Thermosonic flip chip interconnection using electroplated copper column arrays

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Review of Direct Metal Bonding for Microelectronic Interconnections

Microelectronic interconnections require advanced joining techniques. Direct metal bonding methods, which include thercomsonic and thermocompression bonding, offer remarkable advantages over soldering and adhesives joining. These processes are reviewed in this paper. The progress made in this area is outlined. Some work concerned with the bonding modeling is also presented. This model is based on the joint interface mechanics resulting from compression. Both bump and substrate deformation are taken into account. The improved understanding of the relationship between the deformation and bonding formation may provide more accurate joint evaluation criterion.Singapore-MIT Alliance (SMA

Design of high frequency ultrasonic transducers with flexure decoupling flanges for thermosonic bonding

This paper presents the design of high frequency ultrasonic transducers for micro/nano device thermosonic bonding. The transducers are actuated by piezoelectric ceramics and decoupled with their connecting parts through novel flexure decoupling flanges. Firstly, the initial geometric dimensions of the transducers were calculated using electromechanical equivalent method, then dynamic optimization design was carried out based on 3D finite element method (FEM), and the geometric dimensions of the transducer were finally determined. Flexure decoupling flanges were presented, and the decoupling principle of the flanges was explained through compliance modeling using compliance matrix method and FEM. After that the dynamic characteristics of the transducers were analyzed through finite element analysis (FEA). The vibration frequencies and modes of the piezoelectric converter, concentrators and transducers were obtained through modal analysis, and the displacement nodes were determined. The longitudinal ultrasonic energy transmission was presented and the decoupling effects of the flexure flanges were compared. Finally, the transducers were manufactured and experimental tests were conducted to examine the transducer characteristics using an impedance analyzer. The experimental results match well with the FEA. The results show that the longitudinal vibration frequencies of the transducers with ring, prismatic beam and circular notched hinge based flanges are 126.6 kHz, 125.8 kHz and 125.52 kHz, respectively. The decoupling flange with circular notched hinges shows the best decoupling effect among the three types of flanges

Peripheral soldering of flip chip joints on passive RFID tags

Flip chip is the main component of a RFID tag. It is used in billions each year in electronic packaging industries because of its small size, high performance and reliability as well as low cost. They are used in microprocessors, cell phones, watches and automobiles. RFID tags are applied to or incorporated into a product, animal, or person for identification and tracking using radio waves. Some tags can be read from several meters away or even beyond the line of sight of the reader. Passive RFID tags are the most common type in use that employ external power source to transmit signals. Joining chips by laser beam welding have wide advantages over other methods of joining, but they are seen limited to transparent substrates. However, connecting solder bumps with anisotropic conductive adhesives (ACA) produces majority of the joints. A high percentage of them fail in couple of months, particularly when exposed to vibration. In the present work, failure of RFID tags under dynamic loading or vibration was studied; as it was identified as one of the key issue to explore. Earlier investigators focused more on joining chip to the bump, but less on its assembly, i.e., attaching to the substrate. Either of the joints, between chip and bump or between antenna and bump can fail. However, the latter is more vulnerable to failure. Antenna is attached to substrate, relatively fixed when subjected to oscillation. It is the flip chip not the antenna moves during vibration. So, the joint with antenna suffers higher stresses. In addition to this, the strength of the bonding agent i.e., ACA also much smaller compared to the metallic bond at the other end of the bump. Natural frequency of RFID tags was calculated both analytically and numerically, found to be in kilohertz range, high enough to cause resonance. Experimental investigations were also carried out to determine the same. However, the test results for frequency were seen to be in hundred hertz range, common to some applications. It was recognized that the adhesive material, commonly used for joining chips, was primarily accountable for their failures. Since components to which the RFID tags are attached to experience low frequency vibration, chip joints fail as they face resonance during oscillation. Adhesives having much lower modulus than metals are used for attaching bumps to the substrate antennas, and thus mostly responsible for this reduction in natural frequency. Poor adhesive bonding strength at the interface and possible rise in temperature were attributed to failures under vibration. In order to overcome the early failure of RFID tag joints, Peripheral Soldering, an alternative chip joining method was devised. Peripheral Soldering would replace the traditional adhesive joining by bonding the peripheral surface of the bump to the substrate antenna. Instead of joining solder bump directly to the antenna, holes are to be drilled through antenna and substrate. S-bond material, a less familiar but more compatible with aluminum and copper, would be poured in liquid form through the holes on the chip pad. However, substrates compatible to high temperature are to be used; otherwise temperature control would be necessary to avoid damage to substrate. This S-bond would form metallic joints between chip and antenna. Having higher strength and better adhesion property, S-bond material provides better bonding capability. The strength of a chip joined by Peripheral Soldering was determined by analytical, numerical and experimental studies. Strength results were then compared to those of ACA. For a pad size of 60 micron on a 0.5 mm square chip, the new chip joints with Sbond provide an average strength of 0.233N analytically. Numerical results using finite element analysis in ANSYS 11.0 were about 1% less than the closed form solutions. Whereas, ACA connected joints show the maximum strength of 0.113N analytically and 0.1N numerically. Both the estimates indicate Peripheral Soldering is more than twice stronger than adhesive joints. Experimental investigation was carried out to find the strength attained with S-bond by joining similar surfaces as those of chip pad and antenna, but in larger scale due to limitation in facilities. Results obtained were moderated to incorporate the effect of size. Findings authenticate earlier predictions of superior strengths with S-bond. A comparison with ACA strength, extracted from previous investigations, further indicates that S-bond joints are more than 10 times stronger. Having higher bonding strength than in ACA joints, Peripheral Soldering would provide better reliability of the chip connections, i.e., RFID tags. The benefits attained would pay off complexities involved in tweaking

