Electroless Nickel Immersion Gold Research Articles

Pixel detector hybridisation and integration with anisotropic conductive adhesives

A reliable and cost-effective interconnect technology is required for the development of hybrid pixel detectors. The interconnect technology needs to be adapted for the pitch and die sizes of the respective applications. This contribution presents recent results of a newly developed in-house single-die interconnection process based on Anisotropic Conductive Adhesives (ACA). The ACA interconnect technology replaces solder bumps with conductive micro-particles embedded in an epoxy layer applied as either film or paste. The electro-mechanical connection between the sensor and ASIC is achieved via thermocompression of the ACA using a flip-chip device bonder. The ACA technology can also be used for ASIC-PCB/FPC integration, replacing wire bonding or large-pitch solder bumping techniques. A specific pixel-pad topology is required to enable the connection via micro-particles and create cavities into which excess epoxy can flow. This pixel-pad topology is achieved with an in-house Electroless Nickel Immersion Gold (ENIG) process. The ENIG and ACA processes are qualified with a variety of different ASICs, sensors, and dedicated interconnect test structures, with pad diameters ranging from 12 μm to 140 μm and pitches between 20 μm and 1.3 mm. The produced assemblies are characterized electrically, with radioactive-source exposures, and in tests with high-momentum particle beams. This contribution introduces the developed interconnect and plating processes and showcases different hybrid assemblies produced and tested with the above-mentioned methods. A focus is placed on recent optimization of the plating and interconnect processes, resulting in an improved plating uniformity and interconnect yield.

Investigation of isothermal aged Sn-3Ag-0.5Cu/Sn58Bi-Co hybrid solder joints on ENIG and ENEPIG substrate with various mechanical performances

The stability of solder joints is of utmost critical in electronic equipment. The integration of packaged devices in these systems often leads to the generation of heat, which might have a significant impact on how long the equipment lasts when exposed to high temperatures. It is imperative to perform an isothermal aging test on the solder joints in order to replicate the extended use of these joints. This research looked into how the mechanical characteristics and IMC growth of hybrid solder joints were affected by isothermal aging over 240 h, 480 h and 720 h. The reliability of solder joint under isothermal aging condition was improved by adding Pd coating and Co element. The results revealed that Pd coating over ENIG (Electroless Nickel Immersion Gold) pad reduced the growth rate of IMC in SAC305/SnBi hybrid solder joints during isothermal aging. ENIG solder joint strength dropped by 57.66% after 720 h of isothermal aging, and practically all fractures were IMC fractures. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) shear strength lowered 31.01%, IMC fracture mode 20%, strong reliability at high temperature. IMC development was suppressed in SAC305/SnBi-Co hybrid solder joints by higher Co content. After 720 h isothermal aging, SAC305/SnBi-Co solder joints had better shear strength than non-Co joints. According to fracture morphology, SAC305/SnBi-Co hybrid solder joints reliability improves owing to the change of IMC morphology and grain refining.

Investigating the Impact of Final Finishes on the Insertion Loss in As Received and After Aging

The task of the final finish on the printed circuit board is to protect the copper surface and at the same time keep it active for the subsequent assembly steps. In high frequency applications, additionally the impact on the signal loss becomes of interest. Typically, the choice of the final finish is defined by the target application. The demands on solderability and bondability need to be considered and the acceptable price level needs to be defined. For the use in high frequency applications the signal integrity becomes of interest and adds on the reliability criteria such as solder joint reliability or corrosion resistance. In this study various surface finishes were tested towards their performance on insertion loss at high frequency. In contrast to earlier studies the frequency range was investigated up to 150 GHz and the impact of thermal or humid aging of the final finish was evaluated. Overall, seven different finishes have been selected including EPAG (Electroless palladium/Autocatalytic gold), I-Sn (Immersion tin), I-Ag (immersion silver), DIG (direct immersion gold), Au/Pd/Au (Gold/Palladium/Gold), ENIG (Electroless Nickel/Immersion Gold) with high and medium phosphorous Nickel and ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold). The paper compiles the data for insertion loss versus frequency for the different final finishes at layer thicknesses as commonly used in the PCB industry. The results in the insertion loss performance are correlated to other properties of the different final finishes such as reliability and assembly performance to identify the best candidates for high frequency applications.

