Chip Encapsulation Research Articles

High‐Power AlGaN‐Based Ultrathin Tunneling Junction Deep Ultraviolet Light‐Emitting Diodes

AbstractTunnel junctions (TJs) offer a unique approach to utilizing nonequilibrium tunneling injection of holes and have demonstrated potential applications in ultraviolet (UV) emitters. However, high operating voltage caused by the wide bandgap of the III‐nitrides has impeded the further promotion of TJ. Here, 275 nm n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N ultrathin tunnel junction (UTJ) deep‐UV light‐emitting diodes (LEDs) are developed for minimizing the electrical losses and achieve, to the current knowledge, the lowest operating voltage (5.7 V) at 300 mA reported to date. This study discovers that Mg‐Si co‐doping occurs in the n+‐Al0.45Ga0.55N layer during the growth of n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N UTJ due to the memory and diffusion effect of Mg, which leads to the formation of Ohmic contact between high‐work‐function Ni/Au and Mg‐Si co‐doped n+‐ Al0.45Ga0.55N layer in UTJ. The Zener diode, fluorine resin, and optimally designed glass lens are incorporated into the chip encapsulation to enhance the reliability and optical performance of the device. Furthermore, a high‐efficiency sterilization deep‐UV light source is developed integrated with 120 UTJ deep‐UV LED chips. Complete surface inactivation at a long irradiation distance (20 cm) is achieved within only seconds using the high‐power sterilization deep‐UV light source. These results indicate that UTJ is promising in developing deep UV LEDs and their integrated light sources.

Flip chip bonding on stretchable printed substrates; the effects of stretchable material and chip encapsulation

Stretchable printed electronics have recently opened up new opportunities and applications, including soft robotics, electronic skins, human-machine interfaces, and healthcare monitoring. Stretchable hybrid systems (SHS) leverage the benefits of low-cost fabrication of printed electronics with high-performance silicon technologies. However, direct integration of silicon-based devices on conventional stretchable substrates such as thermoplastic polyurethane (TPU) and polydimethylsiloxane (PDMS) is extremely challenging due to their restricted low-temperature processing. In this study, a recently developed thermoset, stretchable substrate (BeyolexTM) with superior thermal and mechanical properties was employed to realize SHS via direct flip chip bonding. Here, ultra-thin chips (UTC) with a fine-pitch, daisy-chain structure was flip-chip bonded by using anisotropic conductive adhesives, while the complementary circuitry was facilitated via screen-printed, stretchable silver tracks. The bonded samples successfully passed reliability assessments after being subjected to cyclic 30% stretch tests for 200 cycles. The potential benefits of chip encapsulation after integration with the stretchable substrate to withstand larger strains were demonstrated by both mechanical simulation and experimental results.

Effects of Moisture Diffusion on a System-in-Package Module by Moisture-Thermal-Mechanical-Coupled Finite Element Modeling.

Epoxy molding compounds (EMCs) are commonly used in electronic products for chip encapsulation, but the moisture absorption of EMC can induce significant reliability challenges. In this study, the effects of hygrothermal conditions and structure parameters on moisture diffusion and the consequent influences (such as moisture content on die surfaces and stress distribution) on a system-in-package module have been systematically investigated by moisture–thermal–mechanical-coupled modeling. Hygroscopic tests were carried out on a new commercial EMC at 60 °C/60% RH and 85 °C/85% RH, followed by evaluations of diffusion coefficients by Fick’s law. It was found that the moisture diffusion coefficients and saturation concentrations at 85 °C/85% RH were higher than those at 60 °C/60% RH. From the modeling, it was found that the consequent maximum out-of-plane deformation and stress of the module at 85 °C/85% RH were both higher than those at 60 °C/60% RH. Influences of thicknesses of EMC and PCB on the moisture diffusion behavior have also been studied for design optimization. It was found that the maximum moisture concentration on die surfaces and resultant stress increased notably with thinner PCB, whereas the effects of EMC thickness were limited. This can be attributed to the comparison between the thicknesses of EMC and PCB and the shortest existing diffusion path within the module. These findings can provide helpful insights to the design optimization of electronic modules for hygrothermal conditions.

Trifluoromethanesulfonic acid-assisted one-pot catalytic synthesis of phenyl silicone resins

A facial one-pot method for the preparation of phenyl silicone resins is introduced. Vinyl end-capped phenyl silicone resin (i) and phenyl hydrosilicone resin (ii) were successfully prepared by selecting trifluoromethanesulfonic acid (TfOH) as the catalyst. The structures and the thermo-optical performances of the resulting resins were investigated by IR, NMR, GPC, TGA and UV–Vis, respectively. The results show that the yield, the transparency and the refractive index for resin (i) are 95%, 90.4% and 1.5253, respectively, and for resin (ii) are 94%, 92.8% and 1.5201, respectively. The properties of the cake obtained by hydrosilylation of resin (i) and (ii) in the ratio of 1:1 for LED chip encapsulation were studied, and the packaging effect was examined preliminarily.

