Reworkable anisotropic conductive adhesives (ACAs) are of interest when the material is used for assembling electronic modules with high value, such as in medical devices. Adhesive matrices comprising a blend of a thermosetting epoxy and a thermoplastic polymer are selected because it has shown potential to ensure good electrical and mechanical integrity whilst still allowing reworkability for ACA assemblies. Our previous work demonstrated the feasibility of using blends of an epoxy and a thermoplastic polysulfone as an adhesive matrix for the reworkable ACAs. The rework temperature, however, is relatively high (190°C) which causes disadvantages for the rework process and safety of sensitive electronic components nearby. ACA material with lower rework temperature is thus of interest. This paper presents the findings of favorable mixing ratio between an epoxy compatible with ACA applications and a thermoplastic polymer that offers good mechanical strength combined with reworkability at a temperature as low as 100°C. The results show that the adhesive blends with a high concentration of thermoplastic polymer (35–65 wt%) exhibit satisfactory die shear strength at temperatures relevant for production/storage (23°C) and operation of medical devices (50°C). Furthermore, successful rework at temperature as low as 100°C is confirmed for such adhesive blends.
Thermal performance of an epoxy resin reinforced by carbon fibers is studied by numerical simulation method. Various carbon fiber structures are taken into consideration and the effective thermal conductivity of the composite carbon fiber waved structure is obtained. The influences of the number, size, shape, spacing and arrangement of the carbon fibers on the thermal conduction of the composites are analyzed. The deformation of the composite under mechanical loading and the corresponding the thermal conductivity of the carbon fiber-reinforced epoxy resin are also investigated.
Corrosion occurs on metallic materials in the presence of moisture and reactive gases, such as H2 S, SO2 or NO2 and is enhanced by different substances like e.g. chlorides. Particularly electronic materials and components are affected, because they are exposed to increasingly harsh environmental conditions, resulting in damage to corrosive media (e.g. off-shore systems, electronic components in engine compartment of automobiles). Especially for the materials and contacts of micro-and power electronic components, corrosion induced processes are significant reliability limiting. Additionally, depending on process parameters, corrosion can also be induced during manufacturing (e.g. nickel corrosion in pad metallization). We developed an innovative method, based on electrochemical measurements by a local measuring cell that allows corrosion sensitivity analyses of typical metallization systems for substrates and electrical contacts in a very short time. In this paper, we present and discuss the results for electrochemical corrosion testing by the new method in correlation to standard reliability tests, like mixed flow gas testing (MFG) and neutral salt spray testing as well as standardized electrochemical testing for metallization systems pf printed circuit boards. High resolution microstructural analyses after standard testing as well as after miniaturized cyclovoltammetric testing are giving evidence for the running corrosion mechanism. The investigations carried out in this study show the application potential of the new local electrochemical test method and the usability for quality inspection of PCBs.
In recent years, laser-reduced graphene oxide (LRGO) has received widespread interest, however, the thermal properties of graphene films obtained by laser reduction of GO are rarely reported. In this paper, a pulsed laser was used to reduce the prepared GO films. The obtained LRGO films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS). The thermal diffusivity of the LRGO was measured as 7.3 mm 2 /s, higher than that of GO measured as 5.9 mm 2 /s. The heating performance of LRGO was performed under different DC voltages and the results show that the temperature can reach up to 91 °C with a response time of 14 s under the voltage of 18 V. The excellent electrothermal performance of LRGO films indicate that the LRGO films are promising as heating elements for various application such as defoggers.
In this work, laser-induced graphene (LIG)/ polyimide (PI) films with good thermal properties were prepared by directly inducing graphene on the bare PI substrates by a computer numerical control (CNC) laser engraving machine. The obtained samples were characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed that the laser energy density has a significant impact on the microstructures of the samples. Moreover, the thermal diffusivity of LIG/PI was increased from 0.5 mm2/s to 1.6 mm2/s, which is 3 times higher than bare PI. Finally, the electrothermal properties of the LIG films were investigated and the results showed that under a 12 V power supply, the equilibrium temperature of LIG films increases from 45°C to 74°C with the increase of laser energy density from 1.8 J/mm2 to 2.4 J/mm2. Our results indicate that this time-saving, low-cost, and environment-friendly method is promising for fabricating excellent graphene-based materials.