Improvement of Solder Joint Shear Strength under Formic Acid Atmosphere at A Low Temperature.

With the continuous reduction of chip size, fluxless soldering has brought attention to high-density, three-dimensional packaging. Although fluxless soldering technology with formic acid (FA) atmosphere has been presented, few studies have examined the effect of the Pt catalytic, preheating time, and soldering pad on FA soldering for the Sn-58Bi solder. The results have shown that the Pt catalytic can promote oxidation-reduction and the formation of a large pore in the Sn-58Bi/Cu solder joint, which causes a decrease in shear strength. ENIG (electroless nickel immersion gold) improves soldering strength. The shear strength of Sn-58Bi/ENIG increases under the Pt catalytic FA atmosphere process due to the isolation of the Au layer on ENIG. The Au layer protects metal from corrosion and provides a good contact surface for the Sn-58Bi solder. The shear strength of the Sn-58Bi/ENIG joints under a Pt catalytic atmosphere improved by 44.7% compared to using a Cu pad. These findings reveal the improvement of the shear strength of solder joints bonded at low temperatures under the FA atmosphere.

An electroplated Ag/AgCl quasi-reference electrode based on CMOS top-metal for electrochemical sensing

The integration and mass-production of reference electrodes for CMOS-based electrochemical sensing systems pose a challenge for the accessibility and commercial-viability of such devices. In this paper, a method of electroplating an Ag/AgCl quasi-reference electrode using CMOS top-metal as a base is presented for the first time. The aluminium bond-pad of a CMOS microchip was zincated, an electroless nickel immersion gold layer applied, and a thick silver layer electroplated and chemically chlorinated. The resulting reference electrode was able to provide a stable potential with a drift rate of 0.3mV/h for up to 18h. This validates the approach of a fully electroplated bond-pad reference electrode, which offers simplified post-processing and greater scalability of production. Further work towards an entirely electroless process is envisaged.

Preparation, characterization and mechanical properties analysis of SAC305-SnBi-Co hybrid solder joints for package-on-package technology

The development of hybrid solder for chip packing is a major 3D packaging research topic. The incorporation of with low melting points into solder materials has been used in low-temperature packaging. Hybrid solder struggles to enhance low-temperature solder efficiency and mechanical properties. This work examined the effects of Co elements on microstructure and intermetallic compound (IMC) morphology of SAC305-SnBi-Co hybrid solder generated by reflow welding on electroless nickel immersion gold (ENIG) substrate at various temperatures. Moreover, mechanical characteristics and failure behavior of joints were extensively explored. The results show that the addition of 0.05% Co reduced the melting range of the solder and improved the wettability of SnBi solder. According to transmission electron microscopes (TEM) and electron diffraction spots (EDS), the crystal structure of the grains is (Cu, Co)6Sn5 basic-centered monoclinic crystal. Fourier analysis of high-resolution images yields deformed lattice fringes, indicating that Co atoms fill Cu6Sn5's lattice gap in IMC. The shear strength of SAC305/SnBi hybrid solder joints with Co rises and then decreases with temperature. The highest shear strength of solder joints with 0.05% Co (40 nm) is 73.2 MPa at 220 °C, whereas joints with 0.1%Co are 67.5 MPa. Combined with the fracture morphology, the proportion of IMC fracture mode decreased after adding Co element.

