Bridging Solder Balls and Mid-Chip Balling
The first three defects bridging, solder balls and mid-chip balling can arise from the solder selection process. Since preheats are higher with lead-free, the hot slump character of the paste is critical; solder pastes with good hot slump at higher temperatures such as 185 °C are needed. Traditional 63/37 paste has already melted and flowed at these temperatures; the gelling materials have broken down. The example below, demonstrates this quite well, two SAC solder pastes are shown.
Both pastes were run through a reflow oven at 180 °C. Paste B has better hot slump properties than Paste A and would less likely cause bridges, solder balls or mid-chip balling. For fine pitch components it is critical to select a lead-free paste with a heat stable gelling agent.
Increasing need for superior hot slump