The issue of limited wetting is often taken for granted in Europe and the U.S., as most engineers print solder paste smaller than the size of the pads. Almost all examples of fillet failures to date have been associated with the wave soldering process. Sony has been using the multi-spot soldering technique for over 10 years on its Discman assembly lines. The solder fillet lifts from the surface of the board, as shown in Figure 2. The three different types of fillet failures were highlighted and their effects on reliability are discussed in the full report.
Lift is allowable up to one pad thickness.
Most examples of this type of defect are above this height and would be considered unacceptable. Visually, the defect looks like a small quantity of solder paste that has fully reflowed and coalesced together, but has failed to form a ball. When under a chip or small active component, particles will emerge from under the side of the part and be called a solder bead. Originally, this type of defect could be seen in the early days of surface-mount when poor quality paste was used or incorrect process conditions were employed. Even with a coating of flux, the oxide coating will hinder flow.