This paper presents results of an empirical investigation on drop feature optimization, subsequent fine trace printing and influence of sintering atmosphere on printed traces on polyimide substrate with Ag nanoparticle ink. The impact of printing parameters such as waveform features, jetting voltage and printhead height on drop formation and fine trace printing is investigated. The experiments revealed that drop diameter decreased by 33% after decreasing jetting voltage and overall waveform duration to a minimum functional range. From further studies, it is found that to print accurate fine traces an optimum drop spacing value in consideration with drop count should be selected which prints traces close to the required trace width dimensions. Based on these optimizations, trace width of 30 ± 2 μm can be printed with minimum pitch of 35 μm. Sintering progresses faster under air compared to nitrogen due to the efficient removal of the organic capping agents. The resulting sheet resistance under air and nitrogen were 0.1312 Ω/π and 1.8586 Ω/π respectively.
A new generation of devices that connects or contains hybrid mechanical-, electrical-, and optical elements on the nano scale are being developed, with potential applications ranging from quantum-enabled hardware to different types of sensors. However, these hybrid optomechanical devices require innovative packaging with not only stress and strain free assembly with high vacuum and hermetic sealing typical of MEMS, but also optical access through windows, fiber alignment and pigtailing. Detailed description of such a module is provided.
Conformal coatings are applied to protect printed circuit boards and components mounted on them from the deleterious effects of moisture, particulate matter and corrosive gases. The conventional method of testing the effectiveness of these coatings is to expose the conformally coated hardware to a corrosive environment for extended periods of time — often lasting many months — and determine the mean time to failure. iNEMI’s Conformal Coating Evaluation for Improved Environmental Protection project team is recommending a quicker test method that takes less than a week to evaluate conformal coatings. This method uses the corrosion rates of conformally coated thin films of copper and silver exposed to a sulfur gas environment as a measure if the coating performance. The project team investigated how temperature and humidity impact the corrosion rates of conformally coated copper and silver thin films compared to uncoated films. Performances of acrylic, silicone and atomic layer deposited (ALD) coatings were studied as a function of temperature and relative humidity. The team found that temperature affected the corrosion rates of conformally coated copper and silver thin films, whereas relative humidity had a lesser influence. The team also discovered significant differences in corrosion protection provided by the three coatings that were tested.
Non-touch interface technology will be used more and more in future electronic systems. This paper describes the development of a novel sensor element for such interfaces. In this work, we apply a systematic approach, the M3-approach (methodologies, models, measures), to design miniaturized, scalable and low-cost 60 GHz MIMO (Multiple Input Multiple Output) radar front-ends for the integration of a novel non-touch human interface into a table. The packaging technology applied for the development of this radar module is suitable for mass production. The complete application of the M3-approach requires the implementation of three key steps, namely methodologies, models and measures. However, in this paper, we focus on the first two steps (i.e., methodologies and models) and describe these steps in detail.
This paper focuses on the design and fabrication of a new programmable thermal test chip as a flexible and cost-effective solution for simplification of characterization/prototyping of new packages. The cell-based design format makes the chip fit into any modular array configuration. One unit cell is as small as 4x4 mm2, including 6 individually programmable micro-heaters and 3 resistance temperature detectors (RTDs). All micro-heaters and sensors have 4-point Kelvin connections for improved measurement accuracy. The chip contains 2 metal layers: 100 nm thin-film Titanium to create micro-heaters and RTDs, and 2 μm Aluminum to add single bump measurement units and daisy chain connections. These structures facilitate bump reliability investigations during thermal/power cycling tests in flip-chip assembly technology. The calibration curves of RTDs show a sensitivity of 12 $\Omega$/K which is improved by 50 percent compared to the state-of-the-art TTC. The proposed design provides higher spatial resolution in thermal mapping by accommodating 3 RTDs per cell. The dense configuration of micro-heaters increases the uniformity of the power dissipation, which enhances the accuracy of thermal interface material (TIM) characterizations. The steady-state infrared (IR) thermography of a 20x20 mm2 TTC, including 150 active micro-heaters, verifies the promising uniformity of the heat profile over the chip surface.
