Gold Bumps Research Articles

Optimization of Bonding Force, Sinking Value, and Potting Gap Size in COF Inner Lead Bonding Process

The thermo-mechanical properties of the polyimide film used in chip-on-film (COF) packaging have a significant effect on the inner lead bonding (ILB) mechanism. Furthermore, specifying an appropriate bonding force is essential in establishing a suitable compromise between a good bonding strength in the ILB process and a suitable gap size for the subsequent resin potting process. This paper commences by performing a series of experimental micro-force tensile tests to investigate the stress-strain relations of two polyimide films, namely Kapton-EN and Espanex-M. The tests are performed using formfitting specimens from actual COF products at various temperatures ranging from 25degC to 200degC . Based on the experimental results, constitutive equations are developed to model the temperature-dependent stress-strain characteristics of the two films. A finite element model of the COF/system comprising the thermo-compression tool, the polyimide film, the copper lead and the gold bump is constructed and is used to simulate the ILB process under various bonding forces. A good agreement is found between the experimental and numerical results obtained for the deformed profile of a single polyimide/copper lead following the bonding process and the variation of the bump sinking value with the bonding force, respectively. The numerical results are used to construct COF/ILB parameter design charts for the Kapton-EN and Espanex-M polyimide films which enable suitable values of the bonding force to be selected for given gap size and bump sinking values. In general, the results show that Kapton-EN has a broader working range than Espanex-M as a result of its higher elastic stiffness. When performing the ILB process using Espanex-M polyimide film, it is necessary to carefully control the bonding force to ensure that the minimum potting gap size is maintained.

Dynamics of Ultrasonic Transducer System for Thermosonic Flip Chip Bonding

Thermosonic flip chip bonding is used in certain fine pitch IC packaging for its unique features. By using this bonding process in this paper, 1 mm times 1 mm chip with 8 gold bumps has been bonded onto a silver-coated substrate. Dynamical properties of transducer system, which is the key component for providing the ultrasonic energy, have been investigated using finite element model (FEM) and measurement using impedance analyzer and laser Doppler vibrometer (LDV). The simulation results show that the ultrasonic transducer vibrates by coupling with all excited modes, therefore resulting in complicated motions during bonding. The third axial mode, which includes 1.5 wavelengths and 3 nodes, is the dominant working vibration. However, this axial mode is severely disturbed by undesirable non-axial modes such as flexural modes. There are some other unwanted parasitic modes close to dominant mode. Measured velocities of the transducer horn show that the system vibrates under several modes simultaneously. The impedance measurements reveal additional frequencies overlapping the working frequency. All non-axial modes of the ultrasonic transducer disturb the bonding process and degrade the bonding quality. A subtle control is needed to obtain unique axial mode and stable vibration for high bonding quality.

Gold Stud Bumps Flip-Chip Bonded onto Copper Electrodes with Titanium and Silver Layers

The purpose of this study was to develop the thermosonic flip-chip bonding process for gold stud bumps bonded onto copper electrodes on an alumina substrate. Copper electrodes were deposited with silver as the bonding layer and with titanium as the diffusion barrier layer. Deposition of these layers on copper electrodes improves the bonding quality between the gold stud bumps and copper electrodes. With appropriate bonding parameters, 100% bondability was achieved. Bonding strength between the gold stud bumps and copper electrodes was much higher than the value converted from the standards of the Joint Electron Device Engineering Council (JEDEC). The effects of process parameters, including bonding force, ultrasonic power, and bonding time, on bonding strength were also investigated. Experimental results indicate that bonding strength increased as bonding force and ultrasonic power increased and did not deteriorate after prolonged storage at elevated temperatures. Thus, the reliability of the high-temperature storage (HTS) test for gold stud bumps flip-chip bonded onto a silver bonding layer and titanium diffusion barrier layer is not a concern. Deposition of these two layers on copper electrodes is an effective and direct method for thermosonic flip-chip bonding of gold stud bumps to a substrate, and ensures excellent bond quality. Applications such as flip-chip bonding of chips with low pin counts or light-emitting diode (LED) packaging are appropriate.