Epoxy composite with significantly improved thermal conductivity by constructing a vertically aligned three-dimensional network of silicon carbide nanowires/ boron nitride nanosheets

The trend of miniaturization, integration and multi-function of modern electronics leads to the speedily increased power density which makes heat dissipation a crucial issue. Herein, epoxy based composites with highly enhanced through-plane thermal conductivity were successfully prepared by constructing a free-standing and vertically aligned silicon carbide nanowires (SiCw)/functionalized boron nitride nanosheets (f-BNNS) framework through modified filtration strategy. The synergetic effect between hybrid fillers ensured highly efficient channels for phonons in vertical direction that the maximum thermal conductivity reached 4.22 W/mK at a low hybrid filler loading of 21.9 vol%, increased by 1658% in comparison to that of pure epoxy. Due to the entanglement effect of SiCw/f-BNNS framework and the enhanced interface interaction by BNNS modification, the interfacial thermal resistance between filler/filler and filler/matrix were both reduced a lot compared with random dispersion method. Exceptional heat dissipation capability of the vertically orientated architecture was also demonstrated by theoretical simulation and chip encapsulation applications. In addition, the coefficient of thermal expansion of composite reached as low as 41.1 ppm/°C, only half of the epoxy resin (83.5 ppm/°C). This work provides a novel strategy for fabricating polymer based composites with superior through-plane thermal conductivity in thermal management applications.

Sugarcane Bud Chip Encapsulation for ex vitro Synthetic Seed Formation

Altough the sugarcane crop had a huge world importance the planting system stil the same since the development, needing changes to increase the procution potential. So, the objective of this study was to assess the effect of sugarcane bud chip encapsulation on the initial growth of seedlings. To provide informations for a new planting system, using small stem pieces of sugarcane to produce the seedlings. Two experiments were conducted in a completely randomized design. In the first, bud chip encapsulation was assessed with six concentrations of sodium alginate (0, 10, 20, 30, 40, and 50 g L-1) cross-linked with 300 mM calcium chloride, with the encapsulated chips being kept in a greenhouse. In the second experiment, the capsules resulting from the different sodium alginate concentrations were tested for the dry mass adhered to the bud chip, moisture, swelling index, biodegradability, and solubility. Emergence greater than 70% was obtained at sodium alginate concentrations of 0, 10, and 20 g L-1. The 30, 40, and 50 g L-1 concentrations inhibited seedling emergence and initial growth; however, when the capsule was removed, the bud chips formed viable seedlings. Encapsulation inhibited emergence because the capsule acts as a physical barrier; however, encapsulation may be used for bud chip preservation. The study of new capsules and encapsulation methods may enable the ex vitro production of synthetic sugarcane seeds.

Reduction of Sodium Migration in Polymers by Addition of 15-Crown-5 Ether as Getter Substance

Already small amounts of ion contamination within sensitive areas of high voltage transistors can cause severe damage to semiconductor devices. Therefore, it is of utmost importance to hinder mobile ions from reaching critical regions (e.g., the gate oxide layer). To protect semiconductor devices from environmental influences and contamination, polymers are used for chip encapsulation. Reduction of ion migration within these polymers can increase the reliability of the semiconductor device. Crown ethers are well known to form stable complexes with alkali cations. Within this work the influence of 15-crown-5 ether on the sodium migration in poly-methyl methacrylate and polyimide will be evaluated by means of ion conductivity measurements.

