• Wright, Daniel Nilsen

Abstract

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

Topics

• impedance spectroscopy
• polymer
• copper
• electrical conductivity
• plasma etching