Transient Stress Distributions in NCF-Bonded COG Packages due to Moisture Diffusion

The nonconductive film (NCF) adhesives have potentials to replace the anisotropic conductive film (ACF) adhesive in the chip-on-glass (COG) or other packages in liquid crystal displays (LCDs) due to their feasible features of finer bump pitch and cost reduction. Prior to applying the NCF to the COG packages, reliability issues, such as interfacial delamination and increase of bump contact resistance under thermal and moisture loads, have to be overcome. The objective of this paper is to investigate the effect of moisture ingression on stress field of the COG packages with NCF bonding. The coefficients of moisture diffusion and the hygro-strains of the NCF adhesive have been determined by an innovative approach of measuring the out-of-plane deformation of the bimaterial specimens during moisture absorption at the condition of 30degC/85%RH using Twyman-Green interferometry, associated with a finite element analysis and bimaterial theory. In this study, the results show that the average of the moisture diffusion coefficients and saturated hygro-strains of the NCF under 30degC/85%RH are 1.09 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s and 1.51 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. After being validated by the diffusion theory, the moisture- transient finite element analysis reveals that the moisture absorption of the COG package under 30degC/85%RH is nearly saturated after 1000 h. It is also found that there are large tensile stresses at pad corner and mediate ones across bump joint, and these tensile stresses gradually increase with moisture absorption and up to the maximum at the state of moisture saturation. Through the parametric study by the validated finite element model, the results show that the tensile bump-joint and interfacial stresses increase nonlinearly with the NCF elastic modulus and linearly with the saturated moisture-induced strain (SMIS) of the NCFs. Furthermore, the COG package with polyimide bumps is found to have lower moisture-induced tensile stresses than that with gold bumps. It is further shown that the tensile stresses from the longitudinal-section and transverse-section models are similar only with a few percent difference in values. It can be concluded that the stress states of the NCF-bonded COG packages induced by transient moisture ingression have been fully addressed through experimental, theoretical and numerical approaches in this study.

Improvement of Contact Resistance in Gold Bump by Advanced Barrier Metal Etching Solution

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スパッタ膜形成条件がフォトレジストのプロファイルおよび金めっき後のバンプ形状に与える影響

Gold bumps are conventionally used for interconnection of TAB (Tape Automated Bonding), COF (Chip on Film), or COG (Chip on Glass) in the driver LSIs of liquid crystal displays for mobile devices. These bumps are generally formed using cyanide-free gold electro-plating, since electrolytes containing cyanide tend to damage organic photoresist. However, a cyanide gold-plating bath for printed writing boards is also sometimes used to make gold bumps. This study investigated the gold sputtering influence on photoresist profiles and bump shapes after gold deposition.

A Low Cost Bumping Method for Flip Chip Assembly and MEMS Integration

In this paper, we present the development of a low cost chip/wafer bumping technique for flip chip assembly and microelectromechanical systems integration using a bump transfer approach. In this method, high melting temperature bumps such as copper, nickel, and gold bumps are fabricated on a low cost, flexible carrier and then transferred onto the target chip/wafer/board for flip chip assembly. The aluminum pads on test chips were remetallized with electroless nickel and gold layers to facilitate the bonding of bumps to the chips using thermocompression bonding. Parallel bonding and transfer of bumps has been achieved. Thermal cycling and bump shear test showed that reliable bonding was obtained between the bumps and the pads. Surface study using laser ionization mass analysis found no traces of of the polymer carrier on the surface of the bumps on the test chip after the completion of the bump transfer process.

Observations on HRTEM features of thermosonic flip chip bonding interface

The bonding interface features in thermosonic flip chip (FC) bonding are of interest to researchers in microelectronics packaging. In this study, a die with Al pads and eight gold bumps was bonded to a silver-coated pad on our lab test bench. The interface of the sample was analysed by using a high-resolution transmission electron microscope (HRTEM). For FC bonding parameters (e.g. ultrasonic power 2 W, bonding time 350 ms, heating temperature 150 °C, and bonding force 3.2 N), the thickness of atom diffusion at the Au–Ag interface is about 200 nm and that at the Au–Al interface is about 500 nm. In addition, ultrasonic vibration of FC bonding leads to the growth of the dislocation density in bonded materials and the formation of a cluster of dislocations at the interface. Therefore, short circuit diffusion plays a major rule during ultrasonic bonding when the temperature rise is relatively low. These observations will be helpful for further analysis.

