Stencil Printing Method Research Articles

Towards Single-Polymer-Based Fully Printed Textile-Based Flexible Ag2O-Zn Battery for Wearable Electronics

Printed textile-based flexible batteries are gaining attention in several applications, but they are becoming more relevant to the health care industry in terms of realizing wearable and skin-conformable electronic devices. A flexible battery must ideally be deformable along multiple directions. In this work, with an aim to develop a fully printed omnidirectional deformable battery, we report the fabrication process of a novel single-polymer-based flexible non-rechargeable planar Ag2O-Zn battery on a textile substrate using the stencil printing method. Except for the electrolyte, all the components of the battery, including the current collectors, the anode, the cathode, and the separator membrane, are fabricated using a single polymer, namely styrene–ethylene–butylene–styrene (SEBS). To fabricate the SEBS separator, we introduce the solvent evaporation-induced phase separation (SEIPS) process. In the SEIPS method, toluene and dimethyl sulfoxide (DMSO) are selected as the solvent–nonsolvent pair. The SEBS: toluene: DMSO system with a wt% ratio of 6:85:9 showed improved performance regarding the OCV tests. A polyacrylic acid (PAA)-based alkaline polymer gel is used as an electrolyte. The demonstrated process is simple, and, with suitable modifications, it should find its use in the development of digitally printed alkaline batteries.

Microstructural and micromechanical characterization of sintered nano-copper bump for flip-chip heterogeneous integration

Copper nanoparticles (CuNPs) sintering for flip-chip interconnects is a promising solution for 3D and heterogeneous integration to overcome the limitation of solder materials. To this end, we perform the photolithographic stencil printing method to pattern CuNPs, and the form of flip-chip interconnects is completed after CuNPs sintering process. This paper aims to study the effect of sintering processing parameters (time, pressure, temperature) on the mechanical properties of CuNPs bumps when applying the novel method to approach the Cu interconnects. We fabricated seven groups of specimens of sintered CuNPs bumps, built with a diameter of 100 μm and sintered. The nanoindentation tests assessed the mechanical property to get Young's modulus and hardness. Results clarify that Young's modulus is strongly affected by pressure. An suggested combination of parameters (the 25 MPa and 260 °C for 15 min) give the highest modulus of 126 GPa and the hardness of 1.76 GPa. Moreover, the observations by scanning electron microscopy (SEM) reveal the microstructure and porosity evolution versus different processing parameters.

Fabrication of paper-based analytical devices by a laminating method with thermal ink ribbons, sticky notes, and office appliances.

A stencil printing method utilizing sticky notes, a thermal transfer ink ribbon, and office appliances for paper-based analytical device (PAD) fabrication was proposed. A sticky note was attached to a filter paper, and a mask pattern was cut using a cutting machine. A commercially available thermal ink ribbon was then placed over the mask and laminated. We have characterized the fabricated devices. This approach could be used for the fast and mass prototyping of PADs using simple office appliances with no need for a wax printer.

Stencil Printing of Liquid Metal upon Electrospun Nanofibers Enables High-Performance Flexible Electronics.

Flexible electronics as an emerging technology has demonstrated potential for applications in various fields. With the advent of the Internet of Things era, countless flexible electronic systems need to be developed and deployed. However, materials and fabrication technologies are the key factors restricting the development and commercialization of flexible electronics. Here we report a simple, fast, and green flexible electronics preparation technology. The stencil printing method is adopted to pattern liquid metal on the thermoplastic polyurethane membrane prepared by electrospinning. Besides, with layer-by-layer assembly, flexible circuits, resistors, capacitors, inductors, and their composite devices can be prepared parametrically. Furthermore, these devices have good stretchability, air permeability, and stability, while they are multilayered and reconfigurable. As proof, this strategy is used to fabricate flexible displays, flexible sensors, and flexible filters. Finally, flexible electronic devices are also recycled and reconfigured.

Printed Textile-Based Ag2O-Zn Battery for Body Conformal Wearable Sensors.