(General Student Poster Award Winner, 1st Place) Accelerated Estimation of Chemical and Sensory Liquid Attributes Using an AI-Assisted Electrochemical Electronic Tongue

Electronic tongues based on arrays of potentiometric sensors bears promises for enabling portable and fast analysis of complex liquids by leveraging low-selectivity and high-sensitivity of the inherent sensing materials. Thus, they have been studied and developed to target multiple applications, including analysis of chemical compositions or for untargeted quality control in the food industry. A key element to enable practical use of electronic tongues, however, has often been the analysis and interpretation of their combinatorial response due to the cross-sensitivity of the employed sensors. Indeed, disentangling the effect of multiple liquid constituents on the potentiometric response is the main associated challenge. The present contribution aims at studying the feasibility and accuracy of employing this class of sensors to both the quantification of multiple analytes in water as well as prediction of sensory attributes in coffee samples. A proof-of-concept system comprises an integrated array of miniaturized polymeric sensors and an electronic board records 15 differential voltages, produced by the interaction with both ionic and organic constituents of liquids, and can transmit them via Bluetooth to an external device. An automated pipeline performs pre-processing, hand-crafted feature extraction and training of machine learning models that can be then deployed on the cloud or edge device (e.g., smartphone). The same array can be then reconfigured to address a new specific task by leveraging new potentiometric data collection and applying the automated data analysis. In particular, the device presented herein employed 16 polymeric sensors that were electrodeposited on conventional electroless nickel immersion gold (ENIG) electrodes. The conductive polymers (PEDOT, PPy, PANI and PAPBA) were synthesized by chronoamperometry or cyclic voltammetry and enriched with doping agents for enhanced sensitivity. Differential voltages between these polymeric sensors were measured during the transition of the sensor array from a reference solution to a test solution, thereby obviating the need for a conventional reference electrode. Indeed, the use of low-selective sensors for potentiometric measurements does not require integration of reference electrodes, which are known to be unpractical for remote sensing applications. The analysis of the potentiometric response revealed deviations from linear sensitivities, showing non-Nernstian voltage trends when varying concentration of target analytes. Training data were obtained by alternatively immersing the sensor array in reference (120 s) and test (60 s) solutions continuously using an automated test rig. In addition to the conventional approach of measuring the equilibrium potential after a certain settling time, the complete evolution of the potentiometric signal in time could provide richer information that could be useful for building calibration models. Hence, hand-crafted features were extracted from time series recorded during sensor training in order to reduce the dimension of data and describe the complete voltage perturbation of the sensor during transition between reference and test solution. If exposed to enough “known liquid examples”, the sensor array can learn patterns to perform accelerated qualitative and quantitative analysis. The device has demonstrated enhanced discrimination of various mixtures and was proved able to predict beverage sensory properties after intensive training with samples with known sensory attributes. Firstly, the sensitivity to multiple ionic species was estimated in model mixtures obtained following an Orthogonal Experimental Design (OED) for a set of four metal ions, including Al3+, Cu2+, Na+ and Fe3+, and another set comprising Ca2+, Mg2+ and Na+. The former experiment demonstrated that non-linear regression models, such as a Random Forest of Extra Trees, can cope with non-linearity of the sensor response and yielded better performances than widely used multivariate regression model i.e., Multiple Linear Regression (MLR). The mean relative quantification error varied between 1% for Fe3+ and 44% for Na+ in the concentration range 1-10 mg/L. The second mixture set was used instead to unveil the cross-sensitive response of the sensor array, build a quantification model to predict major cations in mineral water and proving the importance of feature selection for data-driven sensors. Overall, the sensor showed comparable accuracy to ICP-MS (MRE 12-24%) but with a drastic reduction of cost and processing time. Finally, the same sensor array configuration was also used to test 12 coffee samples, which were also evaluated by a sensory panel through 5 taste attributes. Regression models were built to map instrument response to sensory data and Extra-Tree algorithm yielded the highest performances with a MRE varying from 8% to 21% depending on coffee sample and validation scheme. Thus, it was demonstrated that data-driven sensor arrays could be also valuable alternative tools for accelerated evaluation of beverage sensory characteristics and support food innovation processes. Figure 1

Leveraging an AI-Assisted Electrochemical Sensor Array to Visualize Digital Fingerprints of Complex Liquids

