Flip-chip-on-flex Research Articles

Toward Synthetic Vascular Graft Monitoring Using a Flip-Chip-on-Flex Impedance Spectroscopy Sensor

Synthetic vascular grafts are used in a wide range of clinical applications. However, they can become prone to occlusion over time, due to the growth of vascular smooth muscle cells (VSMCs). This phenomenon is associated with graft failure. Here, we describe novel techniques for packaging and integration of a miniature sensor of VSMC growth. The sensor was based on a microfabricated array of interdigitated platinum electrodes on a silicon substrate. It was assembled using flip-chip-on-flex (FCOF) technology to create a low-profile package that can be manufactured using industry-standard tools and is suitable for integration with a synthetic vascular graft. The packaged sensor responded to changes in solution impedance and accurately detected the growth of VSMC in vitro. We also demonstrated successful proof-of-concept integration of the sensor with a custom synthetic graft. Together these technologies have the potential to allow early detection of VSMC growth prior to graft occlusion, enabling more timely and effective clinical interventions to be made.

Failure modeling of ACA-glued flip-chip on flex assemblies

This paper addresses the reliability of flip-chip on flex (FCOF) assemblies glued with an Ag-particle filled anisotropic conductive adhesive (ACA). As the description of FCOF failure gives still much scope for speculation, a physics of failure based approach is developed here, taking into account the changing thermo-mechanical properties of the ACA under temperature and moisture. A failure hypothesis is formulated based on the loss of contact pressure. Material analysis, material characterisation, finite element (FE) modeling and lifetime tests have been employed to establish correlations to support this failure hypothesis. It was found, that moisture plays the most important role for interconnect failure. The model is able to predict quantitative changes of force as function of loading parameters and correlate them qualitatively to the experimental mean time to failure. New insights are provided about the stress fields at the ACA bump. The model is discussed with respect to a direct prediction of failure versus time.

Mechanics of Adhesively Bonded Flip-Chip-on-Flex Assemblies. Part II: Effect of Bump Coplanarity on Manufacturability and Durability of Non-Conducting Adhesive Assemblies

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.

Mechanics of Adhesively Bonded Flip-Chip-on-Flex Assemblies. Part I: Durability of Anisotropically Conductive Adhesive Interconnects

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 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.

The effect of bonding force on the electrical performance and reliability of NCA joints processed at a lowered temperature

The effect of the bonding force during flip chip-on-flex (FCOF) assembly on the electrical performance of the nonconductive adhesive (NCA) interconnects was investigated in this study, under the precondition of a reduced processing temperature in order to minimize thermally-induced damage to the low-cost flexible substrates. Pressure cooker tests (PCT) were performed to assess the reliability performance of the adhesive joints in high temperature and high humidity conditions. The assembly process was modified and the processing temperature and the bonding force were adjusted according to the experimental results to enable the use of low cost substrates, such as poly(ethylene terephthalate) (PET) materials in smart card fabrication.

Effect of bump characteristics and temperature variation on the online contact resistance of anisotropic conductive joints

Anisotropic conductive film (ACF) suffers a major drawback in regard to reliability even though it has merits, such as reduction in interconnection distance, high performance, and environmental friendliness. The factor of thermal warpage may lead to a highly unreliable electrical connection in the assembly. The work presented in this paper focuses on the online contact-resistance behavior of the ACF joint during thermal shock and compares the results of two different types of dies (Au/Ni bump and bumpless). For this work, we used a flip chip of 11 × 3 mm2 in dimension. The flex substrate used was made of polyimide film with an Au/Ni/Cu electrode and daisy-chained circuit for a matching die-bump pattern. The ACF that was used is an epoxy resin in which nickel and gold-coated polymer balls are dispersed. Tests for three different thermal-cycling profiles (125°C to −55°C, 140°C to −40°C, and 150°C to −65°C) were carried out. The samples bonded at a temperature of 180°C, and a pressure of 80 N was used. The initial contact resistances of Au/Ni bump and bumpless samples were 0.25 ω and 0.4 ω respectively. A comparative study was carried out from the results obtained. The results showed that for the flip-chip-on-flex (FCOF) packages having an Au/Ni bump, the increase in online contact resistance is higher than that of the FCOF packages having bumpless chips. For example, in the thermal-cycling profile of 140°C to −40°C, the online contact resistance for the Au/Ni bump raised to 4.6 ω after 180 cycles, whereas it was only 1.3 ω for the bumpless sample. The bump height and bump materials were found to be the main factor for such variation. Results show that, above the glass-transition temperature (Tg), the ACF matrix becomes less viscous, which reduces its adhesive strength and lets the higher bump height of the chip result in a higher standoff of the package and thus sliding is easier to take place. The responses by the assemblies in hot and cold conditions are examined, and in-chamber behavior of the assembly is studied and explained.