Wearable electronics are playing an important role in the health care industry. Wearable sensors are either directly attached to the body surface or embedded into worn garments. Textile-based batteries can help towards development of body conformal wearable sensors. In this letter, we demonstrate a 2D planar textile-based primary Ag2O–Zn battery fabricated using the stencil printing method. A synthetic polyester woven fabric is used as the textile substrate and polyethylene oxide material is used as the separator. The demonstrated battery achieves an areal capacity of 0.6 mAh/cm2 with an active electrode area of 0.5 cm × 1 cm.

Influences of low dose gamma radiation on hardness and microstructure properties of green solder joint

Microelectronic packaging is frequently used in almost every application involving radiation-related industry. In the present work, the effect of the low dose gamma radiation on hardness and microstructural properties of Sn3Ag0.5Cu solder was explored using triboindenter test and optical microscope. The solder paste was placed onto the printed circuit board utilizing stencil printing method and reflow soldering process. The solder samples were exposed to the gamma irradiation with various low doses (from 5 to 25 Gy). Prior to indentation test, the sample undergoes the standard metallographic procedure. The hardness properties were determined through indentation test while the microstructureof the sample was observed using an optical microscope. In this study, indentation technique was used to understand the evolution of hardness properties of solder sample subjected to gamma radiation. The results show that the hardness increased after exposure to gamma radiation. However, the hardness decreased as the increment of radiation dose. Exposure to the gamma irradiation resulted the microstructure instability of solder joint which increases the intermetallic compound layer and reductionin the properties of hardness.

Fabrication of Novel Printable Electrically Conductive Adhesives (ECAs) with Excellent Conductivity and Stability Enhanced by the Addition of Polyaniline Nanoparticles.

Electrically conductive adhesives (ECAs) are one of the low temperature bonding materials. It can be used to replace toxic Sn-Pb solder. The key issue for the application of ECAs is how to improve their electrical properties. In the present study, we develop an effective method to promote the electrical properties of ECAs by addition of polyaniline (PANI) nanoparticles. PANIs were synthesized via a facile one-step chemical oxidative polymerization method. After adding 0.5 wt% PANI nanoparticles, the conductivity of ECAs increased dramatically by an order of magnitude. The bulk resistivity of 8.8 × 10−5 Ω·cm is achieved for 65 wt% silver fillers with 0.5 wt% PANIs loaded ECAs. Besides, this improvement has no negative effect on the shear strength and the aging life of ECAs. Moreover, the use of PANIs not only lowers the percolation threshold of ECAs, but also reduces the cost and improves the bonding reliability. Finally, PANIs enhanced ECAs patterns were successfully printed by a stencil printing method, which proved their potential applications in replacing conventional solder pastes and printing functional circuits.

Effects of Solder Volume and Reflow Conditions on Self-Alignment Accuracy for Fan-Out Package Applications

The effects of solder volume and reaction time between molten solder and a metal pad at the peak temperature of reflow on the self-alignment effect have been investigated in flip chip bonding. A glass die with two different pad designs and a flame retardant-4 (FR-4) organic substrate were used. Sn-3.0Ag-0.5Cu and Sn-3.5Ag solders were formed on Cu-organic solderability preservation (Cu-OSP) and electroless nickel electroless palladium immersion gold (ENEPIG) pads on FR-4 substrates using the stencil printing method. To assess the effect of solder volume, the thickness and opening size of the stencil mask were controlled. Reflow experiments were performed at 250°C with wetting times of 40 s, 55 s, 65 s, and 75 s. After flip chip reflow soldering, the bonding areas were cross-sectioned to inspect the shape of the interconnected solder using scanning electron microscopy. The results revealed that using an insufficient solder volume on the pad was responsible for die shifts larger than 1 μm, while a sufficient solder volume on the pad and a stable solder joint shape could ensure misalignment less than 1 μm. The Sn-3.0Ag-0.5Cu solder showed a lower die shift value than the Sn-3.5Ag solder because the Sn-3.0Ag-0.5Cu solder has stronger surface tension than the Sn-3.5Ag solder. Using a longer wetting time between the solder and the pad at the peak temperature also improved the die shift value because the increased reaction time changed the interconnected solder shape between the die and substrate from concave to convex, moving the die to a more accurate position. Furthermore, the restoring forces on die self-alignment influenced the die shift value. A stronger solder surface tension and a larger volume of solder on the pad produced stronger restoring forces for die self-alignment, thereby improving the die shift value.

