Head in Pillow Defects

Formation Mechanism

Chemical and physical changes occur during the reflow process that prevent the solder sphere and the paste from melting together. When the component warps, the solder ball lifts away from the paste deposit, allowing the exposed solder surfaces to oxidize at high temperatures. As the board cools and the warpage decreases, the oxidized ball sinks back into the molten paste but cannot break through the oxide layer.

This action leaves a round ball resting on a flat paste deposit instead of a unified joint.

Inspection Method

Detecting these failures is notoriously difficult because the physical shape of the joint is obscured by the component body. Standard automated optical inspection systems cannot see underneath the packages, and basic electrical testing often passes because the metals are in physical contact. Engineers use high resolution x-ray inspection to analyze the solder joint geometry from multiple angles.

This method reveals the telltale boundary between the sphere and the pad that indicates a failed joint.

Preventive Measure

Eliminating these assembly failures requires optimizing both the paste chemistry and the thermal profile of the reflow oven. Using a solder paste with high activity fluxes helps to dissolve the oxides that form when the metals are separated. Modifying the thermal profile to reduce the duration of the soak phase also limits oxide buildup on the metal surfaces.

These process controls ensure that the solder remains active and coalesces when the joints make contact.