Stabilization of the tilt motion during capillary self-alignment of rectangular chips

• We model the spreading of liquid on a square pad crossed by lyophilic bands.
• We model the alignment of a chip on the corresponding pad in presence of lyophilic bands.
• We show that the tilt mode is now restoring under the condition that the chip weight is small.
• It is shown that the other displacement modes are still restoring.
• Tilt mode will still be unstable (not restoring) for heavy chips.

Capillary self-alignment (CSA) has emerged as a convenient technique to assemble solid objects. In this technique a liquid droplet forces a mobile solid plate or chip to align with its counterpart on a solid substrate. It has been widely investigated for applications such as 3D microelectronics and assembly of optical components. It is now thought that it could be a solution for surface mounting and packaging technologies. For 3D microelectronics, where square or rectangular chips are used, it has been found that amongst the four displacement modes, i.e. shift, twist, lift and tilt, only the tilt mode was unstable (not restoring). In particular, tilting of a floating square or rectangular chip may trigger a direct contact between the plate and the pad that impedes alignment. In this text, an analysis of the tilt mode is first presented. Second, it is demonstrated that tilt can be stabilized by incorporating specific geometrical features such as lyophilic bands patterned on the substrate.