Experimental Demonstration of Printed Graphene Nano-flakes Enabled Flexible and Conformable Wideband Radar Absorbers.

In this work, we have designed, fabricated and experimentally characterized a printed graphene nano-flakes enabled flexible and conformable wideband radar absorber. The absorber covers both X (8–12 GHz) and Ku (12–18 GHz) bands and is printed on flexible substrate using graphene nano-flakes conductive ink through stencil printing method. The measured results show that an effective absorption (above 90%) bandwidth spans from 10.4 GHz to 19.7 GHz, namely a 62% fraction bandwidth, with only 2 mm thickness. The flexibility of the printed graphene nano-flakes enables the absorber conformably bending and attaching to a metal cylinder. The radar cross section (RCS) of the cylinder with and without absorber attachment has been compared and excellent absorption has been obtained. Only 3.6% bandwidth reduction has been observed comparing to that of un-bended absorber. This work has demonstrated unambiguously that printed graphene can provide flexible and conformable wideband radar absorption, which extends the graphene’s application to practical RCS reductions.

Stencil printing behavior of lead-free Sn-3Ag-0.5Cu solder paste for wafer level bumping for Sub-100 μm size solder bumps

Stencil printing for flip chip packaging using fine particle solder pastes is a low cost assembly solution with high throughput for fine pitch solder joint interconnects. The manufacturing challenges associated with both solder paste printing increases as electronic device size decreases due to trend of miniaturization in electronic components. Among multiple parameters, the two most important stencil printing parameters are squeegee pressure and printing speed. In this paper, the printing behavior of Pb free Sn-3Ag-0.5Cu solder paste with a particle size distribution of 2–12 μm for wafer level bumping using a stencil printing method (stencil opening dimension −30 μm) was evaluated by varying the printing speed and squeegee pressure to fabricate solder bumps with a sub 100 μm size. The optimal squeegee pressure and print speed for the defect free printing behavior and fairly uniform size distribution of reflowed paste were found to be 7 kgf and 20 mm/s, respectively. The average size of the reflowed printed paste decreased with the increasing squeegee pressure.

A FCOB packaged thermal wind sensor with compensation

A two dimensional wind sensor was designed, fabricated and packaged on ceramic substrate instead of silicon substrate. The Ti/Pt heater and thermistors were fabricated using single lift-off process. The gold bumps were then sputtered and patterned on the chip using lift-off process again. Correspondingly, the Pb/Sn bumps were fabricated on the FR4 substrate using stencil printing method after metallization. The sensor chip was flip-chip packaged on the FR4 substrate, and the gap was filled with epoxy-based underfill to improve the structure strength. The packaged sensor was tested in wind tunnel in constant power mode. The wind velocity and direction offsets of the sensor were compensated using software and hardware calibration. Both the simulation and test results show that the thermal wind sensor can measure wind speeds up to 8 m/s with an accuracy of 0.3 m/s, and wind direction in a full range of 360° with a resolution within 5°.

Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn–37Pb and Sn–3.0Ag–0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni 3Sn 4 intermetallic compound (IMC) layer formed at the Sn–37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn–3.0Ag–0.5Cu solder, (Cu,Ni) 6Sn 5 and (Ni,Cu) 3Sn 4 IMCs were formed at the interface. The shear force of the Pb-free Sn–3.0Ag–0.5Cu flip chip solder bump was higher than that of the conventional Sn–37Pb flip chip solder bump.

