Development of mechanically compliant flip chip interconnect using single metal coated polymer spheres

Most available fine pitch interconnects, like micro bumps and copper pillars, are not particularly compliant
whereas available compliant interconnects, like plastic core solder balls, are not fine pitch. Using Ag plated
polymer spheres (MPS) in conjunction with a nano-Ag conductive ink has the potential to achieve mechanically
compliant flip chip interconnects since the structural integrity is maintained by the flexible polymer core while
the electrical conductivity is maintained by the Ag plated shell. Additionally, the low processing temperature
means that it is relevant for systems that require low temperatures or that are very sensitive to thermo -
mechanical stress, like MEMS sensors.
Previous work has shown that a major challenge in the proposed process was the confinement of the conductive
ink onto the Au pad. This work focuses on finding an oleophobic coating that can be patterned to confine the ink
on the contact pads. Two materials were tested, a fluoroacrylate additive for photoresists and a fluoropolymer
that needed to be patterned separately. The latter showed superior oleophobicity and was therefore chosen.
Patterning by positive and negative photoresist was tested. Using positive photoresist as a masking layer for
reactive ion etching proved incompatible with the desired output. The use of negative photoresist with a lift-off
technique showed potential, but needs to be optimized. Using reactive ion etching through a stencil mask showed
the best results.