Preparation Of Nano-Metal Pastes For Printed Electronics And Die Attachment And Studies Of Their Low-Temperature Sintering Performance And Reliability

Recently,printed flexible electronics have been developing rapidly and show enormous potential in applications of flexible displays,radio frequency identification tags and wearable sensors.In manufacturing processes of these electronic devices and products,highly conductive metal nanoparticle（NP）inks are used to fabricate flexible circuits on less heat-resistant organic substrates by printing and sintering.However,even for the most intensively used Ag NP inks,a high-temperature sintering process（>150°C）is still required to ensure high electrical conductivity,not to mention the cost-effective Cu NP inks which have more intrinsic problems,such as oxidation of Cu NPs,poor storage stability and rigorous sintering conditions.Therefore,it is highly important to improve the low-temperature sintering performance of Ag and Cu NP inks and clarify the underlying sintering mechanisms so as to manufacture cost-effective and high-reliability flexible electronics.Apart from making conductive inks for printed electronics,the metal NPs with superior sintering performance can also be used to prepare sinterable pastes for die attachment,in particular for those capable of low-temperature sintering,which can help meet the requirement of die attachment processing at low temperature while the joints working at high temperature.Noteworthily,the above two types of materials,i.e.,metal NP ink and paste,both consisting of metal fillers and solvents,can be collectively referred to as nano-metal pastes.This thesis work performs systematic and in-depth studies on fabrication of Ag and Cu NP inks and Cu NP paste,as well as their low-temperature sintering performance and reliability.Firstly,the Ag particle ink capable of low-temperature sintering is fabricated by precisely adjusting capping agent contents of Ag NPs.Then,inspired by the above results,the interaction between hybrid capping agents and Cu particles is taken into account for synthesizing Cu powder with bimodal-sized distribution,and the synthesis and sintering mechanisms of the Cu powder are revealed to help achieve optimal fabrication of the Cu NP ink with good oxidation resistance and sinterability at low temperature;in the meanwhile,the low-temperature sinterability of bimodal-sized Cu powder is explored to facilitate the fabrication of the Cu NP paste capable of pressureless sintering for die attachment in power electronics,and its reliability is characterized.At first,the capping agent content of Ag NPs is adjusted with different strategies to achieve low-temperature sintering driven by surface energy,making Ag NP ink suitable for us on organic flexible substrates.The results show that the content of polyacrylic acid（PAA）absorbed on the surface of Ag NPs is reduced significantly after a cleaning treatment using NaCl solution with a concentration of 40 m M,leading to a sharp decrease in sintering temperature from 160°C to 80°C.Additionally,the growth mechanism of Ag NPs during sintering process changes from oriented attachment to Oswald ripening due to reduction of the capping agent.The studies also indicate that Ag NPs capped by negatively charged PAA can be sintered at 100°C when using positively charged cetyltrimethylammonium bromide capped Ag NPs as sintering assisted agent.Then,a liquid metal-like Ag NP ink with good conductivity is prepared using high-concentration NaCl solution which acts as a trigger for the room-temperature sintering among Ag NPs.Specifically,hydrogen bonds formed between PAA absorbed on surface of Ag NPs and carboxymethyl cellulose dissolved in solvent make the sintered Ag aggregates contact with each other,resulting in formation of continuous conductive networks.More importantly,such conductive network structures can always preserve good electrical conductivity under different strain levels.The strain sensor fabricated by 3D direct writing technique shows a large tensile strain range,excellent long-term cycle reliability,good linear sensing performance and short response time,and has been successfully employed to detect the movement of human knuckles.Further,inspired by the interaction between Ag NPs and the capping agents,the cost-effective Cu powder with bimodal-sized distribution is synthesized successfully through a one-step chemical reduction method based on the synergistic effect of hybrid capping agents.It is found that when the molar ratio of lactic acid（LA）to iso-propanol amine（IPA）is increased appropriately,large-size particles in Cu powder transform from microflakes（MFs）to submicroparticles（SMPs）,while small-size ones always remain as Cu NPs with a diameter of～9 nm.This can be attributed to the synergistic effect of IPA and LA,where IPA can form a complex with Cu ions to reduce the reduction reaction rate while LA may accelerate the reduction reaction,then small Cu NPs can be preserved due to electrostatic stabilization effect.From in-situ transmission electron microscopy observation results,the sintering rate of Cu power with various sizes changes significantly due to the big difference in sintering driving force.Further,the surface energy release in coalescing process of Cu NPs can lead to local temperature rise as confirmed by molecular dynamics simulations.In addition,using bimodal-sized Cu NPs,highly conductive Cu–Ag hybrid film circuits can be obtained through two different methods by means of the displacement reaction between Ag ions and Cu NPs.For the first method,core-shell structured Cu–Ag（Cu@Ag）powder containing small Ag NPs（7 nm）,Cu@Ag NPs（20 nm）and large Cu@Ag SMPs（180 nm）are synthesized by a two-step method.The second method is developed by immersing the printed Cu NP film in Ag NO₃ solution for several seconds to form regularly arranged Ag nanoplates on surface of Cu NP film.After drying at room temperature,the immersion plated Cu–Ag hybrid films can be obtained which show a low resistivity of 1.4×10^-7Ω·m.During immersion plating process,large-size Cu SMPs act as nucleation sites for Ag nanoplates,and small-size Cu NPs with high surface activity have strong catalytic effect and accelerate the displacement reaction between Ag ions and Cu NPs,providing enough Ag atoms for the growth of Ag nanoplates.Finally,to explore the low-temperature sinterability of bimodal-sized Cu particles,the Cu paste is prepared to achieve die attachment in power electronics.Results show that the sintering process of Cu NPs consists of two steps:the first step sintering occurs below 200°C mainly by coalescence between unoxidized small Cu NPs and submicron-sized large particles,the second step takes place from 200°C by enhanced sintering process triggered by pure Cu NPs from Cu lactate decomposition.By using the Cu paste formulated by solvents composed of diethylene glycol and glycerol,sintered Cu paste joints after pressureless sintering at 280°C for 10 min show shear strength of 72 MPa,the highest reported by far.In the matrix of sintered Cu paste joints,there is a large amount of Cu bulk,and continuous yet dense diffusion layers form at the interfaces,which show strong strengthening effects on mechanical properties of the joints and meanwhile can be regarded as the standard microstructure to determine whether a sintered joint has high strength or not.The dense sintered matrix of sintered Cu paste joints enables the joints to have excellent high temperature（250°C）oxidation resistance reliability.