 |
|||||||||||||||||||||||||||||
Reflow Soldering Defects - Solder Beading The formation of solder balls around a device, at the side of the component and not at the joint surface, is called "solder beading" and may be caused by a range of manufacturing issues, or it can be caused by a change in the component supplier. A variation in the plastic body height from some suppliers gives a different stand-off height from the board. During assembly, solder paste gets under the body of the component. As the board passes through the reflow oven and into the reflow zone, the paste turns into a liquid. All of the solder balls coalesce together to form a solder joint. In Figure 1, the capillary effect of paste moving under the component is exaggerated and then squeezes out during reflow.
The same thing occurred in Figure 2. Paste under the part lifts the part, allowing liquid to escape. The component then lowers back onto the board, leaving the solder bead. To determine when the paste enters under the parts, check paste printing quality. Remove parts prior to reflow and check for paste. Pass a fully loaded board through reflow, changing the final zone temperature to prevent paste reflow. When the board has exited, check for paste under parts. By finding out when the paste gets under the parts, it is possible to eliminate the problem.
With leadless ceramic chip carriers (LCCC), it is more likely that any solder beading is due to placement. When the component is placed onto the solder paste, the paste is forced under the body of the device. Because the LCCC termination and ceramic body are all relatively flat, some paste displacement must occur.
Remember to also look at paste slumping as a cause.
Reflow Soldering Defects:
Text and photos courtesy of Bob Willis |
