Deformation and Fracture of Micron‐Sized Metal‐Coated Polymer Spheres: An In Situ Study

Abstract

Micron-sized metal-coated polymer spheres are the dominant conductive component in anisotropic conductive adhesives for electronic interconnect. The interconnect is formed under compression, so an understanding of the deformation and fracture of the metal-coated polymer spheres is paramount in particle design for fine pitch applications. The fracture of metal-coated polymer spheres has previous been measured but not observed in-situ. Using a nanoindentation set-up mounted in a scanning electron microscope, 10 µm polymer spheres with Ni, Ni/Au, Au and Ag coatings were compressed and their behavior observed in-situ.The metal systems were observed to lead to fracture in two different modes, brittle and ductile.The fracture mode was not dependent on the type of metal, and not on the coating thickness.Additionally, the rate dependency of the mechanical behavior due to the viscoelastic polymer core was studied, and found to influence the deformation and stress-strain behavior around the fracture point.

Contact area measurement of micron-sized metal-coated polymer particles under compression

“…The Ni-coated MPS exhibit stressdrops during loading, which have been linked to brittle fracture events, while the Au and Ag MPS have monotonic loading curves. The loop around the stress drops indicates cracking, but the magnitude of the stress drop is an artefact of the transducer attempting to maintain constant deformation rate with force actuation and not the result of material properties [34]. A thicker metal layer is generally expected to result in stiffer stress-strain response; we see in this case the opposite is true and the Ni90 is apparently stiffer than Ni120.…”

“…A major trend is that Ni MPS (Figure 5 b & d) fracture in a brittle manner, with pronounced global delamination, while the Ag and Au MPS (Figure 5 a & c) deform in a ductile manner. This has been discussed in detail [34]. Previous simulation work has predicted that the fracture of the metal coating in MPS will initiate at the equator of the particle and propagate towards the contact area [36].…”

Mechanical stability and insulation reliability of polymeric layers constructed by thermal fusion of polymer shell particles heterocoagulated on conductivenickel‐platedcore particles

“…According to previous reports, ACFs containing conductive particles coated with an insulating layer are known to have a low incidence of electrical shorts 48,59 . In addition, the properties of composite particles with multilayered structures have also been analyzed in detail by the nanoindentation method, which simultaneously evaluates the mechanical and conductive properties of metal‐coated conductive particles 60–65 . To clarify the relationship between the state of the insulation layer of heterocoagulates and their insulating properties, the nanoindentation method was applied to evaluate the morphological changes in the insulation layer of heterocoagulates.…”