The change of solder materials is driven by ever new requirements and legislations. Thus, lead-free tin based solders like tin-silver-copper (SAC) have become a standard in the electrical industry, especially in high-power modules. In terms of processing and behavior, they are still different to lead-based solders. In order to obtain lead-free solders with the familiar behavior and proven performance of lead-based solders, their development continues. Micro-additives in conventionally used SAC solder can improve mechanical properties, processability and durability. To evaluate these properties, we soldered different solder alloys with three individual micro-additives each and compare them with standard SAC solder. Following shear tests and optical inspection of the solder joints provide information about the differences in reliability, durability and phase growth caused by the individual micro-additives. By comparing two solders with combined micro-additives, the interaction of the additives can be estimated. In conclusion, it is clear that micro-additives can improve the standard solder. By adding different micro-additives, solder can be adapted to the requirements of different applications.
Here, we report flip-chip bonding processes based on Anisotropic Conductive Film (ACF) and Sn-Ag-Cu (SAC) solder implemented for bonding three test chips (two Silicon, Si ICs on either side of an Indium Phosphide, InP IC) in series on $25{\mu}\mathrm{m}$ thick polyimide foil substrates. Si ICs were bonded only with SAC solder, whereas both SAC and ACF were applied for the flip-chip integration of the fragile InP chips. RF measurements were then performed on the impedance controlled paths across the polyimide foil of the fabricated assemblies to evaluate as well as to compare the RF performance of ACF and SAC solder in terms of the differential S 21 parameter. The measurements revealed that when ACF is employed for interconnecting the InP chip and SAC solder for the two Si chips, a higher 3 dB bandwidth of 42 GHz was obtained whereas a 3 dB bandwidth of only 31 – 34 GHz was measured when only SAC solder was used for bonding all three ICs. These measurement results confirm that a higher bandwidth can be obtained from the assemblies when ACF is used as the interconnection material. Therefore, it can be concluded that the ACF based integration technology could be favored over solder processes for bonding high performance multi Gbit/s subassemblies, especially with higher density of contact pads.
Additive manufacturing of copper structures with selective laser melting offers promising possibilities for prototyping or production of unconventional structures for electronic assemblies. There are various copper powders from different manufacturers available that are suitable for processing with SLM technology. Simple structures produced with copper powder in SLM technology are increasingly being used. However, more complex structures with small dimensions are still the exception. As part of a public funded project, we are researching the potentials and limits of the copper materials and processes currently used for additive manufacturing in electronic systems using the example of a heat sink for microelectronic assemblies. Within the project we focus on the aspects of microstructure formation after processing (e.g. particle sintering, pore formation, binder residues, etc.). In our paper we summarize the research results achieved so far. In comparison to reference assemblies from conventional production, we show impressive high-resolution microstructural results of SEM on copper powder in initial state and manufactured structures and correlate these results to each other. The result is a current state of the art for the use of copper materials and SLM processes in additive manufacturing in the field of electronic systems.
The results developed in a research project resulted from the cooperation between Fraunhofer IKTS in the field of ceramic multilayer technology and InfraTec GmbH in the field of infrared sensors and measurement technology and were generated within the project’s duration of 4 years. New types of miniature heaters embedded in ceramics were developed and their properties regarding modulation, spectral emission, long term and high temperature stability were characterized and optimized. A new process chain based on ceramic multilayer technology was developed to produce the IR emitter, which is based on a combination of sacrificial paste technology using screen or stencil printing and Low Temperature Cofired Technology (LTCC) processing. The generated dielectric layer thicknesses of 15-20 µm are half as thick as with conventional LTCC tape manufacturing processes. To optimize the emission coefficient towards a black body, new ceramic pastes suitable for screen printing have been developed. In this way, black surfaces could be created on the radiators, which are characterized by high temperature stability and low thermal mass. First long-term studies of the IR emitter up to 2000 hours show functionality at 650 °C with low degradation.
