Magnetic Force‐Induced Self‐Assembly and Flip‐Chip Alignment on Electroplated Multilayer Solder with Tailored Melting Points

Abstract This communication demonstrates fluidic self‐assembly and self‐alignment of flip‐chips applying magnetic field force for chip agitation and single angular orientation. The receptor solder bumps are Pb‐free electroplated core and core‐shell metal layers with tailored transformation imprinted melting points. While the capturing step takes place on the low‐melting shell Bi_33.7In_66.3 (72 °C) of the capturing Cu‐pad, the high‐melting core tin on capturing as well as on non‐capturing Cu‐pad remains solid. The flip‐chips are provided with a nickel bar on top and are agitated over the substrate using magnetic field force in water as an assembly medium. The reduction of the free surface energy of the shell‐water interface provides the driving force for the capturing step in heated water. Once the capturing step on all shell receptors is completed, the external magnetic field causes the single angular orientation of the flip‐chips. A short reflow causes the transformation of the core and the shell in a high‐melting point solder on capturing receptors. Additionally, the formation of a solder joint between the core on the non‐capturing Cu‐pad and the corresponding Au‐binding side on the chip succeeds in the same reflow step yielding the second stable high melting interconnect of the flip‐chips to the substrate.