
• Avoid old and new flux being used together in a single application
• Adjust the pressure of solder paste printing as well as the pick-and-place nozzle to prevent splashing.
• Ensure there is a zero print gap between the PCB and the stencil.
Hope these inputs will help you in getting better results from your SMT reflow processes. If you are still experiencing solder balls or bridging defects in your PCBs, make sure you consult an expert for corrective action.
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