High viscosity paste dosing for microelectronic applications

AbstractToday's microelectronics packaging especially for SiPs relies on the processing of a wide variety of materials Such as materials for components, substrates, contact materials (solder & adhesives) and encapsulants. Most materials are processed as bulk material but precision dosing of pastes is key to many assembly processes. Examples are dosing of solder paste, typically done by stencil printing, underfilling for Flip Chip encapsulation, typically done by dispensing or jetting, or glob top encapsulation of Chip on Board assemblies, where also dispensing is the typical process. When working with those paste materials, viscosity is one of the key parameters for processing, and viscosities too high do not allow dosing of the materials, not even to transport the material from a reservoir to the dosing head, which may be a simple needle or a jet valve. [1,2]To overcome this obstacle, i.e. to dose materials of high viscosity precisely and homogeneously from a syringe to the dosing head, a research program has been set up, where Vermes microdispensing as a valve manufacturer and TU Berlin/IZM as a research institute are cooperating. TU Berlin is working on material rheology effects and flow models; Vermes is researching valve modifications and material flow path optimization. Core of the research is to find methods that allow a reduction of paste viscosity without leading to irreversible changes in the material, as would be the case when simply applying heat to the paste.As reference process for material dosing, FO-WLP has been chosen, materials selected for the investigations are glob top dam and fill material and liquid molding compound – using both rheological experiments as well as actual material dosing and processing. Apart from temperature, mechanical and ultrasonic stimulation of the material have been evaluated to achieve optimized dosing of high viscous pastes,As a result, a first description of paste behavior during processing is given, being the basis for future work towards homogeneous precision dosing of high viscous pastes for microelectronic applications.

Underfill flow simulation based on lattice Boltzmann method

Underfill process is carried out mainly to prevent interconnection failures caused by the mismatch of CTE between the die and the substrate in flip chip encapsulation. Owing to troubles in calculating the capillary force, i.e., result-based and interface reconstruction, the underfill flow cannot be well characterized by current simulation methods. In this paper, we present a mesoscale underfill simulation method based on three-dimensional LBM (lattice Boltzmann method) with D3Q19 velocity set and LBGK (lattice-Bhatnagar–Gross–Krook) evolution model. In this method, the GIPM (generalized interparticle-potential model) is first proposed to model the capillary flow, which can solve the fluid–fluid interaction and the solid–fluid interaction in a unified manner. A geometric model for underfill simulation is then developed. The solid wall is divided into three parts, i.e., the substrate, the die and the solder bump, and each part is allowed to have a different wettability by assigning a mesoscale interaction parameter. For verification purpose, three underfill cases, which are different in wall wettability, are examined. In each case, besides the experimental results, we also present numerical results predicted by a VOF multiphase method with CSF capillary model. The results show that the proposed method has a good performance in the underfill simulation.

Three-Dimensional Measurement System Based on Micro Stereovision

With vision feedback the binocular micro stereovision system based on stereo light microscope (SLM) makes it possible to achieve 2D/3D high accuracy auto-positioning, 3D information extraction, 3D shape reconstruction and 3D measurement. Therefore, it is extensively used in micro robot navigation, micromanipulation, micro assembly and bioengineering, etc. To improve the key problems of low accuracy, refraction and occlusion in micro stereovision measurement, a novel micro stereo vision system is built based on optical theory and digital image processing. Then, the nonlinear correlation between optical paths of the micro stereo vision system is studied and the environment adaptive nonlinear model is established. On this basis, the coupling mechanism of multi-refraction and occlusion with micro-projection nonlinear model is proved, and multi-refraction variable refractive index correction and micro stereo occlusion reconstruction algorithms are developed. It realizes synchronization observation in larger field of view, rapid non-contact accuracy positioning and 3D measurement based on stereovision to solve the pivotal problems of accuracy positioning and measure in the chip encapsulation, micro assembly and the micro manipulation.

Mechanical durability of RFID chip joints assembled on flexible substrates

PurposeThe purpose of this paper is to investigate the durability of radio‐frequency identification (RFID) chips assembled on flexible substrates (paper and foil), with materials evaluated with regard to mechanical stresses and dependence on the applied substrate, antenna materials, chip pad printing and chip encapsulation.Design/methodology/approachRFID chips were assembled to antennas screen printed on flexible substrates. Shear and bending tests were conducted in order to evaluate the mechanical durability of the chip joints depending on the materials used for mounting the RFID chip structures. X‐ray inspection and cross sectioning were performed to verify the quality of the assembly process. The microstructure and the resistance of the materials used for chip pads were investigated with the aim of determining the conductivity mechanism in the printed layers.FindingsAddition of carbon nanotubes to the conductive adhesive (CA) provided a higher shear force for the assembled RFID chips, compared to the unmodified conductive adhesive or a polymer paste with silver flakes. However, this additive resulted in an increase in the material's resistance. It was found that the RFID substrate material had a significant influence on the shear force of mounted chips, contrary to the materials used for printing antennas. The lower shear force for chips assembled on antennas printed on paper rather than on foil was probably connected with its higher absorption of solvent from the pastes. Increasing the curing temperature and time resulted in an additional increase in the shear force for chips assembled to antennas printed on foil. A reverse dependence was observed for chips mounted on the antennas made on paper. An improvement in the durability of the RFID chip structures was achieved by chip encapsulation. Bending tests showed that a low‐melting adhesive was the best candidate for encapsulation, as it provided flexibility of the assembled structure.Research limitations/implicationsFurther studies are necessary to investigate the mechanical durability of RFID chips assembled with a conductive adhesive, with different addition levels and types of carbon nanotubes.Practical implicationsThe results revealed that the best candidate for providing the highest RFID chip durability related to mechanical stresses was the low‐melting adhesive. It can be recommended for practical use, as it simplified the assembly process and reduced the curing step in the encapsulation of the RFID devices. From the results of shear testing, conductive adhesives with carbon nanotubes can be used in RFID chip assembly because of their ability to increase the shear force of joints created between the antenna and the chip.Originality/valueIn this paper, the influence of the materials used for antenna, chip pads, encapsulation and the curing conditions on the mechanical durability (shear and bending) of RFID chips was analyzed. Commercial and elaborated materials were compared. Some new materials containing a conductive adhesive and carbon nanotubes were proposed and tested in RFID chip assembly to antennas printed on flexible substrates (paper and foil).

