This is Part II of a two-part paper on the mechanics of adhesively bonded Flip-Chip-on- Flex (FCOF) assemblies. Part I dealt with the use of anisotropic conductive adhesives (ACAs) while this paper deals with the use of non-conducting adhesives (NCAs). The central concern here is the influence of bump coplanarity on the manufacturability and durability of the assembly. The assembly is first analyzed with global/local nonlinear finite element models to assess the effect of bump coplanarity on the maximum force needed to achieve reliable bonding. The number of bumps at the low end of the manufacturing height tolerance is parametrically increased to quantify the effect on the bonding force. Results confirm that the bonding force required is inversely proportional to the number of ‘short’ bumps in the assembly. Next, the bonding and adhesive curing process is simulated in detail with finite element models, in order to assess the residual pre-stress between matching interconnect bumps, since this compressive contact stress may be important to the long-term performance of the FCOF assembly. The nonlinearities addressed in the model include elastic-plastic properties of gold, viscoplastic properties of the NCA and evolution of contact area between the mating bumps. Results show that it is necessary to model the viscoplasticity of the NCA to obtain realistic predictions, and that the residual compressive pre-stress between the mating bumps increases as
the percentage of ‘short’ bumps increases in the assembly.

Keywords: FCOF, non-conductive adhesive, gold bump, copper bump, fabrication pressure, residual pre-stress, contact force, viscoplasticity, elastic-plastic deformation, curing shrinkage, contact surface area

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