Intermetallic Formation between Sn-Ag(-Cu) Solder Bumps and Au/Ni/Ti UBM and It’s Effects on the Shear Force of the Solder Bumps

The shear force of Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder bumps formed by the stencil printing method was measured and the effects of intermetallic formation on the shear force of solder bumps were investigated. The Sn-Ag(-Cu) solder paste was printed on the Au/Ni/Ti under bump metallurgy (UBM) and then reflowed repeatedly. The shear force of the solder bumps was measured as a function of the reflow times. The intermetallic formation in the Sn-Ag(-Cu) solder/UBM was characterized using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray diffractormeter. Ni3Sn4 phase was formed in the Sn-3.5Ag solder/UBM interface and (Cu,Ni)6Sn5 phase formed at the Sn-3.8Sg-0.7Cu solder/UBM interface. The shear force of solder bumps was sensitive to the depletion of Ni layer and the intermetallic thickness at the solder/Ni interface. The shear forces of Sn-3.5Ag solder bumps decreased rapidly after the fifth reflow due to the depletion of Ni layer in the UBM. The shear forces of Sn-3.8Ag-0.7Cu solder bumps decreased after the tenth reflow due to extensive growth of intermetallic layer in the solder/Ni interface.

Flip Chip Bump Formation of Sn–1.8Bi–0.8Cu–0.6In Solder by Stencil Printing

Flip chip bumping by stencil printing method using a new composition of solder paste, Sn–1.8%Bi–0.8%Cu–0.6%In, all in mass%, was investigated. Sn–3.5%Ag, Sn–37%Pb and Sn–36%Pb–2%Ag were selected as references for the experiment. The solder pastes were printed on the under bump metallization (UBM) of a Si-wafer using a stencil, where diameter and thickness of the stencil opening were 400 and 150 mm, respectively. The UBM deposit comprised 0.4 mm each of Al, Ni and Cu, and 20 nm of Au from bottom to top of the metallization, sequentially. The printed paste bumps were reflow soldered in air, and the peak soldering temperature of Sn–1.7Bi–0.8Cu–0.6In and Sn–3.5Ag was 523 K and of Sn–37Pb and Sn–36Pb–2Ag was 503 K. From the experimental results the solder bumps of Sn–1.8Bi–0.8Cu–0.6In alloys were well-formed with a mean height of 260 mm. The shear strength of Sn–1.8Bi–0.8Cu–0.6In at 523 K (as-reflowed) showed the highest value of 6.5N followed by those of Sn–3.5Ag, Sn–37Pb and Sn–36Pb–2Ag solders. After 1000 h aging, while the shear strength of the Sn–1.8Bi–0.8Cu–0.6In showed 27% decrease compared to as-reflowed conditions, it was still 15–30% higher than those of Sn–37Pb, Sn–36Pb–2Ag and Sn–3.5Ag solders. Intermetallic compounds (IMCs) formed on the interface between solder and UBM were (Cu,Ni)6Sn5. As aging time went on up to 1000 h, the content of Ni in the IMC changed from 6.6% at initial stage (as-reflowed) to 13.5% at final stage (1000 h aging).

Microstructure and Mechanical Properties of Sn–0.7Cu Flip Chip Solder Bumps Using Stencil Printing Method

The interfacial microstructure of Sn–0.7Cu solder with ENIG (electroless Ni/immersion Au) was studied using scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and transmission electron microscopy (TEM). (Cu,Ni)6Sn5 intermetallic compound (IMC) layer was formed at the interface between the solder and Ni–P under bump metallurgy (UBM) upon reflow. The thickness of the (Cu,Ni)6Sn5 IMC layer increased with isothermal aging time. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from TEM observation. Analytical studies using energy dispersive spectrometer (EDS) equipped in TEM revealed that the composition of the P-rich layer is close to that of a mixture of Ni3P and Ni, while that of the Ni–Sn–P layer is analogous to the P-rich layer but containing a small amount of Sn in it. The shear force of the joints decreased during isothermal aging. The decrease of the shear force should be mainly due to the microstructural degradation within the solder.