Flip-chip on flex integrated power electronics modules for high-density power integration

Three-dimensional flip-chip on flex (FCOF) integrated power electronics modules (IPEMs) have been fabricated for high-density power applications. In this FCOF-IPEM structure, solder-bumped devices were flip-soldered to a flexible substrate with electrical circuits etched on both sides. One side of the flex provides interconnection to power devices while the other is used to construct a simple gate-drive circuit; via holes through the flex integrate the power stage and gate-drive together. Solder-bumped MOSFET devices were obtained by a metallization processing and were used in the FCOF power module construction to improve thermal performance, power density, and integration. With this packaging approach, the multiple solder bumps, instead of the thin, long bonding wires were utilized to connect the power devices to the flex substrate and to improve heat dissipation, lower parasitic oscillations, and reduce package size. Reliability of solder joints has been dealt with through selection of materials, such as use of flexible substrates and underfill encapsulation, and design of joint shape for lower thermomechanical stresses. A comparative study of continuous switching test results have shown that the FCOF-IPEMs have better electrical performance than commercial wire bonded power modules.

Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly II. Different bonding pressure

The effects of different bonding pressure during flip-chip-on-flex (FCOF) assembly in relation to the performance of anisotropic conductive film (ACF) interconnect were investigated. Two types of ACF were used in this study. ACF 1 is designed to create good interconnection when the connecting bumps and pads are in close contact while ACF 2 can give good connections when the conductive particles are in contact with the bumps and pads, hence the deformation of conductive particles within ACF 2 FCOF packages were less than that within ACF 1 packages. ACF 2 gave much better interconnection performance when compared to ACF 1 indicating that ACF 2 is more flexible and can tolerate a wider range of bonding pressure.

Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly I. Different bonding temperature

The effects of different bonding temperatures during flip-chip-on-flex (FCOF) assembly in relation to the performance of anisotropic conductive adhesive (ACF) interconnect were investigated. Two types of flip chips were used in this study. It was found that Ni bumps formed better interconnections than bumpless FCOF packages. Aluminium oxide was observed and was thought to be the main cause of the increased in contact resistance after the moisture-soak tests. The conductive particles were not fully compressed by the bumps and pads and gaps were observed between the conductive particles and Cu pads in bumpless packages. Conductive particles in the Ni bump FCOF packages were tightly trapped between the bumps and pads and hence gave better connections. The performance of the ACF interconnects were affected by the degree of curing of the ACF, which was determined by the bonding temperature.

Nondestructive methodology for standoff height measurement of flip chip on flex (FCOF) by SAM

Flip chip technology is the emerging interconnect technology for the next generation of high performance electronics. One of the important criteria for reliability is the width of the gap between the die and the substrate, i.e., the standoff height. A nondestructive technique using scanning acoustic microscopy (SAM) for the standoff height measurement of flip chip assemblies is demonstrated. The method, by means of the implementation of a pulse separation technique, time difference of the representative signals of the die bottom and water interface and water and substrate surface interface from the A-scan image can he found. Then, the corresponding standoff height can be calculated. When compared to the traditional destructive measurement method (SEM analysis on sectioned sample), this nondestructive technique yields reliable results.

FLIP CHIP ON FR-4, CERAMICS AND FLEX

This paper discusses flip chip on FR-4, ceramics and flex investigated in our research institution and the industry. The reliability performance of the processes of flip chip on FR-4 and ceramics using gold bumps with anisotropic conductive film or anisotropic conductive paste, and eutectic solder bumps with underfill epoxy was studied. Flip chip on ceramic substrate gives much better reliability than flip chip on FR-4 boards. Most of the samples assembled on ceramic boards using eutectic solder with underfill epoxy survived after 9,500 cycles of the thermal cycling test (-40°C to 125°C). Also, flip chip on FR-4 substrate using eutectic solder with underfill epoxy showed reasonable reliability performance. Flip chip on flex (FCOF) was also studied as FCOF can be the next generation of flex circuit packaging for cheaper and faster speed performance. The FCOF utilized eutectic solder bumps for the chip interconnection. Prototypes of the FCOF were assembled and their reliability was tested with satisfactory results.