Surface Of Bumps Research Articles

Process Window of Mini-LED Display Panel Packaging using Laser Assisted Bonding Technology

For high-resolution display panels using mini-LEDs, high alignment accuracy and high yield of LED transfer and bonding technology are very important parameters of packaging technologies. These parameters of packaging technologies are highly dependent on the interconnection material and bonding process. The use of laser assisted bonding (LAB) technology and epoxy-based interconnection materials has been introduced in mini-LED display panel packaging to improve bonding accuracy and yield. The bonding process of the entire display panel was performed by a tiling SITRAB process because the laser irradiation area is smaller than the LED bonding area on the substrate. For the SITRAB process, an epoxy-based SITRAB material to remove oxide on the surface of solder bump has been developed as a solvent free material. For good wetting of the solder bump during the SITRAB process, the temperature of the solder bumps must be higher than the melting temperature of solder. In general, the stage temperature of the laser process is determined to be around 100C in order to minimize the laser power. However, if the stage temperature is high, the initial properties of the SITRAB material (such as low viscosity and deoxidation function) may be lost due to the high temperature. In this study, the process window of the SITRAB material was determined for a stable SITRAB process. The process window was identified according to three different pot lifes: room temperature pot life (RPL), stage pot life (SPL), and laser pot life (LPL).

Reliability Assessment of Indium Micro Bumps for 2.5D/3D Electronic Packaging Through Cryo-Argon Milling Technique

Indium bumps are widely used in electronic packaging for interconnecting microelectronic devices due to their low melting point of 156.6 degrees C, excellent bonding capabilities at room temperature, high thermal conductivity, ductility at extremely low temperatures, and ability to compensate for the mismatched coefficient of thermal expansion between different materials. However, cross-sectioning indium bumps can be challenging due to the softness and low melting point of indium. The cutting tools and techniques used must be optimized for the specific sample being analyzed, and specialized preparation techniques such as cryogenic freezing or chemical etching may be required, which can be expensive and time-consuming. Cryo-focused ion beam (FIB) is also a technically demanding process that requires careful optimization of milling parameters, multiple iterations of milling and imaging, specialized equipment, and expertise, which can make it too expensive. It should be noted that the maximum achievable crosssectional area with cryo-FIB is limited, typically in the range of hundreds of micrometers. Cryo-Argon milling is an effective technique for obtaining high-quality cross-sections of indium bump arrays and other materials prone to deformation or melting during traditional crosssectioning. This process involves freezing the sample using liquid nitrogen and bombarding it with a stream of Argon gas ion, which reduces the deformation and melting of the indium. This paper presents challenges in the sample preparation and mechanical polishing of indium micro bumps. We are showcasing an artifact: polishing particles embedded in the indiummicro bumps, hindering the revelation of their microstructure. To address this issue, we are incorporating cryo-Ar milling to remove the embedded particles from the surface of indium micro bumps. This work presents data before and after the application of cryo-Ar milling, demonstrating a significant improvement in the microstructure of indium micro bumps. Millimeter-wide, damage-free cross-sections of arrays of indium micro bumps with a clean and smooth surface were achieved through cryo-Argonmilling at270 degrees C.

Research on flexible skin technique for adaptive bump inlet structure

PurposeAdaptive bump inlet can adaptively change the shape of inlet bump surface according to the flight speed of aircraft, ensuring that the inlet has good inlet-engine match performance in a wide speed range. This paper aims to use a composite flexible skin reinforced by shape memory alloy (SMA) fiber as the deformable structure at bump surface to realize the adjustable bump surface of adaptive bump inlet.Design/methodology/approachAccording to the deformation and load-bearing requirements of adaptive bump, SMA is applied to the design of adaptive bump inlet due to its characteristic of super-elasticity. A kind of SMA fiber is studied. A composite flexible skin reinforced by SMA is proposed, and its mechanical properties are analyzed. On this basis, an adaptive bump inlet is designed in which the composite flexible skin reinforced by SMA is used as bump surface, and the shape of the bump surface is adjusted by way of pressuring. The design scheme and specific parameters of the adaptive bump are given.FindingsAn adaptive bump surface that meets the design requirements of the inlet is designed, which can effectively adjust the inlet throat area with a throat area change rate of 20%.Originality/valueAn adaptive bump inlet with composite flexible skin as a deformable structure at bump surface is designed, and SMA is applied as the reinforcing fiber.

