Level III-case control study.

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.

No abstract

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 anisotropically 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 200-2500 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 AuAu 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.

One fifth of idiopathic clubfoot deformities cannot be fully corrected by Serial Ponseti casting and deformity recurs in 20%–30% of cases. To avoid x-ray exposure, the joints with largely unossified bones are diagnosed with magnetic resonance images (MRI). Typically, geometric measurements are made in the MRI planes; however, this method is inaccurate compared to measurements on three-dimensional (3D) models of the joint. More accurate measurements using the 3D bone shapes may be better at identifying differences between groups; and therefore, improve diagnosis. The entire set of shape features from MRI can be analysed simultaneously through statistical shape modelling (SSM) which assesses bone morphology of clubfoot in a more sensitive way. A method for SSM of the talus is developed in this study and the shape of the normal talus is compared with the one in clubfeet with residual deformity through both geometric measurements and SSM. Significant differences between two groups were found by both methods; and therefore, might contribute to improve diagnosis of clubfoot.