Electrochemical sensor arrays have shown potential for fast and untargeted chemical analysis of multi-component media, enabling simultaneous quantification of multiple analytes and estimation of liquid properties that could correlate with human sensory perception. In particular, potentiometric electronic tongues (e-tongues) have been explored as promising alternative tools for chemical analysis in various applications, ranging from traceability of goods to characterization of food products for quality control and innovation processes. The reduced complexity of non-selective sensor fabrication and the ease of potentiometric transduction make these devices suitable for portable and decentralized chemical analysis. However, the interpretation of the sensor array response has always been a challenge due to the inherent cross-sensitivity of the sensing materials and the corresponding combinatorial signals arising upon sensor interaction with a liquid. Machine learning can help recognizing and mapping signal patterns and various data processing, unsupervised and supervised techniques have been proposed to calibrate e-tongue devices. These methods usually require exposing the sensor array to a set of liquids with known properties that serve as training base to build a calibration model. Therefore, the quality and quantity of tests is crucial to boost sensor performances. Nevertheless, performing an extensive number of measurements could be extremely time-consuming and results to be tedious to achieve in practice for certain use-cases. In this context, advances in deep learning and machine learning models have shown potential to accelerate chemical and materials discovery when combined with high-throughput experimentation, highlighting the benefits of AI-assisted research practices. Moreover, the recent advent of multi-domain and multi-task models trained by self-supervision, so-called foundation models, bears also promises for extending learnt representations across multiple fields, thus counteracting the reduced data availability in certain applications, and benefiting from information exchange across domains. Thus, in the present contribution we propose extending this approach to data-driven chemical sensors. More specifically, we leverage transfer learning based on fingerprints pretrained in other domains to model new instrument/sensor data representations. Herein, we demonstrate how the output of a model system comprising an integrated electrochemical sensor array for analysis of multi-component liquids can be encoded as image representations to leverage existing deep learning computer vision models pretrained on large collections of image data. The models effectively extract features from these representations and feed specific model heads to perform downstream tasks. Firstly, an integrated sensor array comprising 16 polymeric sensors was fabricated through electrodeposition on conventional electroless nickel immersion gold (ENIG) electrodes. The conductive polymers (PEDOT, PPy, PANI and PAPBA) were synthesized by chronoamperometry or cyclic voltammetry in a three-electrode configuration and were enriched with doping agents for enhanced sensitivity. 15 linearly independent differential voltages between these polymeric sensors were measured during the transition of the sensor array from a reference solution to a test solution, thereby obviating the need for a conventional reference electrode. Indeed, the use of low-selective polymeric sensors for potentiometric measurements does not necessarily require integration of reference electrodes, which are known to be unpractical for remote sensing applications. Training data were obtained by alternatively immersing the sensor array in reference (120 s) and test (60 s) solutions continuously using an automated test rig. The 15 raw time-series data from the sensor array were processed and concatenated to yield a spectral response, which was smoothed by means of the moving average technique and standardized using Standard Normal Variate (SNV). The obtained spectra were encoded into image representations using the Gramian Angular Summation Field transformation. Off-the-shelf features were generated leveraging pretrained neural networks developed to classify natural images and applied to the “sensor images”. Dimensionality reduction through Principal Component Analysis (PCA) yielded a set of features that could then be used to train machine learning classifiers and regressors. The pipeline was applied to generate visual fingerprints of multiple beverages, proving full discrimination of liquid types (mineral waters, coffees, fruit juices, soft beverages and wines). On a model dataset comprising 11 Italian red wines, it was demonstrated that image fingerprints of samples enabled class identification with a mean accuracy ~95%. The results demonstrate the successful creation of a new representation of the chemical sensing space which achieves comparable performance as domain-specific hand-crafted feature selection. The present contribution represents an example of integration of data processing techniques and publicly available libraries/models to support transfer of methodologies across domains. We believe this approach could be disruptive in the field of electrochemical sensor arrays, especially for processing e-tongues and e-noses response and enhancing capabilities of data-driven chemical sensors. Figure 1

Electrochemical detection of myeloperoxidase (MPO) in blood plasma with surface-modified electroless nickel immersion gold (ENIG) printed circuit board (PCB) electrodes