Intermetallic compounds formation and joints properties of electroplated Sn–Zn solder bumps with Cu substrates

Sn–Zn solder bumps are prepared using electrodeposition method, and the interfacial compounds formation and joints properties of solder bumps with Cu substrates are also investigated in this paper. The DSC results indicate that the melting temperature of electroplated Sn–9Zn solder bumps declines markedly with Zn alloying element added into pure Sn solder. XRD results suggest that the content of Zn on the surface of solder bumps decreases significantly after solder reflowing. EPMA results indicate that in Sn–9Zn/Cu interconnection joints, Zn diffuses from the surface of micro-bumps to the interface during solder reflowing. The BSEM images and EDX results of the solder bumps reveal that there was only one layer of Cu–Zn intermetallic compound(IMC) formed at the interface which is safely assumed as Cu5Zn8. Under the condition of aging tests at 150 °C for 400 h, joints interface between electroplated Sn–9Zn solder bumps and Cu substrate was Cu5Zn8 layer invariably, and no other significant phase transition occurs. However, interfacial intermetallic compound between pure Sn bump and the Cu substrate changes from Cu6Sn5 to CuSn3 under the same condition of aging tests. The shear strength of electroplated Sn–9Zn solder joints is higher than that of the pure Sn solder joint due to small Cu5Zn8 sizes and thin IMC layer, and shear strength of the solder joints decreases with the increase of aging time.

Effects of α-Fe2O3 Additions on Assembly Reliability of Electroplated Sn-Based Solder Cap on Cu Pillar Bump During Thermal Cycling

Composite solder alloys reinforced by different nominal concentrations of α-Fe2O3 nanoparticles are examined and the performance of corresponding copper (Cu) pillar bumps during thermal cycling (TC) tests is investigated. Firstly, the spherical α- Fe2O3 nanoparticles (50 nm in diameter) were successfully prepared by a chemical method. Then, the Sn-xFe2O3 composite solders with different concentrations of α-Fe2O3 were deposited on the surface of Cu pillar bumps by electroplating. During electroplating, the concentration of α-Fe2O3 nanoparticles in the plating solution ranged from 0 g/L to 0.04 g/L. TC tests of fabricated joints were conducted from − 65°C to 150°C for different numbers of cycles. The microstructure, evolution of interfacial intermetallic compounds (IMCs) and the shear strength of the solder joints were investigated following TC tests. Cross-sectional analysis indicates that adding α-Fe2O3 nanoparticles can remarkably suppress the overgrowth of the interfacial IMCs. Compared to the strength of joint without nanoparticles after the TC test, the strength of joints increases with the addition of α-Fe2O3 and an optimum nominal concentration of 0.032 g/L in the electroplating solution was identified.

Low Temperature Flip Chip Bonding Using Squeegee-Embedded Au Nanoporous Bump Activated by VUV/O3 Treatment

This paper describes low-temperature bonding realized by squeegee-embedded Au nanoporous bumps that were activated by vacuum ultraviolet in the presence of oxygen (VUV/O3). The VUV/O3 technology is confirmed to be a suitable surface treatment technique for Au nanoporous bump bonding because it maintains the highly reactive surface of the Au nanoporous bumps during the treatment. X-ray photoelectron spectroscopy confirmed that the VUV/O3 treatment was capable of removing organic contamination on the nanoporous surface, and scanning electron microscopy images showed that the ligament size of the nanoporous bumps stayed the same. After bonding, the ligament size of the VUV/O3-treated nanoporous structure grew to 54 nm compared with 27 nm for the untreated samples. This increase in ligament size was attributed to the improvement in nanoporous coalescence by removing organic contamination that obstructed Au atom diffusion. Furthermore, the highest strength of the VUV/O3-treated samples reached 8.9 MPa at a low temperature of 200°C, which was three times higher than that of the untreated sample. This technology is expected to assist manufacturing of future 3-D integrations.

Microstructure and reliability of hybrid interconnects by Au stud bump with Sn-0.7Cu solder for flip chip power device packaging

With miniaturization of the interconnect solder bumps, high current density causes serious reliability issues (stress, electromigration etc.) in electronic packages. Through Au stud bumping on the chips and following reflow of solder to produce hybrid interconnects, the eletromigration resistance may be improved by the intermetallics formed inside them due to their barrier effects on the atoms migration. Here, microstructures and reliabilities of Au stud with serial amounts of Sn-0.7Cu solder paste were studied through controlling size of stencil printing aperture. After reflow, AuSn, AuSn2 and AuSn4 formed from the surface of Au stud bump to the solder. A layer of (Cu,Au)6Sn5 with thickness of 3μm existed at the interface near the Cu substrate with a scallop shape similar to Cu6Sn5. The fraction of intermetallics to the mixed joints varied with the solder amount. Shear strength decreased slightly when comparing with the sole solder joint due to large amounts of brittle intermetallics. Thermal aging resulted in many Kirkendall voids generated at the interfaces of Au stud and the solder, which further decreased the shear strength. The effect of solder amount on microstructural evolution and fracture modes was discussed. The hybrid interconnects showed a good electromigration resistance.

