A Thermal Model for Ultrasonic Welding of Thermoplastics

Ultrasonic welding is one of the most popular techniques for joining thermoplastics and plays an important role in MEMS applications such as fabrication and packaging of MEMS devices. In this paper, an attempt was made to further understand the heating mechanism during ultrasonic welding. Firstly, the equation governing heat generation was derived assuming adiabatic heating. A thermal equivalent circuit model was also developed to describe the heat transfer process from the joint interface into the surroundings, and the governing equation of temperature distribution in the welding sample was deduced. Finite element method was then engaged to solve these equations to reveal the transient heating behaviour. Lastly, temperatures of the joint interface and the point adjacent to the joint were measured. The temperatures of the point adjacent to the joint calculated from finite element model are matched well with the experimental results. Based on the correlation, the temperature distributions of welding samples can be derived from the finite element model. Since the new developed model can be used to obtain the dynamic temperature distributions of welding samples during ultrasonic welding, the model provides an effective way for detailed understanding of the thermal behaviours and monitoring of the ultrasonic welding process.

Defect Detection of Flip Chip Solder Bumps Through Local Temporal Coherence Analysis of Laser Ultrasound Signals

Microelectronics packaging technology has evolved from through-hole and bulk configuration to surface-mount and small-profile ones. In surface mount packaging, such as flip chips, chip scale packages (CSP), and ball grid arrays (BGA), chips/packages are attached to the substrates or printed wiring boards (PWB) using solder bump interconnections. Solder bumps, which are hidden between the device and the substrate/board, are no longer visible for inspection. A novel solder bump inspection system has been developed using laser ultrasound and interferometric techniques. This system has been successfully applied to detect solder bump defects including missing, misaligned, open, and cracked solder bumps in flip chips, and chip scale packages. This system uses a pulsed Nd:YAG laser to induce ultrasound in the thermoelastic regime and the transient out-of-plane displacement response on the device surface is measured using the interferometric technique. In this paper, local temporal coherence (LTC) analysis of laser ultrasound signals is presented and compared to previous signal processing methods, including Error Ratio and Correlation Coefficient. The results show that local temporal coherence analysis increases measurement sensitivity for inspecting solder bumps in packaged electronic devices. Laser ultrasound inspection results are also compared with X-ray and C-mode Scanning Acoustic Microscopy (CSAM) results. In particular, this paper discusses defect detection for a 6.35mm×6.35mm×0.6mm PB18 flip chip and a flip chip (SiMAF) with 24 lead-free solder bumps. These two flip chip specimens are both non-underfilled.

Aligned Carbon Nanotube Polymer Composites

Carbon nanotube (CNT) polymer nanocomposites are promising new materials with a unique combination of mechanical and transport properties. As physical properties such as mechanical behavior, dielectric relaxation, thermal and electrical conductivities depend strongly on morphological structures of composites, we illustrate in this study the structure/property relationship in vertically aligned CNT (VACNT)/polymer nanocomposites. We have prepared VACNT/polystyrene composites and characterized their morphologies and properties. We have reported previously the continuous variation of alignment order along the height of CNT, which remain unaltered upon forming composites as revealed by small angle neutron scattering (SANS). Nanoindentation shows that both the elastic modulus and hardness vary along the CNT growth direction due to the varying tube density, alignment order and entanglement.

Warpage Prediction of Ball Grid Array Packaging

The effect of material property modeling on the warpage calculation is studied in this paper. A chip level packaging of ball grid array was considered for the simulation model. The mechanical properties of thin FR-4 and molding compound were modeled as elastic and viscoelastic based on the experimental data, and warpage developments during cooldown were calculated using different types of the mechanical property modeling. It was found that the viscoelastic characters of the FR-4 and the molding compound had significant influence on the warpage development. The results were compared with those of elastic cases, and discussions were given to analyze the deformation mechanism.

A Study of Transient Thermal Spreading of VLSI Packaging With Multi Level Scaling

Trend of VLSI chip power consumption sounds switch over from the Moore’s law to more moderate curve by the “multi core processing” paradigm. Many of the recent advanced VLSI chips adopt the multiple processing units since clock enhancement is no longer feasible to gain the expected performance based on realistic range of power consumption. Even though, heat flux may keep increasing by further fine semiconductor process and may keep localizing by further complex logics. In this study, thermal impact of hot spot size relative to chip size or the dimension of heat sink is investigated by analytic modeling as well as numerical analysis. The analytic transient thermal spreading model in a solid with transfer function has already proposed and was validated in our previous work. In this study, we have considered the impact of thermal interface between the heat source and conductive and spreading component to the sink. Thermal response in wide rage of scales is discussed from transistor level to a millimeter scale. Each level of such various sizes can be investigated individually and can be built up with some sort of cascade manner. Based on this model, thermal diffusion in silicon substrate, which has the thermal coupling with spreader and thermal interface, will be discussed for a further fine process generation of the chip. The result implies that passive thermal spreading can be achieving to the limit.