The photothermal effect is the most direct and simple way to use solar energy. Different color materials have different thermal effects under light exposure, which will have wide range of influences on people’s daily life, such as the production of clothing and the choice of paint. This article innovatively proposes a way of using MEMS micro-thermoelectric generator (μ-TEG) to distinguish the photothermal effect of different materials after exposure to light. The design and manufacture of MEMS μ-TEG is the focus of this paper, in which the micro-nano manufacturing technologies such as ultraviolet lithography and magnetron sputtering are mainly used. A hybrid fabrication method combining the non-contact lithography and photoresist melting is used to prepare a top electrode with good electrical contact. In the 5 cm*5 cm area, we integrated 46000 P-N thermoelectric units in series on a silicon wafer and its ultra-thin structure offers very high temperature sensitivity. In our experiment, by changing the substrate materials’ type and color, the light-to-heat effects were studied under monochromatic LED light illumination. The results demonstrated that the difference of the photothermal effects can be used to distinguish the various materials. The experimental samples include cotton, linen, chemical fiber, and paper. Each material includes red, yellow, green, and white. For each sample, ultraviolet/visible/near infrared spectrophotometer was used to measure the absorptivity in the visible light range. At room temperature (298K) and in an enclosed test platform (to minimize air flow), the open circuit voltage of 1~2 mV can be detected by the irradiation of monochromatic LED light source, which can accurately measure and distinguish samples of different colors and materials. It has broad application prospects in the field of photothermal effect temperature measurement and reaction process temperature monitoring.
This paper presents a novel approach for removing out-of-plane deformation in metal interconnects by adding a fractional structure to the original meander shape and using the optimised fabrication stack. In thermoformed electronics in cases where copper is used as the conductor, the twisting of meander-shaped structures caused by excessive mechanical stress can cause a non-uniform surface, delamination of the metal interconnect from the substrate, and in some cases, a short circuit to the adjacent tracks. Typically, stretchable electronics designers use various shapes and widths of the copper interconnect to tackle this issue. Using conventional meander shapes such as horseshoes and U shapes is not universally practical, especially when stretching is higher than 30 percent leading to significant out-of-plane deformation. Limiting this out-of-plane deformation by reducing the track width is not always applicable, as a minimum width is needed from a technology and conductivity perspective. The presented approach is inspired by computational and experimental studies of multiple meander shapes and fabrication methods. A geometry-based and fabrication-based approach is presented, which can reduce the mechanical stress of almost all possible meander shapes by increasing the meander’s path length to accommodate the metal track’s produced torque during stretching. An analytical approach is provided for calculating the optimal meander parameters, and the optimal fabrication stack is achieved based on simulation results. Experiments and finite-element modeling for an industrial case study show the improvement in the stress distribution and reduction of out-of-plane.
Electrically conductive adhesives offer a low-temperature alternative for soldering process in electronics assembly. Adhesive bonding technology is especially advisable when integrating components on printed structures on soft substrates with low heat-tolerance. This study investigates mechanical durability of surface mounted chip resistors bonded on a flexible substrate with an isotropic conductive adhesive. The test specimen are manufactured in fully automated roll-to-roll processes by rotary screen printing a conductive circuitry, and subsequently dispensing silver epoxy, pick-and-placing components and heat-curing the adhesive. The adhesive bonding strength is studied with an automated test setup pulling a component off from the substrate. This destructive test method is used to evaluate the mechanical strength when varying adhesive volume at the interconnection areas with different component sizes. The results prove that the adhesive bonding strength for printed and hybrid electronics can be improved by increasing the adhesive volume in bonds when using a silver epoxy. At the same time, spreading of adhesive dots during assembly process limits the applicable adhesive volume to avoid short circuits and to maintain the electrical functionality. The study provides guidelines to optimize dispensing routines to achieve stable electro-mechanical interconnections.