Flow visualisation of a normal shock impinging over a rounded contour bump in a Mach 1.3 free-stream

An experimental study has been conducted to visualise the instantaneous streamwise and spanwise flow patterns of a normal shock wave impinging over a rounded contour bump in a Mach 1.3 free-stream. A quartz-made transparent shock generator was used, so that instantaneous images could be captured during the oil-flow visualisation experiments. Fluorescent oil with three different colours was used in the surface oil-flow visualisation experiment to enhance the visualisation of flow mixing and complicated flow features that present in the flow field. Experimental data showed that the rounded contour bump could split the impinging normal shock wave into a or a series of lambda-shaped shock wave structure(s). In addition, it was found that the flow pattern and the shock wave structures that appeared over the rounded contour bump depended highly on the impinging location of the normal shock wave. The flow pattern shown in this study agreed with the findings documented in literature. Moreover, it was observed from the instantaneous oil streaks that the normal shock impinging location also affected the size and the formation location of the spanwise counter-rotating vortices downstream of the bump crest. Finally, it was concluded that the terminating shock could distort the oil streaks that left over the surface of the contour bump. Therefore, the use of the transparent normal shock wave generator is recommended when conducting experiments with normal shock wave impingement involved.Graphical abstract

Particle on Bump (POB) technique for ultra-fine pitch chip on glass (COG) applications by conductive particles and adhesives

Chip on glass (COG) technology is widely used in liquid crystal display (LCD) modules for connecting driver ICs to the displays especially for middle and small size panels. The most common COG technology currently used in display applications is based on anisotropic conductive films (ACF). As the increasing demand in higher resolution and cost reduction, the bump pitch of the driver ICs becomes finer and finer. With the reduction of bump pitch, the current ACF based COG technology is confronted with two issues: one is the increase of the chances of open circuit; the other is the increase of the chances of forming shorts. A new approach for ultra-fine pitch chip on glass (COG) bonding, named ”Particle on Bump (POB)”, is proposed in this paper. In this technique, conductive particles are planted on the top surface of bumps of a driver IC through Au–Sn intermetallic connection. The driver IC is then assembled on the glass substrate of a LCD panel with an insulated adhesive by thermal press. The new method ensures that electrical connections are established only between bumps and corresponding pads. The Au–Sn reflow process for particle planting and corresponding COG bonding process were investigated in detail. The results showed that reliable connections were formed between particles and bumps through an Au–Sn intermetallic layer and final COG interconnections thus formed performed well in reliability tests. It is concluded that the POB technique overcomes the shortcomings of current ACF technique and has good potential to provide a viable ultra-fine pitch flip chip on glass solution for display applications.

Electroplating Characteristics of Sn–Bi Microbumps for Low-Temperature Soldering

In this paper, the characteristics of eutectic Sn-Bi microsolder bumps fabricated by electroplating are investigated. The underbump metallization (UBM) layers on a Si chip consist of Al, Cu, Ni, and Au, sequentially from bottom to top. The desired Sn-Bi bump size has a diameter of 22 μm and pitch of 44 μm. In order to obtain the optimal conditions for the eutectic Sn-Bi solder bumps, the polarization curves of Sn, Bi, and Sn-Bi electrolytes are analyzed, and the variation of the Sn- Bi composition as a function of the current density is measured. Experimentally, from the polarization curve, Bi and Sn start to deposit below -0.12 and -0.54 V, respectively, and Sn-Bi codeposition occurs below an electropotential of -0.54 V. The Bi content of the electroplated bumps decreases from 92.4 to 38.2 wt% when the current density is increased from 20 to 50 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and near-eutectic composition of the Sn-61 wt% Bi bump is obtained by plating at 40 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 5 min. The surface of the Sn-Bi microsolder bumps show plate-like structures with acicular shapes, and the grain size increases with increasing current density. An intermetallic compound layer, estimated as AuSn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> with a thickness of about 0.5 μm, was observed between the UBM layers and the as-plated Sn-Bi bumps.

