This is Part I of a two-part paper on adhesively bonded flip-chip-on-flex (FCOF) microelectronic assemblies. This paper examines the use of an-isotropically conductive adhesives (ACAs), and the companion paper (Part II) addresses the use of non-conductive adhesives (NCAs). Two types of FCOF dies, bonded with ACAs, were subjected to temperature cycling and repeated temperature shock durability tests. The specimens have Au-plated bumps and the ACA contains Au-plated polymer particles. Specimens were fabricated with different bonding pressures. Scanning Electron Microscopy (SEM) investigations were performed to characterize the distribution and shape of the conducting particles.

Contact resistance measurements, conducted throughout the temperature cycling durability test for 1000 temperature cycles between 20°C and 115°C, showed that resistance varied cyclically with each temperature cycle, but there was no increase in the average resistance over the duration of the test. In contrast, cyclic thermal shock tests between -50°C and 115°C produced failures within 200 to 2500 cycles, depending on the specimen configuration.

Studies were conducted to examine the effect of specimen configuration and temperature on: (i) interconnect resistance, based on electro-thermal modeling; and (ii) mechanical contact stresses, based on thermomechanical modeling. Modeling techniques used detailed finite element analysis (FEA) as well as simpler rapid-assessment models based on 2D variational methods (for example, the Raleigh-Ritz method) and 1D electrical resistance networks. These models were used to parametrically investigate the influence of interconnect design, bonding pressure, temperature and interconnect degradation mechanisms, on the contact resistance and
contact stresses.

Results suggested that the thermal expansion of the ACA during temperature cycling did not cause any changes in the contact resistance at the contact interface, thus suggesting the possibility of bonding at Au-Au interfaces. During thermal shock, the expansion mismatch stress between the die and Printed Wiring Board (PWB) at the low temperature (-50°C) was probably sufficient to fracture this bond. The issue of gold-to-gold bonding will be addressed in a future paper.

Keywords: flip-chip-on-flex, an-isotropically conductive adhesives, thermal cycling test, thermal shock test, failure mechanisms, bonding pressure, interconnect resistance, finite element model, Raleigh-Ritz model, parametric studies.

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