A Preliminary Electron Backscatter Diffraction Study of Microstructures and Microtextures Evolution during Au Stud and Flip Chip Thermosonic Bonding

Microstructures and microtextures of the gold wire, free air ball, Au stud bumps and flip chip bonding bumps were analyzed using Electron Backscatter Diffraction (EBSD). It is demonstrated that process parameters, such as bonding power, force and temperature have significant influences on the microstructure and microtexture of gold bumps. The non-uniform deformation, the associated microstructure defects and the local textures of the Au bumps under the vertical force and the horizontal ultrasonic wave applied are presented and discussed.

Ultra fine pitch flat panel display packaging using 3 μm conductive particles

In this research, dependency of the contact resistance on the hardness of the gold bump and process parameters such as the contact area and the load pressure of the extra fine pitch chip-on-glass (COG) flat panel display (FPD) packaging is investigated. The FJ2530 anisotropic conductive film (ACF) by Sony Inc. containing the currently smallest 3 μm conductive particles is implemented to conduct the experiments. Feasibility of the 3 μm ACF in ultra-fine pitch COG applications is further examined. From the experimental results, we found that no special process is required to obtain a satisfied contact resistance for the ultra fine pitch packaging based on the 3 μm conductive particles. It can be concluded that the combination of the 3 μm conductive particles and the IC gold bumps with hardness 70 Hv can lead to a miniaturized device with less cost. For mass production, test sample with pitch space being larger than 10 μm is suggested to retain a better reliability.

Improvement of LDI BUMP Stripping Process by Development of Room Temperature PR Stripper

ABSTRACTBump process using gold bump led to many cleaning problems like polymer residues, metal and polyimide consumption after stripping process. First of all, stripping process of bump photoresist is one of the difficult technologies since thickness of PR pattern of bump is thicker than that of general metal line at least 100 times. We investigated the improvement of LDI BUMP stripping Process at 25 with new chemical. We found that the wettability may be improved when the additive was added to the chemical and it improves the stripping ability. It was found that new chemical was superior to commercial chemical in terms of chemical stability, removal efficiency of polymer residue and decrease in metal and polyimide consumption. Also, we could obtain removal mechanism of photoresist pattern by measured Raman equipment and enhancement of yield in mass production line of semiconductor.

Increasing bondability and bonding strength of gold stud bumps onto copper pads with a deposited titanium barrier layer

A flip-chip assembly is an attractive scheme for use in high performance and miniaturized microelectronics packaging. Wafer bumping is essential before chips can be flip-bonded to a substrate. Wafer bumping can be used for mechanical-single point stud bump bonding (SBB), and is based on conventional thermosonic wire bonding. This work proposes depositing a titanium barrier layer between the copper film and the silver bonding layer to achieve perfect bondability and sufficiently strong thermosonic bonding between a stud bump and the copper pad. A titanium layer was deposited on the copper pads to prevent copper atoms from out-diffusing during thermosonic stud bump bonding. A silver film was then deposited on the surface of the titanium film as a bonding layer to increase the bondability and bonding strength for stud bumps onto copper pads. The integration of the silver bonding layer with a diffusion barrier layer of titanium on the copper pads yielded 100% bondability between the stud bump and pads. The strength of bonding between the gold bumps on the copper pads significantly exceeds the minimum average values in JEDEC specifications. The diffusion barrier layer of titanium effectively prevents copper atoms from out-diffusing to the silver bonding layer surface during thermosonic bonding, which fact can be interpreted with reference to the experimental results of energy dispersive spectrometry (EDS) and analyses of Auger depth profiles. This diffusion barrier layer of titanium efficiently provides perfect bondability and sufficiently strong bonding between a stud bump and copper pads with a silver bonding layer.

A novel contact resistance model of anisotropic conductive film for FPD packaging

A novel contact resistance model for the flat panel display (FPD) packaging based on the within layer parallel and between layers series resistance concepts is proposed in this research. The FJ2530 anisotropic conductive films (ACF) by Sony Inc., containing the currently smallest 3 μm conductive particles, was used to conduct experiments to verify the accuracy of the proposed model. The calculated chip-on-glass (COG) packaging resistance using the proposed model is 0.165 Ω. It is found that a gold bump with 0.162 Ω resistance play a major role in the overall resistance. Although the predicted resistance using the proposed model is only one third of the experimentally measured value a threefold improvement is obtained compared to existing models.