Optimization of Ag Composition in Cu Pillar Bumps with Sn-xAg Solders

In order to minimize the large plate growth of Ag3Sn intermetallic compounds (IMCs) adversely affecting the mechanical behavior and reducing the reliability of solder joints, the Ag content of less than 3wt% has been suggested in ball grid array (BGA) Sn-Ag solders. The suggestion (&amp;lt; 3wt% Ag in the solders) is invalid in small solder bumps, and moreover there is no available guideline of the Ag content in the small Sn-Ag solders. In this study, the optimum level of Ag content in small Sn-Ag Cu pillar bumps (CPBs) of less than 50um in diameter was investigated to suppress the large plate growth of Ag3Sn. Since the undercooling of Sn-Ag solders increases with a decrease of solder ball size, the Ag3Sn IMCs are more prone to become largely plate-like in small solders. Actually, in small Sn-Ag CPBs, the large plate growth of Ag3Sn was observed on the top surface of solder bumps even though the Ag content was less than 3.0wt%. For suppression of Ag3Sn plates in Sn-Ag CPBs, two ways were suggested; increasing the cooling rate of the reflow profile and reducing the Ag content in the solders. Reducing the Ag content was effective on reducing the large plate growth of Ag3Sn, while a high cooling rate was not effective. Through the thermal analysis and thermodynamic calculations, the optimized Ag content in Sn-Ag CPBs of less than 50um was suggested, and the large plate growth of Ag3Sn was effectively reduced within the suggested Ag content.

Fabrication on uniform plating-based flip chip solder bumps after reflow process using a polishing mechanism

This paper presents an innovative polishing process aimed at leveling rough surface of plating-based flip chip solder bumps so as to get uniform coplanarity across the whole substrate after both electroplating and reflow processes. This polishing mechanism is characteristic of combining mechanical-dominated polishing force with slight chemical reaction together. A large number of extremely but inevitably rugged mushroom-like structures after electroplating are drastically smoothed down with the help of this newly-developed polishing process. Nearly 70 μm solder bumps in height with two different profiles as square and circle on the substrates reach as flatly as ±3 μm between different substrates after reflow process; ±2.5 μm in single substrate; and even ±1 μm in die, respectively. Besides, surface roughness among the solder bumps is simultaneously narrowed down from Ra 0.6 to Ra 0.03 along with the coplanarity improvement. Excellent uniformity and smooth surface roughness in solder bumps are absolutely beneficial to pile up and deposit in the following steps in MEMS and semiconductor fields.

EFFECTS OF HEMATOCRIT LEVEL ON THE BLOOD FLOW IN THE CORRUGATED VESSEL DUE TO THE IMPLANTATION OF INTRAVASCULAR STENTS

The vascular stenting has been accepted as a very effective treatment of occlusive vascular disease. But the implantation of vascular stent will change the geometric shape of stenosed vessel to be corrugated and create some local hemodynamic features characterized by the flow separation, recirculation in the area close to the stent. Under the physiological conditions of left coronary artery and specific geometric shapes created by the implantation of stent, the numerical method has been used to solve the blood flow and the shear stress distribution patterns in the vicinity of the stent. The non-Newtonian fluids with different levels of hematocrit (H), 25%, 45%, 65% are employed for the numerical simulation and compared with the numerical results of the Newtonian fluid in which the blood viscosity is assumed to be constant. The simulations have revealed that the surface of stent and atheroma bumps experience lower time-averaged wall shear stress by the non-Newtonian fluid with lower level of hematocrit. The value of wall shear stress on the peak of atheroma bumps has also been studied because it may associate with the cause the rupture of atheroma plaque. The numerical results of Newtonian fluid were compared with those of non-Newtonian fluid with hematocrit level of 45%, the average hematocrit value inherent in normal person. It is noted that non-Newtonian effect on the time-averaged wall shear stress distribution along the stent surface is not significant. The outline of bulged atheroma experiences almost same time-averaged shear stress distribution acting by the fluid with or without the non-Newtonian property. It means that for the non-Newtonian fluid with hematocrit level of 45% the property of changing viscosity in the flow field didn't affect the results of wall shear stress distribution on the surfaces of stent and atheroma bump resulted from the simulation of Newtonian fluid flow. These results will provide insight into the effects of different levels of hematocrit on the blood flow in the corrugated vessel due to the implantation of stent. Our findings suggest that the level of hematocrit is closely associated with the value of wall shear stress distribution. It is particularly significant to acute stage of atherogenesis, intimal hyperplasia, platelet deposition, thrombosis, endothelial cell orientation and the rupture of atheroma plaque after the implantation of stent. Some of the results presented can be used to explain the clinical performance of vascular stenting.