Printed circuit board (PCB) based biosensors have often utilized hard gold electroplating, that nullifies the cost advantages of this technology as compared to screen printed electrodes. Electroless nickel immersion gold (ENIG) is a popular gold deposition process widely used in PCB manufacturing, but vulnerable to pinhole defects and large surface roughness, which compromises biosensor performance. In this work, we present a method to address these challenges through electrodeposition of methylene blue (MB) to cover surface defects and improve electroactivity of ENIG PCB electrodes. We also demonstrate a process to realize in situ synthesis of gold nanoparticles (AuNPs) using acid-functionalized multi-walled carbon nanotubes (MWCNTs) as scaffold, that are used to immobilize antibody for the target molecule (myeloperoxidase: MPO, early warning biomarker for cardiovascular diseases) through a modified cysteamine/gluteraldehyde based process. The processing steps on the electrode surface are developed in a manner that do not compromise the integrity of the electrode, resulting in repeatable and reliable performance of the sensors. Further, we demonstrate a cost-effective microfluidic packaging process to integrate a capillary pump driven microfluidic channel on the PCB electrode for seamless introduction of samples for testing. We demonstrate the ability of the sensor to distinguish clinically abnormal concentrations of MPO from normal concentrations through extensive characterization using spiked serum and blood plasma samples, with a limit of detection of 15.79 ng/mL.

How to Tailor Immersion Tin Plating for IC Substrate Applications

Immersion tin finishes are widely used in the printed circuit board (PCB) industry. They provide reliable connections with high solder joint reliability compared to other final finishes. Due to the direct intermetallic formation between copper and solder, the intermetallic compounds (IMC) formed during the assembly exhibit ductile properties and are less prone to brittle fractures than e.g. solder joints formed with nickel finishes such as electroless nickel/immersion gold (ENIG). With the given high solder joint reliability, the immersion tin finish therefore is one of the first choices for the application in the IC substrate industry. It provides already a robust and nickel free finish with high process efficiency for lower cost. But due to the trends to increased I/O counts, reduced lines and spaces, improved signal integrity and further lowering the cost in the packaging applications, the adjustment of the immersion tin process became necessary to cope with the changed technology demands. Topic of this paper is the introduction of an immersion tin plating process specifically designed for IC substrate applications. It is focusing on three major aspects: the plating process itself, the equipment solutions to realize best quality control and process monitoring and finally the performance of the final finish itself in comparison to other common end finishes. The trend to more and more reduced lines and spaces requires the adjustment of the plating chemistry. A good solution flow needs to be ensured to prevent excessive attack of the copper tracks or disproportionate solder mask undercut. The study presented here compares conventional immersion tin plating solutions with a modified process for IC substrate plating. It shows, how the variation of solution viscosity and flow can impact the copper attack and solder mask undercut. Due to the better solution exchange and adjusted plating conditions, the copper dissolution can be significantly reduced and the attack to the solder mask can be minimized. The complimentary horizontal plating equipment supports the specific properties of the plating electrolyte. Insights to the equipment design and adjustments are being discussed to identify the best conditions for the immersion tin plating on fine structures with highest quality demands. Finally, the tin deposit is investigated and compared to conventional immersion tin as well as to other standard finishes such as ENIG, ENEPIG (Electroless Nickel, electroless Palladium, Immersion gold) or OSP (organic solder preservative) regarding its solder wetting performance and solder joint reliability. It can be concluded that the immersion tin finish designed for small L/S applications can validate the benefits known from the long years’ experience with immersion tin. The layer shows comparable solder wetting properties to conventional immersion tin finishes and excellent solder joint reliability when compared to ENIG or OSP with better performance even after thermal aging. The risk for typical IC substrate related defects such as excessive copper corrosion or solder mask undercut as well as missing bumps is lowered compared to the conventional process. The main focus of this paper is to show, that the high demands in substrate industry can be tackled when the plating solution and plating equipment are designed and adjusted to the new requirements in a coordinated approach so that they can complement each other.

