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Surface microstructures of electroless nickel plating, showing characteristic nickel nodules, on silver sintered at 0.1, 10 and 50°C/s (resp. from left to right) via SEM observations.
Source publication
The nanoporous nature of the inkjet printed silver nanoparticles entail low hardness and surface effective contact area for being compatible with pads that are suitable for wire-bonding in electronic packaging. Electroless nickel plating is a selective metal deposition technique which can brings the required thickness and hardness for further pads...
Context in source publication
Context 1
... electroless nickel plating on porous printed silver pads, a high pH nickel bath is used at 65°C since acidic plating bath tends to chemically etch the silver pads. The growth of electroless nickel has been performed during 5 min in order to achieve a 1.7 μm-thick nickel layer. This thickness is sufficient to improve the wear mechanical resistance and the film hardness and tends to clog the film porosities (See Figure 4 and Figure 5). Young’s modulus of inkjet -printed silver nanoparticles has been determined by nanoindentation system in continuous stiffness measurement mode at 45 Hz using Berkovich tip. In order to take into account the high disparity of porosities on silver surface, 64 indentations has been performed on each sample and the resulting average is presented in Figure 6. The Bec model has been used to extract the film Young’s modulus from the reduced one corresponding to the film/substrate composite measured by the equipment ...
Citations
... The material characteristics such as temperature stability, solidity or wettability limits the possible process spectrum for the assembly. Especially conventional ultra-sonic (US) or thermo-sonic (TS) wire bonding, which are using ultrasonic forces combined with either high pressure or thermal-heat, cannot be applied directly for creating stable and reliable interconnections of ink and paste sintered contact structure on printed and injection molded devices due to the soft material characteristics [2]. ...
Traditional methods of wire bonding do not work well on liquid printed contact layers and soft substrate materials. Laser soldered wire bonding i.e. SB²-WB constitutes a novel approach to this problem as it works without the need for high pressures, temperatures or vibration [1]. In this work we qualify different conductive layers of printed, plated, and vapor coated materials containing Au, Cu and Ag with regard to their suitability for forming a stable and reliable interface with a SAC_305 laser jetted solder bump, which is the basis for the laser soldered wire bonding process (SB²-WB). As printing technologies, we have chosen stencil-and screen-printing, alternatively gold was deposited by sputtering and patterned by subsequent lithography and wet-chemical etching. As substrates, we selected Polyethylene-naphthalate (PET) Teonex Q51, Parylene and Glass. Au wire and Ag ribbon were used for forming the wire bonds. Investigations on printed layers have been focused on correlation between paste receipt, printing technology related layer thickness, paste-related post-treatment and compatibility to laser assisted solder ball bumping i.e., SB²-Jet. Using a shear-tester, the mechanical load-capacity of the formed solder joints was measured, and the corresponding fracture modes were inspected using an optical microscope. The characteristic of the interfacial layers, material bulk textures and other metallurgical properties was analyzed with SEM-FIB, EDX, X-ray and optical microscopy. Moreover, we demonstrated the forming of laser soldered wire bonds on two demonstrators showing different pad metallization, contact form and substrate bulk material. Finally, the future prospects of intended feasibility studies for laser soldered wire bonding, with structures consisting of paste and ink are highlighted.
... Besides, extra stiffness provided by a thicker metal layer could reduce strains arising on the electrodes plane due to swelling at high humidity levels. Since the printed silver layer is very thin and porous [31] , nonelectroplated electrodes would result especially vulnerable to all these effects. In any case, more studies are needed to totally understand these phenomena. ...
Functional humidity sensors have been fabricated on polymeric foil PET using all additive processes. The fabrication included inkjet printing and electrodeposition of interdigitated structures followed by the inkjet printing of a test humidity sensing layer. Quantitative and qualitative analysis of the sensor operation led to the understanding and subsequent improvement of the sensor performances by means of the differential measurements method. To the best of our knowledge, it is the first time that electrodeposition of a thicker metallic layer on inkjet printed patterns has shown advantages in the stability and sensitivity of a capacitive humidity sensor. The process exposed in this paper is proposed as a perfect candidate for the fabrication of sensors arrays or disposable sensing platforms where low-cost, light weight and mechanical flexibility are important issues.
