Defect Formation
Surface mount assembly of small passive components like resistors and capacitors can suffer from the formation of unwanted metallic spheres adjacent to the component body. These mid-chip solder beads occur when excess solder paste is squeezed out from the pad during component placement and gathers on the non-wetting ceramic surface of the chip. During reflow, this isolated paste melts and coalesces into a bead that does not merge with the main solder joint.
The resulting bead remains loose or attached to the side of the component, presenting a risk of electrical short circuits. The phenomenon is most common in 0603 and 0805 packages, where the gap between the component body and the board is very small. Adjusting the stencil aperture shape, such as using a home-plate design, reduces the amount of paste in the center, minimizing the defect rate.
Placement Pressure
High placement force on the surface mount machine drives the component too deep into the printed solder paste. This action displaces paste sideways onto the solder mask, where it cannot wet the metal pads during reflow. Calibrating the placement height and force of the pick-and-place nozzles avoids this displacement.
Reflow Profiles
Rapid heating in the preheat zone of the reflow oven causes the flux to boil violently, which sputters solder particles away from the pads. Slowing the ramp rate allows the flux to outgas gently, keeping the solder paste unified. This thermal management reduces the occurrence of beads on the circuit board.