Influence of the filling factor on the spectral properties of plasmonic crystals

Plasmonics crystals (PCs) comprised of finite-size triangular lattices of gold bumps deposited on a gold thin film are studied by means of a near-field optical microscope. The plasmonic crystals fabricated by electron-beam lithography are illuminated by an incident surface plasmon polariton excited in the Kretschmann-Raether configuration at the gold/air thin-film interface for incident free-space wavelengths in the range $740--820\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Based on the measurement of the surface plasmon polariton (SPP) damping distance in the crystals, the existence of a band gap for an incident SPP traveling along the two symmetry axes $\ensuremath{\Gamma}M$ and $\ensuremath{\Gamma}K$ is demonstrated. By increasing the lateral size of the bumps, the influence of the filling factor on the PC spectral properties is investigated. We show that, for the two $\ensuremath{\Gamma}M$ and $\ensuremath{\Gamma}K$ axes, the central gap frequency depends only weakly on the filling factor, whereas we observed a neat band-gap broadening for increasing filling factors. The experimental results are found to be consistent with the PC band structures computed by means of the differential method. In particular, a good agreement between the experimental and theoretical central gap frequencies for the two $\ensuremath{\Gamma}M$ and $\ensuremath{\Gamma}K$ axes is obtained. Finally, we investigate the origin of the band gap in PCs by computing modal reflectivity (or in-plane reflectivity) of semi-infinitely extended crystals. From these calculations, we show that the SPP propagation through a PC is inhibited by distributed reflection and not by losses channels such as absorption or out-of-plane scattering.

Study of Some Parameter Effects on Warpage and Bump-Joint Stresses of COG Packages

The purpose of this paper is to investigate the parameters such as thermal loads, adhesive material properties and fillets, bump materials, and moisture, affecting the warpage and bump-joint stresses of the chip-on-glass (COG) packages with anisotropic conductive film (ACF). Two- and three-dimensional finite-element models (FEMs) are employed for calculating the warpage and bump- joint stresses in terms of such parameters. In addition, the real-time full-field Twyman-Green interferometry is used for measuring the out-of-plane deformations of the specimens and providing validation of the warpage calculated from finite-element models. Also, Suhir's close-form solution of a die-substrate assembly is applied for looking into insight of thermomechanics, especially for adhesive stresses and correlated with finite-element results. The results show that the fillets strongly affect the warpage of the COG specimens by changing the convex shape of warpage for those without fillets to the saddle shape for those with fillets. However, unlike those with fillets, the warpage of the COG is insensitive to those with or without (gold or polymer) bumps. It is also found that the two-dimensional (2-D) FEM in consistence with Suhir's solution can be used for approximately addressing both warpage and bump-joint stress issues. The COG warpage results for thermal loads, from 2-D FEM, indicate that the maximum warpage is very sensitive to the modulus of the ACFs (especially for low modulus) rather than the thermal expansion coefficients of the ACFs, which are consistent with Suhir's solution. The stress results show that the COG with gold bumps provides higher bump-joint stresses (especially for compressive transverse normal stress, which is important for maintaining good electrical contact between bumps and tracks through the conductive particles in the ACF) than that with polymer bumps for the isothermal loaded case. Note that negative isothermal load here applied represents the cooling temperature change from an uniform elevated temperature of the ACF curing in the COG packaging. Furthermore, the transverse normal bump-joint stress increases three fold, when the fillets are present for the COG either with polymer bumps or with gold bumps. The similar mechanism is found in the case of the COGs under thermal-gradient loads. It is also seen that the higher the elastic modulus (E) and thermal expansion coefficient (CTE) of the ACF, the larger the compressive transverse normal bump-joint stress. It is also interestingly found that the volume swelling of the ACF due to moisture absorption provides very relatively large tensile transverse normal bump-joint stresses for the COGs with bumps. These tensile stresses could be one order of magnitude larger than the compressive values resulted from other parameters discussed here. As a result, the approaches to well controlling ACF moisture absorption and selecting low-moisture-expansion ACF materials are very critical to improving the reliability of the COG packages

