Hoang-Vu Nguyen

Spherical polymer particles in isotropic conductive adhesives a study on rheology and mechanical aspects

Isotropic conductive adhesive (ICA) filled with metal coated polymer spheres has been studied as a novel approach to increase the flexibility, and hence the reliability of the adhesive compared to traditional metal filled ICAs. In this paper, we have investigated the rheological properties of the novel ICA to evaluate its applicability in practical use. The current work also involves the investigation of the mechanical properties including shear strength of the novel ICA. Spherical polymer particles (SPP) of sizes Ø6 µm and Ø30 µm were investigated in the present study. The results show minor differences in the rheological properties and the adhesion strength for adhesives filled with particles in different sizes. Filling SPP into the adhesive matrix increases the viscosity of the system monotonically and continuously, in excellent accordance with model systems previously reported in the literature. Furthermore, the novel ICA exhibits high mechanical shear strength, being comparable to the traditional solder joint technology and twice higher than the traditional metal filled ICA.

Au-Sn fluxless SLID bonding: Effect of bonding temperature for stability at high temperature, above 400 °C

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300–350 °C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350–400 °C, with 95 % of the tested samples surviving 400 °C without bond delamination. This is more than 100 °C higher than the melting temperature of the commonly used eutectic Au-Sn bond (80 wt% Au, melting at 278 °C). The Au-Sn system is particularly interesting since it is oxidation resistant, allowing fluxless bonding. With the SLID process, the metal system is applicable for true high-temperature applications.

Die Shear Testing of a Novel Isotropic Conductive Adhesive—Epoxy Filled With Metal-Coated Polymer Spheres

Isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres (MPS) are introduced to improve the mechanical reliability compared with conventional ICAs filled with silver (Ag) flakes. This paper deals with the die shear performance of an MPS-based ICA; an epoxy filled with 45 vol% of Ø30 μm Ag-coated monodisperse polymer spheres. The curing kinetics of the ICA is also studied. The MPS-based ICA is compared with two ICAs filled with Ag flakes: an in-house prepared ICA (with the same epoxy matrix as the MPS-based ICA) and a commercially available ICA. Both ICAs with Ag flakes have a lower particle volume fraction than the MPS-based ICA. Loading 45 vol% of MPS into the epoxy matrix has no significant effect on the curing kinetics and the glass transition temperature (Tg) of the matrix. The MPS-based ICA has 140% higher die shear strength than the in-house prepared ICA with Ag flakes, and 23% higher die shear strength than the commercial ICA with Ag flakes. The MPS-based ICA also has higher shear strain at failure, and less scatter in shear strength between repeated tests. Hence, the MPS-based ICA has a good potential for demanding applications; it has better die shear performance and higher Tg than the commercial ICA with Ag flakes.

Temperature dependence of mechanical properties of isotropic conductive adhesive filled with metal coated polymer spheres

An isotropic conductive adhesive (ICA) filled with metal-coated polymer spheres (MPS) has been studied as a novel approach to increase the flexibility, and hence the reliability, compared to the conventional metal-filled ICA. In this study, the effect of the metal coating on the die shear strength was investigated by comparing ICA materials with coated and uncoated polymer spheres. The other important part of the study was to assess the temperature dependence of the die shear strength of an MPS-based ICA, and also compare this with the behavior of a conventional ICA filled with Ag particles. The results showed that the metal coating does not have a critical effect on the die shear strength of ICA filled with MPS. The die shear strengths obtained for the MPS-based ICA and the conventional Ag-filled ICA have the same temperature dependence in the range of 20 °C to 120 °C. Furthermore, none of the ICA systems has experienced a critical drop in die shear strength at and above the glass transition temperature.

Isotropic conductive adhesive filled with metal-coated polymer spheres — Effects of metal coating on rheological and mechanical properties

An isotropic conductive adhesive (ICA) filled with metal-coated polymer spheres has been studied as a novel approach to increase the mechanical flexibility, and hence the reliability, compared to conventional metal-filled ICAs. In the present paper, effects of the metal coating on the viscosity and the die shear strength were investigated, by comparing ICA materials with metal-coated and uncoated polymer spheres. The sphere diameter is 30 μm, and the metal coating used on the spheres is silver (Ag). The results showed a minor effect of the Ag coating on the die shear strength of the ICA. On the other hand, the Ag coating had an evident effect on the viscosity of the ICA, particularly at low shear rates.

Thermosonic bonding for ultrasound transducers: Low-temperature metallurgical bonding

A low-temperature bonding process for ultrasound transducers is presented: compatible with poling requirements, manufacturability and reliability. In this work, we demonstrate that a thermosonic bonding process can provide a reliable, metallurgical bond at moderate temperatures, even down to room temperature, with bonding times in the order of seconds. Bonding parameters (temperature, compression force, ultrasonic energy) were optimized by evaluating shear strength on Au stud bump bonded Si chips. Model systems have been bonded, mimicking a complete Electro-Acoustic Module (EAM), including a stack of IC emulator / flex interconnection / interface part of the ultrasound transducer.

Anisotropic conductive film interconnects for fine-pitch MEMS

The flip-chip interconnection technology based on anisotropic conductive films (ACFs) has recently become an attractive solution for the assembly of micro-electromechanical systems (MEMS) and application specific integrated circuits (ASIC) in MEMS packages. In the present work, we have studied the fine pitch capability of ACF interconnects for MEMS applications such as fingerprint sensors and capacitive micromachined ultrasonic transducers, in which interconnects spread around MEMS and ASIC surface. The silicon test chips and substrates with different interconnect pitch were assembled using a single layer ACF. The electrical performance of ACF interconnects with varying pitch from 110 to 200 μm was compared. Furthermore, the distribution of conductive particles and the electrical resistance of ACF interconnects at both peripheral and central parts of the chips were evaluated. Effect of thermal shock cycling test (−40 to +125 °C) on samples was investigated. The results showed insignificant difference in the electrical performance between ACF interconnects with pitch varying from 110 to 200 μm. The particle distribution and the electrical resistance of ACF interconnects at different chip regions were similar. No significant effect of the thermal shock cycling test was observed. No failures (open/short circuit) occurred, both before and after the thermal shock cycling test.

Low-Temperature High-Throughput Assembly Technology for Transducer Array in Medical Imaging Applications

Isotropic conductive adhesives (ICAs) based on metal-coated polymer spheres (MPS) have shown high potential for low-temperature, high-throughput assembly of a transducer array on a substrate in ultrasound imaging applications. The process of bonding and subsequently dicing a transducer stack on a flexible substrate was evaluated. The bonding material was MPS-based ICAs containing a commercial epoxy filled with Ø5 µm Au-coated mono-disperse polymer spheres in two different concentrations, the lower being somewhat above the percolation threshold whereas the higher being close to the maximum concentration for processability (such as dispensing, printing). ICA interconnects in test samples undergoing the entire assembly process exhibit high mechanical shear strength and sufficient electrical resistance even after the tough dicing. In addition, a high process yield obtained in this work is very promising for implementation in industry. The present work has demonstrated the applicability of MPS-based ICAs for assembling ultrasound transducer arrays on system substrates.

Low temperature bonding of piezoelectric single crystal materials for miniaturized high resolution ultrasound transducers

The realization of the next-generation miniaturized ultrasound transducers comprising of fine pitch array elements requires packaging techniques used in the microelectronics industry. In addition, new types of piezoelectric materials are used to enhance the acoustic performance of these transducers, but these materials cannot withstand the temperature and pressure applied in conventional bonding technologies. This paper explored two different bonding techniques specifically to address this issue.