Solder Joint Degradation and Electrochemical Metallic Ion Migration Property of Solder Joints with Surface Finish

To confirm the degradation property of solder joints by substrate surface finish, chip resistors are reflow soldered to organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG) finish substrate in a nitrogen atmosphere using the type 6 Sn-3.0Ag-0.5Cu (SAC305) solder paste. To confirm the electrochemical metallic ion migration (ECM) sensitivity with and without photo solder resist (PSR) ink-coated printed circuit board (PCB), comb patterned test vehicles were prepared by the hot air reflow soldering process in nitrogen atmosphere, and the SAC305 solder paste was printed on the conductor of the ECM test PCB. The ECM test was conducted under high temperature and high humidity conditions of 85℃ and 85% RH, respectively. As a result of measuring the void content of the 2012 chip resistor, the average void content of the OSP substrate was higher than that of the ENIG substrate. The thermal shock test (TST) indicated that the degradation rate of the OSP substrate was higher than that of the ENIG substrate. Microstructure analysis showed that the OSP substrate had more crack propagation and thicker intermetallic compound (IMC) thickness than the ENIG substrate. The ECM test revealed that the substrate with PSR-coated PCB coupon generated more dendrites than that without PSR-coated coupon. Because the PSR coated substrate decreased the moisture absorption into the substrate, moredendritesweredeterminedtobe generated.

Fatigue performance and microstructure of lead-free solder joints in BGA assembly at room temperature

One of the crucial factors in evaluating the reliability of an electronic appliance is fatigue failure of the interconnecting solder joints. In most situations, large bulk samples are used to investigate the fatigue characteristics of the solder materials. Only a few studies have examined the solder joints frequently observed in ball grid array (BGA) components. This study implemented a specialized test vehicle in BGA assembly connecting two substrates with nine identical solder joints. Four SAC-based solder alloys were tested using an Instron micromechanical tester in the stress-controlled and strain-controlled methods at room temperature and constant strain rate. At two distinctive stress and strain levels, the fatigue performance of alloys featuring organic solderability preservative (OSP) and electroless nickel-immersion gold (ENIG) surface finishes was investigated. The fatigue results obtained from the stress-controlled and strain-controlled tests were compared. Microstructural analysis was performed using scanning Electron Microscopy (SEM) and energy dispersive spectroscopy (EDS) to identify the precipitates in the bulk solder and the morphologies of IMC layers. The thickness of intermetallic compound (IMC) layers and the failure mode were examined in each case. The results demonstrated that the OSP surface finish significantly outperformed the ENIG surface finish regardless of the testing process and solder alloy. Various failure modes were observed, depending on the solder alloy and IMC layer.

Effects of Multiple Reflow on the Formation of Primary Crystals in Sn-3.5Ag and Solder Joint Strength: Experimental and Finite Element Analysis.

The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.

Cu aggregation behavior on interfacial reaction of Sn-3.0Ag-0.5Cu/ENIG solder joints

Cu aggregation of Sn-3.0Ag-0.5Cu /electroless nickel-immersion gold (ENIG) solder joints generally affect the formation of intermetallic compounds (IMCs) at the interfacial region. Herein, the interfacial microstructure of Sn-3.0Ag-0.5Cu/ENIG solder joint is compared with the joints obtained with Cu-free solder or substrate to reveal the Cu contributor and its metallurgical behavior. Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) were used to observe the IMCs and elemental distribution, respectively. The Gibbs free energy of the Sn-Ni-Cu system was calculated by JMatPro software. The results showed that Cu had significant aggregation in the interface. The solder is the major contributor to Cu aggregation in the interface, while the ENIG substrate also makes a minor contribution. The Cu6Sn5 is then formed at the interface because of its lowest Gibbs free energy in the system. These results are of significance for subsequently controlling the interfacial microstructure and performance of joints.