Adiabatic bends in surface plasmon polariton band gap structures

Propagation and interaction of surface plasmon polaritons (SPPs) excited in the wavelength range 700-860 nm with periodic triangular arrays of gold bumps placed on gold film surfaces are investigated using a collection near-field microscope. We observe the inhibition of SPP propagation into the arrays within a certain wavelength range, i.e., the band gap (BG) effect. We demonstrate also the SPP propagation along a 30 degrees bent channel obtained by an adiabatic rotation of the periodic array of scatterers. Numerical simulations using the Lippmann-Schwinger integral equation method are presented and found in reasonable agreement with the experimental results.

An Accommodative Approach Designed for TCP Gold-to-Gold Inner Lead Bonding

This paper proposes an approach that resolves the tradeoff between bond pad crack, the passivation crack, and lead lift failure modes in gold-to-gold inner lead bonding. The bonding head temperature, stage temperature, and bonding force are addressed as three critical recipes. The proposed scenario determines the bonding head temperature according to the maximum compensation flatness, followed by the corresponding stage temperature and required bonding force in sequence. The relevant gold bump hardness, chip warpage, and flatness variation in the bonding tool were evaluated using the finite-element method. The corresponding bonding force was determined using semianalytical equations with empirical factors. A gate driver IC with 280 I/O pins was adopted to conduct the experimental verification in the engineering pilot run. With the optimal recipes, the obtained results demonstrated success in diminishing these three failure modes. The feasibility of proposed approach was verified experimentally.

Silicon Through-hole Interconnection for 3D-SiP Using Room Temperature Bonding

ABSTRACTIn rapidly growing market sectors, such as mobile information devices, SiP technology, in which multiple LSI chips are stacked three-dimensionally, is attracting attention as a means of greatly reducing the mounting area of electronic components to improve system performance while reducing system size. Hitachi, Ltd. and Renesas Technology developed a new way to interconnect stacked chips using through-hole electrodes with a lower cost and shorter turn around time (TAT). Stacked chips are electrically interconnected by simply applying a compressive force at room temperature to a conventional chip with multiple gold stud bumps. Gold stud bumps on the upper chips are pressed into the through-hole electrodes on the lower chips by applying a compressive force, which causes plastic to flow into the gold bump. That is, the use of a gmechanical caulkingh technique makes possible electrical connections between stacked chips at room temperature. Compared with conventional through-hole electrode interconnection (more than 200°C), this new method drastically reduces the production cost and the environmental load. By using this technology, the package thickness can be 1.0 mm or less even in ten-chip layers, compared with two-chip layers using wire bonding, which are approximately 1.25-mm thick.

Two‐photon luminescence microscopy of field enhancement at gold nanoparticles

Using a reflection scanning optical microscope detecting two-photon luminescence (TPL) we have imaged square gold bumps positioned in a periodic array either on a smooth gold film or directly on a glass substrate. The second-harmonic (SH) and TPL response from these structures show both polarization and wavelength dependence. The gold bumps on gold film showed extremely high sensitivity to the incident field, with the strongest TPL response from the gold bumps being enhanced nearly 103 times compared to the TPL response from the smooth gold surface. For gold bumps directly on glass the similar calculated enhancement was estimated to be ∼102. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of hygro-mechanical and thermo-mechanical stress on delamination of gold bump

The reliability of the FC–CSP (flip chip–chip scaled package) package with gold bump at the MRT (moisture resistance test) reflow temperature, was evaluated by using the finite element method. The moisture properties of EMC (epoxy molding compound) obtained from the test described in JEDEC standard, were used to characterize the local moisture concentration analysis by transient moisture diffusion, the hygro-mechanical analysis by CME, the vapor pressure analysis and the thermo-mechanical analysis by CTE mismatch. Also, after precondition, the package reliability under the reflow process was predicted, by comparing and integrating each factors, package swelling and stress due to by vapor pressure, as well as thermo-mechanical stress. Consequently, the result showed that the effects on hygro-mechanical stress and vapor pressure in a package could not be negligible, when it is compared with that of the thermo-mechanical stress by CTE mismatch, which is recognized as the main effect on the package crack under reflow temperature. The stress was concentrated at interface between gold bump and die, where most of delamination occurred.