Degradation of moulding compounds during highly accelerated stress tests – A simple approach to study adhesion by performing button shear tests

High temperature storage can degrade moulding compounds for chip encapsulation to such an extent that the adhesion to surfaces like copper (lead frames) or polyimide (chip coating) decreases drastically causing delamination. Also during normal operation of electronic components heat is generated locally (bond wire or chip surface) degrading the moulding compound and reducing the adhesion which in extreme cases can destroy the metallisation or the bond wires.

Long-Term Stability of Mold Compounds and the Influence on Semiconductor Device Reliability

Lifetimes of semiconductor devices are specified according to the products and their applications to ensure safe operation, for instance as part of an automobile product. The long-term stability of the device is strongly dependent on the chip encapsulation and its adhesion to the chip and substrate. Molded silicon strips that act as a model system for molded chips inside semiconductor devices were investigated. Four commercially available mold compounds were applied on silicon strips and stored over 5 years at room temperature (RT), and changes in the thermomechanical behavior were analyzed. After storage, all molded strips exhibited warpage reduction in the range of 11% to 14% at RT with respect to the initial warpage. The temperatures for the stress-free state also changed during storage and were located between 228°C and 235°C for each mold. Additional stress applied to the stored modules, by temperature cycling as well as high-temperature storage, increased the warpage of the molded silicon samples. For further interpretation of measured results, finite-element method calculations were performed.

Plastic Ball Grid Array Encapsulation Process Simulation on Rheology Effect

The integrated circuit should be encapsulated for protection from their intended environment. This paper presents the flow visualization of the plastic ball grid array (PBGA) chip encapsulation process considering of the rheology effect. In the molding process, encapsulant flow behavior is modeled by Castro-Macosko viscosity model with considering curing effect and volume of fluid technique is applied for melt front tracking. The viscosity model is written into C language and compiled using User-Defined Functions into the FLUENT analysis . Three types of Epoxy Molding Compound namely case 1, 2, and 3 were utilized for the study of fluid flow inside the mold cavity. The melt front profiles and viscosity versus shear rate for all cases are analyzed and presented. The numerical results are compared with the previous experimental results and found in good conformity. In the present study, case 1 with greater viscosity shows the higher air trap and higher pressure distributions.

Modeling of a non-Newtonian flow between parallel plates in a flip chip encapsulation

The viscosity of the underfill encapsulant may be different under the conditions of different shear rate, filler content, and temperature. Most of the encapsulant is epoxy containing silica fillers. It exhibits non-Newtonian behavior in the underfill flow. The effect of the viscosity variations on the underfill filling flow was investigated in this study. An analytical model of the filling flow is proposed to accomplish the shear-rate depending viscosity. Due to the addition of fillers in the encapsulant, the viscosity may exhibit both shear thinning and thickening behaviors depending on the temperature and filler content. This study proposes a model of the viscosity considering both effects. In the situations demonstrated in the results, the shear thinning and thickening effects may have major influence on the velocity profile and the filling speed.

Modeling manufacturing processes using fuzzy regression with the detection of outliers

Empirical modeling, which involves various common techniques such as statistical regression, artificial neural networks and fuzzy logic modeling, is a popular approach to developing models for manufacturing processes. Among those techniques, statistical regression is the most popular one used to develop the explicit type of empirical models. However, if the experimental data and results contain a substantial degree of fuzziness, fuzzy regression is more appropriate for use in developing empirical models based on such data and results. In recent years, attempts have been made to use fuzzy regression to model manufacturing processes. However, it has been recognized that the existence of outliers can have a great effect on the prediction accuracy of a fuzzy regression model. This problem has not been well addressed in the previous studies on fuzzy regression. In this paper, an algorithm for detecting outliers based on Peters’ fuzzy regression is proposed. The application of the algorithm to developing a fuzzy regression-based process model of the dispensing of fluid for IC chip encapsulation is described. Finally, the results of the validation of the models are discussed.