Development of a Three Dimensional Neural Sensing Device by a Stacking Method

This study reports a new stacking method for assembling a 3-D microprobe array. To date, 3-D array structures have usually been assembled with vertical spacers, snap fasteners and a supporting platform. Such methods have achieved 3-D structures but suffer from complex assembly steps, vertical interconnection for 3-D signal transmission, low structure strength and large implantable opening. By applying the proposed stacking method, the previous techniques could be replaced by 2-D wire bonding. In this way, supporting platforms with slots and vertical spacers were no longer needed. Furthermore, ASIC chips can be substituted for the spacers in the stacked arrays to achieve system integration, design flexibility and volume usage efficiency. To avoid overflow of the adhesive fluid during assembly, an anti-overflow design which made use of capillary action force was applied in the stacking method as well. Moreover, presented stacking procedure consumes only 35 minutes in average for a 4 × 4 3-D microprobe array without requiring other specially made assembly tools. To summarize, the advantages of the proposed stacking method for 3-D array assembly include simplified assembly process, high structure strength, smaller opening area and integration ability with active circuits. This stacking assembly technique allows an alternative method to create 3-D structures from planar components

Design of a Smart Ultrasonic Transducer for Interconnecting Machine Applications

A high-frequency ultrasonic transducer for copper or gold wire bonding has been designed, analyzed, prototyped and tested. Modeling techniques were used in the design phase and a practical design procedure was established and used. The transducer was decomposed into its elementary components. For each component, an initial design was obtained with simulations using a finite elements model (FEM). Simulated ultrasonic modules were built and characterized experimentally through the Laser Doppler Vibrometer (LDV) and electrical resonance spectra. Compared with experimental data, the FEM could be iteratively adjusted and updated. Having achieved a remarkably highly-predictive FEM of the whole transducer, the design parameters could be tuned for the desired applications, then the transducer is fixed on the wire bonder with a complete holder clamping was calculated by the FEM. The approach to mount ultrasonic transducers on wire bonding machines also is of major importance for wire bonding in modern electronic packaging. The presented method can lead to obtaining a nearly complete decoupling clamper design of the transducer to the wire bonder

Mechanics of Non Planar Interfaces in Flip-Chip Interconnects

With the continued proliferation of low cost, portable consumer electronic products with greater functionality, there is increasing demand for electronic packaging that is smaller, lighter and less expensive. Flip chip is an essential enabling technology for these products. The electrical connection between the chip I/O and substrate is achieved using conductive materials, such as solder, conductive epoxy, metallurgy bump (e.g., gold) and anisotropic conductive adhesives. The interconnect regions of flip-chip packages consists of highly dissimilar materials to meet their functional requirements. The mismatches in properties, contact morphology and crystal orientation at those material interfaces make them vulnerable to failure through delamination and crack growth under various loading patterns. This study encompasses contact between deformable bodies, bonding at the asperities and fracture properties at interfaces formed by the interconnects of flip-chip packages. This is achieved through experimentation and modeling at different length scales, to be able to capture the detailed microstructural features and contact mechanics at interfaces typically found in electronic systems

3D packaging for integrated circuit systems

A goal was set for high density, high performance microelectronics pursued through a dense 3D packing of integrated circuits. A {open_quotes}tool set{close_quotes} of assembly processes have been developed that enable 3D system designs: 3D thermal analysis, silicon electrical through vias, IC thinning, mounting wells in silicon, adhesives for silicon stacking, pretesting of IC chips before commitment to stacks, and bond pad bumping. Validation of these process developments occurred through both Sandia prototypes and subsequent commercial examples

Thermo-Mechanical Reliability and Electrical Performance of Indium Interconnects and Under Bump Metallization

This thesis presents reliability analysis of indium interconnects and Under Bump Metallization (UBM) in flip chip devices. Flip chip assemblies with the use of bump interconnections are frequently used, especially in high density, three-dimensional electronic devices. Currently there are many methods for interconnect bumping, all of which require UBM. The UBM is required for interconnection, diffusion resistance and quality electrical contact between substrate and device. Bonded silicon test vehicles were comprised of Indium bumps and three UBM compositions: Ti/Ni/Au (200\xc5/1000\xc5/500\xc5), Ti/Ni (200\xc5/1000\xc5), Ni (1000\xc5). UBM and indium were deposited by evaporation and exposed to unbiased accelerated temperature cycling(-55°C to 125°C, 15°C/min ramp rate). Finite Element Analysis (FEA) simulations were used to gain understanding of non-linear strain behavior of indium interconnects during temperature cycling. Experimental testing coupled with FEA simulations facilitated cycle-to-failure calculations. FEA results show plastic strain concentrations within indium bump below failure limits. It has been demonstrated that fabrication of Ti/Ni/Au, Ti/Ni, and Ni UBM stacks performed reliably within infant mortality failure region