솔더범프와 TiW/Cu/electroplating Cu UBM 층과의 금속간 화합물 형성과 범프 전단력에 관한 연구

The joint strength and fracture surface of Sn-Pb solder bump in photo diode packages after isothermal aging testing were studied experimentally. Cu/Sn-Pb solders were adopted, and aged for up to 900 hours at 12<TEX>$0^{\circ}C$</TEX> and 17<TEX>$0^{\circ}C$</TEX> to analyze the effect of intermetallic compound(IMC). In 900-hour aging experiments, the maximum shea strength of Sn-Pb solder decreased by 20％ and 9％. The diffraction patterns of Cu<TEX>$_{6}$</TEX>Sn<TEX>$_{5}$</TEX>, scallop-shape IMC, and planar-shape Cu<TEX>$_3$</TEX>Sn were observed by Transmission Electron Microscopy (TEM).EM).

Thermomechanical behaviour of PBGA package during laser and hot air reflow soldering

In this paper, the temperature distribution in laser reflowed solder balls and warpage of the package upon temperature change were simulated using a finite element method, and effects of different laser heating means on the temperature distribution have been discussed in detail. The results of simulation show that solder balls reflowed by laser heating power would not damage the silicon chip and the package and that with laser, the advantages of a short heating time, a lower temperature in the package and smaller deformation were obtained as compared with a traditional heating method such as the hot air reflow method. Also, experiments on a plastic ball-grid-array (PBGA) solder ball laser reflow were carried out. Results show that the surface of the solder bumps obtained by the laser reflow method with proper parameters is much smoother than that obtained by the hot air reflow method.

The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 µm)/Ni/Ti UBM, and it was confirmed to be the (Au1−x−yBixNiy)Sn2 phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 µm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.

Research on the contact resistance, reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications

Although there have been many years of development, the degradation of the electrical performance of anisotropically conductive adhesive or film (ACA or ACF) interconnection for flip-chip assembly is still a critical drawback despite wide application. In-depth study about the reliability and degradation mechanism of ACF interconnection is necessary. In this paper, the initial contact resistance, electrical performance after reliability tests, and degradation mechanisms of ACF interconnection for flip-chip-on-flex (FCOF) assembly were studied using very-low-height Ni and Au-coated Ni-bumped chips. The combination of ACF and very-low-height bumped chips was considered because it has potential for very low cost and ultrafine pitch interconnection. Contact resistance changes were monitored during reliability tests, such as high humidity and temperature and thermal cycling. The high, initial contact resistance resulted from a thin oxide layer on the surface of the bumps. The reliability results showed that the degradation of electrical performance was mainly related to the oxide formation on the surface of deformed particles with non-noble metal coating, the severe metal oxidation on the conductive surface of bumps, and coefficient of thermal expansion (CTE) mismatch between the ACF adhesive and the contact conductive-surface metallization. Some methods for reducing initial contact resistance and improving ACF interconnection reliability were suggested. The suggestions include the removal of the oxide layer and an increase of the Au-coating film to improve conductive-surface quality, appropriate choice of conductive particle, and further development of better polymeric adhesives with low CTE and high electrical performance.

Mechanical Properties and Fracture Behavior of the Cu and Cu6Sn5–Dispersed Sn-Pb Solder Bumps Processed by Screen Printing

ABSTRACTThe mechanical properties of the 1-9 vol % Cu and Cu6Sn5-dispersed 63Sn-37Pb solder alloys were characterized with tensile test. Also, the Cu and Cu6Sn5-dispersed 63Sn-37Pb solder bumps of 760 μm size were fabricated on the Au(5 μm)/Ni(5 μm)/Cu(27 μm) BGA substrates by screen printing process, and their shear strength were characterized with variations of the dwell time at the reflow peak temperature(220°C) and aging time at 150°C. The yield strength and ultimate tensile strength of the 63Sn-37Pb solder alloy increased with dispersion of 1-9 vol % Cu and Cu6Sn5. In general, however, the Cu and Cu6Sn5-dispersed solder bumps exhibited lower shear strengths than those of the 63Sn-37Pb solder bumps The failure surface of the solder bumps after ball shear test could be divided into two regions of slow crack propagation and critical crack propagation, and the shear strength of solder bumps was inversely proportional to the slow crack propagation length.

Solder bump inspection by evaluating focusing ratio on surface and two-dimensional features

This paper proposes a method to inspect forms of solder bumps located on the surface of a semiconductor bare chip. The form of a solder bump is inspected by evaluating the two-dimensional features observed from the vertical view and by measuring the change of the optical focus on the surface of the solder bump when the Z-stage is moved once using the vertical coaxial illumination and the dome-typed uniform illumination. As the experimental result, the method for measuring the focus on the surface of the solder bump which evaluates the summary of differential brightness by the Prewitt operator had the best performance. Also, the detection rate of defective solder bumps reached 95.6% and the inspection time was 0.3 second to a solder bump.