Digitally printed conductive structures often need to be electrically connected to batteries, microcontrollers or other devices. A consensus of industry and research is that such hybrid printed electronics will play a major part in the future of printed electronics. To
promote the technology of hybrid printed electronics, surface mount technologies for electronic components using isotropic conductive adhesives (ICA) and low melting tin bismuth solders on inkjet-printed silver structures on injection molded Liquid Crystal Polymer (LCP) substrates were investigated in this publication. The special needs for
inkjet-printed electronics were considered as well as the reliability of assembled surface mounted devices (SMD) and their failure mechanisms. Connected 0603 and 1206 SMDcomponents achieved a characteristic fatigue life of more than 3500 cycles during thermal cycling at + 125 °C / - 40 °C and withstood over 1000 h under damp-heat atmosphere of + 85 °C / 85 % RH. The coefficient of thermal expansion of the substrate and the selection of solder have major impacts on the reliability of the assemblies.
Digital printing technologies are well suited for structured deposition of functional materials such as nanoparticular metal inks on different substrates. Inkjet printing as a maskless and direct writing technology is very suitable for manufacturing of low-cost devices and packages e.g. for highly integrated sensor systems. The availability of printable materials is ever-expanding and enables new capabilities in many applications. Currently, nanoparticular silver inks are very popular for ink-jet printing of conductive paths. However, at present the availability of reliable technologies for the assembly of electrical components is a challenge. In this paper soldering of chip resistors on inkjet-printed silver structures was investigated.
In this paper, we have investigated a laser-assisted fast nano-Ag sintering process for die attach in power electronic applications. The effects of laser power, irradiation time, and load on microstructure and shear performance of the bonded samples are presented. Moreover, samples sintered by a hotplate were also studied as a comparison. The results indicate that the die-attach process using the nano-Ag material can be realized within 1 min in the laser-assisted sintering method. In general, better shear strength was obtained with increasing laser power, irradiation time, and load. The shear strength of samples irradiated by 2-5 min of laser beam was comparable to that of the samples sintered on the hotplate for 80 min.
In this paper, a low temperature flip-chip integration technique for Si bare dies is demonstrated on flexible PET substrates with screen-printed circuits. The proposed technique is based on patterned blind vias in dry film photoresist (DP) filled with isotropic conductive adhesive (ICA). The DP material serves to define the vias, to confine the ICA paste (80 mu m-wide and potentially 25 mu m-wide vias), as an adhesion layer to improve the mechanical robustness of the assembly, and to protect additional circuitry on the substrate. The technique is demonstrated using gold-bumped daisy chain chips (DCCs), with electrical vias resistances in the order to hundreds of milliohms, and peel/shear adhesion strengths of 0.7 N mm(-1) and 3.2 MPa, respectively, (i.e. at 1.2 MPa of bonding pressure). Finally, the mechanical robustness to bending forces was optimized through flexural mechanics models by placing the neutral plane at the DCC/DP adhesive interface. The optimization was performed by reducing the Si thickness from 400 to 37 mu m, and resulted in highly robust integrated assemblies withstanding 10 000 cycles of dynamic bending at 40 mm of radius, with relative changes in vias resistance lower than 20%. In addition, the electrical vias resistance and adhesion strengths were compared to samples integrated with anisotropic conductive adhesives (ACAs). Besides the low temperature and high integration resolution, the proposed method is compatible with large area fabrication and multilayer architectures on foil.
In the last few years, inkjet printing of functional materials for electronics applications has been studied intensively, and significant progress has been made in developing ink materials and deposition techniques. Though sintering, the process of turning liquid nanometallic ink into its final and functional form, has also been studied, the traditional convection-oven-based heat treatment yet remains the common sintering method. High process temperature and long process time both limit the usability of inkjet printing as they restrict the selection of substrate materials and consume most of the overall fabrication time. Several sintering methods have been studied as alternatives for thermal sintering, but so far none of them have been widely adopted. We studied laser sintering of copper nanoparticle ink to gain an understanding of the process and the main variables affecting it. Laser sintering was also studied by thermal modeling, and the results are used to explain the behavior of process. Process parameters comprised substrate material and thickness, application of an insulator layer on top of the silicon wafer, laser scanning speed and optical power, and thickness of the printed structure. Comparison of modeling and test data shows that though the temperature of the printed structure can be used to plan the sintering process, it alone cannot explain the test-based conductivity results.