Thermal reliability of Cu sintering joints for high-temperature die attach

In this study, the thermal reliabilities of sintered Cu joints were investigated under aging and in thermal shock environments. Sintered Cu joints were maintained for 2000 h and 2000 cycles in high-temperature (200 °C) and thermal shock (from −45 to 175 °C) environments, respectively. Subsequently, the microstructural changes, Cu-oxide formation, and bonding strengths of the joints were evaluated. The initial shear strengths of the chips with bare copper and ENIG surfaces were 40.84 and 36.44 MPa, respectively, and after a 2000-h aging test, the strengths decreased to as 39.84 and 18.58 MPa, respectively. With an increase in the aging time under an aging environment of 200 °C, the internal porosity decreased owing to the formation of Cu2O and additional diffusion between the particles. Therefore, the shear strength of the chips with bare copper surfaces was maintained throughout the aging tests, whereas that of the electroless nickel immersion gold chip decreased owing to the formation of Kirkendall voids at the interface, which were attributed to the different diffusion rates of Cu and Au. In addition, Au3Cu, AuCu, and AuCu3 interfacial intermetallic compounds were formed between the Au and Cu layers after the aging test was performed at 200 °C for 2000 h. Copper oxide formed during the thermal shock test caused delamination at the interface because the coefficient of thermal expansion mismatched with that of pure Cu. Understanding this intrinsic Cu oxidation mechanism will widen the prospects for the application of Cu sintering in various devices.

Role of Cu percentage in the Sn-xCu alloy on the mechanical performance at Sn-xCu/ENIG interface produced by laser soldering

PurposePb-free solders have been developed to replace the standard Sn–Pb eutectic solder since the prohibition on Pb used in solders. The Sn–Ag–Cu series of lead-free solders is the most extensively used in the electronics industry. The Ag3Sn, which forms during isothermal ageing, can significantly degrade solder joint reliability. Sn–Ag–Cu solder’s high price further hindered its use in the electronics industry. This paper aims to investigate different copper percentages into Sn–xCu solder alloy to improve its microstructure and strength performance.Design/methodology/approachThe solder alloys used in this work were Sn–xCu, where x = 0.0, 0.3, 0.5, 0.7, 1.0 Wt.%, which was soldered onto electroless nickel immersion gold (ENIG) substrate using carbon dioxide (CO2) gas laser. Then these samples were subjected to isothermal aging for 0, 200, 500, 1,000 and 2,000 h. The Sn–xCu solder alloy was fabricated through a powder metallurgy process.FindingsMicrostructure characterization showed that Cu addition resulted in fine and rounded shape of Cu–Sn–Ni particles. Shear strength of Sn–xCu solder joints was increased with increasing Cu content, but at aging duration of 1,000 h, it dropped slightly. It is believed that the strength improved due to the increment of diffusion rate during isothermal aging.Practical implicationsIn a Cu–Sn solder, the recommended amount is 1.0 Wt.% of Cu. In extensive aging procedures, it was discovered that Sn1.0Cu solder improved the reliability of solder joints. The findings indicated that the innovative solder alloys might satisfy the needs of high-reliability applications.Originality/valueThe study shows that the right amount of Cu enhances the solidification of Sn–Cu solder, increasing the shear force of the Cu–Sn solder joint. The Sn1.0Cu exhibits a ductile fracture on the top microstructure, improving the joint’s average shear strength.

Mixed reaction gold as versatile plating solution for ENIG, ENEPIG and EPAG plating

The surface finish on a printed circuit board (PCB) or substrate serves as surface protection and at the same time enables a variety of assembly techniques. Depending on which final finish is selected it allows for e.g. soldering connections or wire bonding by Al, Au or even Cu-wires. Surface finishes are typically characterized by different metal layers, including tin, nickel, palladium or gold. In this context the electroless or immersion plating of gold has a long history in the electronics industry. The gold plating represents a key step in different finish systems such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium/immersion gold (ENEPIG) and electroless palladium/autocatalytic gold (EPAG). ENIG, ENEPIG and EPAG finishes are characterized by their capabilities to be used as solderable and wire-bondable finish with specific benefits for ENEPIG and EPAG such as enhanced solder joint reliability and gold-wire bondability. In this paper an overview is given on the different types of electroless gold plating electrolytes from fully immersion type and autocatalytic type to mixed reaction type gold electrolytes. The details of the various plating mechanisms are presented, and the different properties and capabilities of these electrolytes are compared and explained based on the respective mechanisms. The presented study investigates the performance of a gold electrolyte of a mixed reaction type looking into the layer properties as well as the soldering and bonding capabilities. The results show that a mixed reaction gold electrolyte can be capable for versatile application- ENIG, ENEPIG and EPAG. The capability of the process was tested with respect to the solder wetting performance, solder joint reliability, wire bonding performance, corrosive attack to the nickel and thickness distribution. It can be shown that this gold process passes or exceeds all related industry standards and target requirements for ENIG, ENEPIG and EPAG final finishes.