A flip–chip encapsulation method for packaging of MEMS actuators using surface micromachined polysilicon caps for BioMEMS applications

In this paper, we present a novel technique for encapsulation of MEMS devices. The technique is demonstrated to address two issues related to the use of in-plane thermal actuators for BioMEMS applications. First, an encapsulation process is described to provide protection to a MEMS actuator from debris and other particulate matter when deployed in a biological environment. The encapsulation structure consists of a multilayer wall around the actuator and a surface micromachined polysilicon cap. A small clearance is provided around a piston that transmits motion from the actuator to the external world. In air, the packaged actuator performance is comparable to that of an unpackaged actuator, thus indicating successful encapsulation without any damage to the actuator. Second, this packaging approach is used to address the issue of reduction in efficiency of the thermal actuator in liquids by coating the packaged actuator with a thin conformal hydrophobic layer. This prevents liquid from entering the encapsulation, thus isolating the hot actuator components from the liquid. Experimental results show that efficiency of the packaged actuator in water improved giving a performance similar to that observed in air, suggesting an isolation of the hot actuator components from the liquid. Although the technique is demonstrated for thermal actuators, it is also applicable to other MEMS devices and in-plane actuators such as electrostatic comb drives for engineering as well as biological applications.

Cost-Effective Chip-On-Heat Sink Leadframe Package for 800-Mb/s/Lead Applications

Chip-on-heat sink leadframe (COHS-LF) packages offer a simple, low-cost chip encapsulation structure with advanced electrical and thermal performance for high-speed integrated circuit applications. The COHS-LF package is a novel solution to the problems of increased power consumption and signal bandwidth demands that result from high-speed data transmission rates. Not only does it offer high thermal and electrical performance, but also provides a low-cost short time-to-market package solution for high-speed applications. In general, there are two main memory packages employed by the most popular high-speed applications, double data rate (DDR) SDRAM. One is the cheaper, higher parasitic leadframe packages, such as the thin small outline packages (TSOPs), and the other is the more expensive, lower parasitic substrate-based packages, such as the ball grid array (BGA). Due to the requirement for higher ambient temperature and operating frequency for high-speed devices, DDR2 SDRAM packages were switched from conventional TSOPs to more expensive chip-scale packages (i.e., BGA) with lower parasitic effects. And yet, by using an exposed heat sink pasted on the surface of the chip and packed in a conventional leadframe package, the COHS-LF is a simpler, lower cost design. Results of a three-dimensional full-wave electromagnetic field solver and SPICE simulator tests show that the COHS-LF package achieves less signal loss, propagation delay, edge rate degradation, and crosstalk than the BGA package. Furthermore, transient analysis using the wideband T-3/spl pi/ models optimized up to 5.6 GHz for signal speeds as high as 800 Mb/s/lead demonstrates the accuracy of the equivalent circuit model and reconfirms the superior electrical characteristics of COHS-LF package.

Simulation of Mold Filling for Non-Newtonian Fluids - Part 2

This paper deals with the flow in the resin transfer molding process commonly used for IC chip encapsulation in the electronic packaging industry. A solution algorithm is presented for modeling the flow of a non-Newtonian fluid obeying a Power-Law model and the algorithm is used to conduct parametric studies in transfer molding. The flow model uses the Hele-Shaw approximation to solve the Navier-Stokes Equations and a pseudo-concentration algorithm for tracking the interface between the resin and the air. The Finite Element Method is employed to reduce the governing partial differential equations to algebraic form. The model is used to study the flow from the transfer ram into the cavity for different dimensions of transfer molding tools. Parametric studies are carried out to obtain balanced filling for transfer molding configuration. Parametric studies could provide a design guideline to optimize the encapsulation process prior to the setting up of an actual manufacturing set.

Encapsulation of ISFET sensor chips

The encapsulation of ISFET sensor chips is a challenge to sensor technology and materials. Recently developed new packaging technologies enable reliable and cost effective chip encapsulation. By this, one of the serious problems, which had hindered the large-scale production and industrial application of ISFET sensors over three decades could be overcome now to a large extent. The paper reports both on ISFET encapsulation methods on laboratory level and advanced industrial production technologies. The influence of the packaging design on special applications and on the dynamic behaviour of the sensor is illustrated. Furthermore, sensor materials are characterised and various testing methods are described.