A thin film Gold/Platinum/Titanium (Au/Pt/Ti) - based microheater with pectination construction and a four-point probe was fabricated on a silica substrate. A standard lithography process was used to transfer the circuit pattern onto the substrate, and then Au/Pt/Ti was deposited on the substrate by an evaporator. Standard calibration was carried out at various temperatures, which can be obtained the relationship between the temperature and the resistance of the microheater, the results show that the Au/Pt/Ti-based microheater has a good linear relationship between the temperature and the resistance, indicating the microheater can also be used as a temperature sensor. Furthermore, the effects of different input powers, the geometry, and the thickness of the thin-film metals of the microheater were investigated and discussed. Finally, a finite element model was set up to see the temperature distribution of the microheater after the electric potential is applied.
Traditional planar microelectrode arrays (MEAs) have contributed significantly to broaden our knowledge on neuronal electrophysiological signaling by enabling simultaneous recording and stimulation of intracellular activities. However, planar MEAs are not suitable for investigating the electrophysiological behavior of complex 3D neuronal cultures. To exploit the potential of these 3D cultures, more advanced tools are needed which can assess the network-wide electrophysiological activity of neurons in 3D space. In this work, we propose a novel approach to develop a multi-level 3D microstructured array built on a well-established planer MEA setup. Initially, a planer MEA is realized using standard photolithography and physical vapor deposition (PVD) technique. During fabrication of the planer MEA, circuitry is added to connect the planar microelectrodes separately into individual groups. In addition to it, an electroplating process is utilized to grow gold micro-pillars on the planar electrode pads using a chemically amplified negative photoresist (KMPR 1050) from Kayaku Microchem as the mold. The circuitry allows independent control of the heights of the individual groups of 3D multi-level gold microelectrodes on the array. The mold is then stripped off. The microelectrodes can be insulated with Parylene-C and crowned with spherical gold beads using the ball bonding technique. The spherical gold beads could act as the interface between the device and the neuronal culture. The spherical shape of the bead would allow omnidirectional growth of neuronal networks, better mimicking the in vivo growth patterns. Experiment work to record and stimulate the electrophysiological activities of neuronal networks is ongoing. All fabrication techniques utilized in this approach are well established, allowing the fabricated devices to be reproducible, cost-effective, and scalable.
One of the most challenging aspects of processing electronic parts is the abundance of influencing factors. These have a significant impact on the quality of the finished product. Particularly the affinity to embed moisture in the plastic matrix can lead to severe damage mechanisms. With a moisture gain of up to 0.5 percent by weight [1] delamination and the so-called “popcorn effect” may occur during reflow soldering [2]. This is due to the explosive vaporization and volume gain the embedded water experiences during the heating process. To prevent these failures the floor time has been established. It specifies the maximum amount of time a Surface Mount Device (SMD) can be exposed to certain conditions (floor life) before failures start to occur. After the floor life ends, time-consuming and costly preconditioning in a warm and dry atmosphere is necessary to restart the floor time. The Moisture Sensitivity Level (MSL) in combination with the JEDEC standard defines the exact conditions and times for preconditioning.The proposed article focuses on modeling cause-effect-relations between influence and target values to create a more efficient way to determine when preconditioning is necessary. A literature review of research papers and industry standards is used to determine the quality defining influence parameters. Transferring these insights into two simulation models, one based on the Fickian diffusion mechanism, the other on the two-stage Fickian diffusion, leads to a promising approach for concluding reversible and irreversible moisture diffusion into the SMD components.An experimental setup is determined using the derived dependence model. The experiments consist of defined loading and drying cycles of different samples of MSL 3 and 4 to quantify the moisture de- and absorbed by the part under defined conditions. Subsequently, several differently moisture-loaded parts are reflow soldered to recreate a variety of failure mechanisms. Which, in turn, are used to verify the critical moisture mass absorbed by the SMD-components