Interfacial Diffusion Mechanism between ALD Al2O3 passivation/Cu in Direct Bonded Copper Substrate for Power Electronics

Direct bonded copper(DBC) substrate requires surface treatments for oxidation prevention and adhesion enhancement to die-attachment. Conventional surface treatment, electroless nickel immersion gold (ENIG) has a cost and environmental problems, and immersion silver is sensitive to temperature and humidity. To solve these problems, this study presents the invention of aluminum oxide (Al2O3) passivation for DBC Cu pads, which meets the requirements of oxidation prevention and adhesion enhancement to die-attachment. Ultrathin Al2O3 passivation layers (~20nm) by atomic layer deposition(ALD) were coated on DBC Cu pads for replacing conventional ENIG and immersion silver surface finishes. This paper presents bondability and interfacial diffusion mechanism between ALD Al2O3 passivation/Cu in DBC substrate.

A Potentiometric Uric Acid Biosensor Integrated With Temperature Correction Readout Circuit on Flexible PCB

This study presents the enzymatic potentiometric uric acid sensor with a temperature correction circuit, sensor set readout, and integration on a flexible printed circuit board (FPCB). Coupled with the uric acid sensor calibrated by the temperature sensor, the readout voltage can respond directly to the sensed uric acid value. Complete circuits are surface mounted on commercial electroless nickel immersion gold FPCBs. Such a miniaturized potentiometric sensor provides the shortest signal path to reduce measurement errors and noise interference. The voltage-time measurement system used to measure the sensing characteristics of the potentiometric uric acid sensor is also analyzed. In addition to high linearity and repeatability, the sensor has an average sensitivity of 19.65 mV/(mg/dL), and the error caused by temperature changes is reduced by 81% after temperature correction.

Microstructures and mechanical properties of ENIG/Sn-3.5Ag/ENIG joints formed by ultrasonic-assisted solder bonding

The effects of temperature and ultrasonic-assisted time on the microstructures and mechanical properties of the electroless nickel-immersion gold (ENIG)/Sn-3.5Ag/ENIG sandwich structure in ultrasonic chip bonding were investigated. Ultrasonic-assisted solder bonding was performed for 0.5–4 s at 25, 150, and 250 °C. The bonding was successful under all conditions. At 25 °C, Au–Sn intermetallic compounds (IMCs) were formed at the upper interface, and AuSn4 was formed on Ni–Sn IMCs at the lower interface. As the bonding time increased, spalling of AuSn4 was increasingly observed at the upper interface. At 150 °C, a trend similar to that at 25 °C was observed, but as the time increased, the thickness of the joint decreased, Au–Sn IMCs at the upper interface disappeared, and thin Ni–Sn IMCs and Cu6Sn5 were formed. The results observed at 250 °C differed from those observed at 25 and 150 °C because ultrasonic-assisted solder bonding was performed with the solder in a molten state. Ni–Sn IMCs were formed at the upper and lower interfaces, residual Sn and Au–Sn IMCs were present inside, and the thickness of the joint was very small. At 250 °C, the shear strength was high regardless of the bonding time. Observations of the top views and cross-sections of the fracture surfaces showed that, at 25 and 150 °C, the fractures initially occurred at the lower interface but moved to the upper interface as the bonding time increased. At 250 °C, the fractures did not significantly change, regardless of the ultrasonic-assisted time.