Influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of solderable anisotropic conductive adhesives (SACAs)

Highlights

• The reliability properties of CNT-filled SACAs were investigated.

• The metallurgically-interconnected SACAs show good electrical reliability.

• MWCNTs can enhance the mechanical reliability properties of SACA.

Abstract

In the present work, the influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of carbon nanotube (CNT)-filled solderable anisotropic conductive adhesives (SACAs) containing low-melting-point-alloy (LMPA) fillers was investigated. To measure the reliability-related properties of SACA assemblies, six types of SACAs with different MWCNT concentrations (from 0 to 2 wt%) were formulated, and two types of reliability tests, thermal shock (TS) and high-temperature and high-humidity (HTHH) tests, were conducted on these samples. All of the CNT-filled SACA assemblies with different MWCNT concentrations showed proper and stable electrical reliability during each aging test, due to the molten LMPA fillers forming a metallurgical interconnection between the corresponding metallization regions. Although the mechanical strength of the CNT-filled SACA joints degraded after aging tests due to excessive intermetallic compound (IMC) layer growth, the CNT-filled SACAs with an MWCNT concentration below 0.1 wt% exhibited better mechanical reliability properties than those of SACAs without MWCNTs due to the initially achieved high bonding strength of the SACA joints, enabled by the reinforcing effects of the MWCNTs within the polymer composites.

Introduction

Conventional anisotropic conductive adhesives (ACAs) consist mainly of polymer composites and non-fusible conductive filler with a relatively low volume fraction (typically 5 to 20 vol%). Due to their low volume fraction of conductive filler, ACAs are deposited over the entire contact region in the form of a paste or film and provide a unidirectional electrical conduction flow along the Z-axis through the conductive fillers, which are trapped between the corresponding electrodes [1,2]. Because of their potential advantages (e.g., low processing temperature, the capability for simple processing, higher flexibility, and excellent compatibility with non-fusible materials), ACAs have been successfully implemented as alternatives to conventional solder materials in flat panel displays (FPDs) modules, such as liquid crystal displays (LCDs), as well as in various packaging technologies in a form of tape carrier packages (TCPs), chip on glass (COG), chip on flex (COF), and flip-chip bonding in electronic packaging industries [[3], [4], [5]]. However, in spite of these advantages, the wide application of ACAs in electronic interconnections is limited due to several critical drawbacks such as low thermal and electrical conductivity, unstable contact resistance, low impact and joint strengths, and decreased conductivity under elevated temperature and humidity aging conditions. These ACA drawbacks arise as a result of the intrinsic conduction mechanisms of the physical and mechanical contacts of the conductive fillers during the curing process [5,6].

Our research team previously developed solderable anisotropic conductive adhesives (SACAs) mainly composed of a functionalized polymer composite with fluxing capability and a fusible low-melting-point alloy (LMPA) filler to overcome the limitations of conventional ACAs. This approach combines the merits of the solder materials (e.g., metallurgical conduction path formation) and the advantages of conventional ACAs (e.g., a low processing temperature and a simplified manufacturing process with fine pitch capability). In the SACA interconnection process, the metallurgical conduction paths were established between the corresponding metallizations through the selective wetting behaviors of molten LMPA fillers within the SACAs [7,8]. We confirmed that SACAs can achieve proper electrical and mechanical properties due to the metallurgical conduction path formation by the molten LMPA fillers. Further, in previous studies, as shown in Fig. 1, we developed carbon nanotube (CNT)-filled SACAs containing multi-walled carbon nanotubes (MWCNTs) as a reinforcing material to improve the interconnection properties of SACAs, and we investigated these interconnection properties [9]. CNTs have attracted considerable interest as ideal reinforcement for the enhancement of epoxy resins due to their superior physical properties such as a novel structure (an extremely high aspect ratio), high electrical and thermal conductivities, and high intrinsic mechanical properties (including stiffness, strength, and toughness) produced by strong bonding between carbon molecules [[10], [11], [12]]. In the interconnection test results using CNT-filled SACAs, MWCNTs were distributed in the polymer composite region, and the interconnection performances (i.e., electrical and mechanical properties) of the CNT-filled SACA joints were enhanced compared with those of SACAs without MWCNTs due to the reinforcing effects of the MWCNTs with unique physical properties.

In this study, to investigate the influence of the MWCNT concentration on the thermo-mechanical reliability properties of CNT-filled SACA assemblies, six types of quad flat package (QFP) assemblies containing CNT-filled SACAs with different MWCNT concentrations (0–2 wt%) were prepared, and then two types of reliability tests (thermal shock (TS) and high-temperature and high-humidity (HTHH) tests) were performed on CNT-filled SACA interconnections. The electrical and mechanical reliability properties of these assemblies were investigated during each of the reliability tests. In addition, the interfacial microstructures and fracture modes of the SACA assemblies were also investigated and compared.