Experimental Verification of Water Ingress in a Void by Quick Diffusion at High Reflow Temperature

The pop-corning failure is known to result from high vapor pressure generation inside cavities at defective interfaces of the electronic package. In order to study the phenomenon, vapor pressure inside a void at high temperature is measured using a specific specimen configuration developed for this purpose. The specimen incorporates a volume-controllable cavity at a polymer-metal interface. A pressure sensor is used to monitor pressure evolution inside the void at high temperature. An underfill material used in the configuration is characterized in terms of hygroscopic properties. The phenomenon is also simulated on a finite element model based on these properties and specimen geometry. The prediction by the numerical model well matches the measurement by pressure sensor. This corroborates the validity of the hypothesis of high vapor pressure employed in numerous existing studies that simulated the pop-corning failure.

Thermal and Mechanical Analysis of High Power LEDs With Ceramic Packages

In this paper we present the thermal and mechanical analysis of high power light emitting diodes with ceramic packages. Transient thermal measurements and thermomechanical simulations were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistances from the junction to the ambient were decreased from 76.1 °C/W to 45.3 °C/W by replacing the plastic mould to the ceramic mould for LED packages. Higher level of thermomechanical stress in the chip was found for LEDs with ceramic packages despite of less mismatching coefficients of thermal expansion comparing with plastic packages. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high power LEDs with ceramic packages is critically important and should be in charge of delaminating interface layers in the packages.

Modeling and Experimental Verification of the Contact Resistance of a Novel Anisotropic Conductive Adhesive

The need to eliminate lead-based materials as a means of interconnection has renewed the electronics industry’s interest in using conductive adhesives for component attach, especially Anisotropic Conductive Adhesives (ACA). Typical ACAs require the application of pressure during the curing process, to establish the electrical connection and also to capture a monolayer of conductive particles between the mating surfaces. The novel ACA discussed in this paper uses a magnetic field to align the particles in the Z-axis direction during curing and eliminates the need for pressure. The application of the magnetic field allows for the formation of conductive chains between the mating surfaces, thereby eliminating lateral conductivity. This uniqueness of the novel ACA also accommodates for any coplanarity error and the formation of effective Z-axis conductivity, with a variety of lead and bump shapes. The novel ACA also enables mass curing of the adhesive, eliminating the need for sequential assembly. As part of the study presented in this paper, the conductive chains were modeled as series and parallel resistor networks in an insulating adhesive matrix. The number of particles in the chain and hence the number of interfaces between the particles is found to influence the initial contact resistance of the joints. The interfacial resistance is derived from the experimental run. Area array packages with and without bumps, reveal varying contact resistances as indicated by the model and experiment. This paper will present a model for the conductive chain formation in the novel ACA, and discuss the experimental results obtained to verify the joint contact resistance.

Reproducible Reliable AuSi Eutectic Wafer Bond Process With High Yield

The AuSi eutectic bond process is a well known and important technique in the field of single chip packaging. When it comes to low-cost and hermetic sealed packages for MEMS/NEMS sensors and actuators this technology has its decisive merits. The AuSi bonding is a low-temperature process with an electric conductive alloy. To achieve a reliable bonding with 100% yield is quite difficult, especially for large areas. In our institute we made several analyses with different process parameters and surface properties variations. The results show that the surface condition of the silicon side of the wafer pair as well as the process parameters are very important factors in relation to the yield of the eutectic bond. We also did investigations on the thickness of the gold layer. Unlike conventional AuSi wafer bonding technologies [1] our technique does not need several μm thick gold layers. We were able to achieve 100% bond yield with 1500nm and even 150nm thin gold layers. Another result we found was that a good bonding process is not only depending on the value of applied temperature and time, there is also an important influence because of the heat flow and applied pressure. In the presentation we would like to introduce our results and experience, plus we will present the coherences of parameter variations for achieving 100% yield.

Effect of Size and Spatial Distribution of Titania Pigments in Injection Molded Parts on Surface Reflectance

Titania pigments are used in molding compounds as a means to improve opacity by increasing the scattering efficiency of the medium and to develop new applications such as liquid crystal displays (LCD) and light emitting diodes (LED). The characteristics of the injection molded products are a function of molding parameters such as gate location and shear rate. In this study, quantitative measures of the particle distribution of titania pigments in polymer composites have been experimentally determined, including area fraction, average diameter, and diameter volume. A 2 × 3 × 3 ANOVA test has been conducted to assess the statistical significance of these parameters. This study deals with the size and spatial distribution of the particles. The important parameters calculated based on the Feret’s diameter are diameter-volume (dv), diameter-number (dn), and area fraction (AF). The term diameter-volume (dv) has been used to give greater significance to the large particles and thus ‘large’ indicates more and/or larger particles. The parameters have been calculated by using Image-J image processing software. MINITAB has been used to assess the statistical significance of these parameters. The results show that titania particles are not uniformly distributed within the final molded parts and they vary along the molding (longitudinal) and transverse directions of plastic flow. The difference of pigment area fraction and diameter volume at different locations within a final molded part has a significant effect on the percentage reflectance of the surface.