Long time reliability study of soldered flip chips on flexible substrates

Due to the requirements of new light, mobile, small and multifunctional electronic products the density of electronic packages continues to increase. Especially in medical electronics like pace makers the minimisation of the whole product size is an important factor. So flip chip technology becomes more and more attractive to reduce the height of an electronic package. At the same time the use of flexible and foldable substrates offers the possibility to create complex electronic devices with a very high density. In terms of human health the reliability of electronic products in medical applications has top priority. In this work flip chip interconnections to a flexible substrate are studied with regard to long time reliability. Test chips and substrates have been designed to give the possibility for electrical measurements. Solder was applied using conventional stencil printing method. The flip chip contacts on flexible substrates were created in a reflow process and underfilled subsequently. The assemblies have been tested according to JEDEC level 3. The focus in this paper is the long time reliability up to 10,000 h in thermal ageing at 125 °C and temperature/humidity testing at 85 °C/85% relative humidity as well as thermal cycling (0 °C/+100 °C) up to 5000 cycles. Daisy chain and four point Kelvin resistances have been measured to characterise the interconnections and monitor degradation effects. The failures have been analysed in terms of metallurgical investigations of formation and growing of intermetallic phases between underbump metallisation, solder bumps and conductor lines. CSAM was used to detect delaminations at the interfaces underfiller/chip and underfiller/substrate respectively.

Under bump metallurgy study for Pb-free bumping

The demand for Pb-free and high-density interconnection technology is rapidly growing. The electroplating-bumping method is a good approach to meet fine-pitch requirements, especially for high-volume production, because the volume change of patterned-solder bumps during reflow is not so large compared with the stencil-printing method. This paper proposes a Sn/3.5Ag Pb-free electroplating-bumping process for high-density Pb-free interconnects. It was found that a plated Sn/Ag bump becomes Sn/Ag/Cu by reflowing when Cu containing under bump metallurgy (UBM) is used. Another important issue for future flip-chip interconnects is to optimize the UBM system for high-density and Pb-free solder bumps. In this work, four UBM systems, sputtered TiW 0.2 µm/Cu 0.3 µm/electroplated Cu 5 µm, sputtered Cr 0.15 µm/Cr-Cu 0.3 µ/Cu 0.8 µm, sputtered NiV 0.2 µm/Cu 0.8 µm, and sputtered TiW 0.2 µ/NiV 0.8 µm, were investigated for interfacial reaction with electroplated Pb/63Sn and Sn/3.5Ag solder bumps. Both Cu-Sn and Ni-Sn intermetallic compound (IMC) growth were observed to spall-off from the UBM/solder interface when the solder-wettable layer is consumed during a liquid-state "reflow" process. This IMC-spalling mechanism differed depending on the barrier layer material.

Wafer level and flip chip design through solder prediction models and validation

In flip chip package applications, bumped dies are flip-chip assembled to substrate metal pads creating joints that serve electrically and mechanically. Resulting solder joint profiles are defined by the solder bump volume, the under bump metallurgy (LTBM) area, and the substrate metal pad size and shape. Solder bump height and diameter was predicted by the geometrical truncated sphere model and surface evolver model at the wafer level, using the known solder volume deposited by stencil printing method. The surface evolver model was used to predict the assembled solder joint height, gap height, collapse height, and maximum bump diameter of flip chip assemblies. In turn, substrate pads were fine-tuned to achieve required gap heights. Collapse heights provided the means to develop assembly tolerances and relative risk of bridging was determined from knowledge of resulting bump diameters. Through validated design of the stencil printing technology and prediction of realistic bump and assembly solder geometries, the results are improved processes and die level design and assembly. Optimized design parameters are incorporated and accurately represented in simulation and experimentally validated with assemblies.

Taguchi design of experiment for wafer bumping by stencil printing

Taguchi experiments are designed and carried out with five critical factors that influence the solder bumping of a 200 mm (8 in) wafer by the stencil printing method. These factors are: paste types, squeeze forces, snap-off heights, aperture shapes, and aspect ratios. They are varied to form a two-level L8 orthogonal array experiment. Analysis of mean (ANOM) and analysis of variance (ANOVA) are used to choose the most influential factors. After fixing the most influential factors, a two-level L4 orthogonal array experiment is followed to optimize the remaining material and process parameters. Important results are summarized in this paper which could be very useful for wafer bumping with the stencil printing method.