Thermo-mechanical reliability of a multi-walled carbon nanotube-incorporated solderable isotropic conductive adhesive

Highlights

• The reliability properties of MWCNT-incorporated SICAs were investigated.

• The metallurgically-interconnected SICAs show good electrical reliability.

• MWCNTs can enhance the mechanical reliability properties of SICA.

Abstract

Carbon nanotubes (CNTs) are considered as ideal candidates for the reinforcement of polymer composites due to their superior physical properties. In this paper, in order to investigate the influence of multi-walled carbon nanotubes (MWCNTs) on the reliability properties of solderable isotropic conductive adhesives (SICAs) with a low-melting-point alloy (LMPA), two types of SICAs (with 0.03 wt.% MWCNTs and without MWCNT) were formulated. Thermal shock (− 55 to 125 °C, 1000 cycles) and high-temperature and high-humidity (85 °C, 85% RH, 1000 h) tests were conducted on these samples. The SICA assemblies with and without MWCNTs showed stable electrical reliability properties during reliability testing; this stability was due to the formation of excellent metallurgical interconnection between corresponding metallization by the molten LMPA fillers. Although the mechanical pull strength of SICA assemblies decreased after thermal aging, due to the excessive layer growth and planarization of the IMCs, the SICA with MWCNTs showed enhanced mechanical reliability properties compared with the SICA samples without MWCNTs. This improvement in performance was caused by the enhancement effect of the MWCNTs. These results demonstrate that MWCNTs within SICAs can enhance the reliability properties of SICA joints due to their outstanding physical properties.

Introduction

Electrically conductive adhesives (ECAs) with non-fusible organic or non-organic fillers have been widely used as an alternative to lead-free solder materials in the microelectronic packaging industry due to their potential advantages such as low processing temperature, high resolution capabilities, more flexible and simpler processing, and compatibility with non-solderable materials [1], [2], [3]. However, non-fusible filler-filled ECAs form a conduction path through the physical–mechanical contact of the conduction fillers between corresponding metallization surfaces during the curing process. Due to this conduction mechanism, the non-fusible filler-filled ECAs have critical limitations when compared with metallurgical interconnections using soldering techniques. These shortcomings include low and unstable electrical and thermal conductivities, increased contact resistance, and low impact and joint strength [4], [5]. To overcome the limitations of non-fusible filler-filled ECA techniques, solderable isotropic conductive adhesives (SICAs) with a polymer matrix (with reduction capabilities) and fusible low-melting-point alloy (LMPA) fillers have been investigated [6], [7]. SICAs can achieve excellent interconnection properties, such as low and stable electrical performance and high mechanical bonding strength, by the formation of metallurgical conduction path in the molten LMPA fillers.

Additionally, in previous works, we formulated SICAs that were incorporated with carbon nanotubes (CNTs) as a reinforcement material to improve the interconnection properties of SICAs. We also developed an interconnection mechanism for these materials, as shown in Fig. 1 [8], [9]. CNTs are considered to be ideal nanofiller materials for the improvement of polymer composites because of their physical properties (e.g., their novel structure, high intrinsic mechanical strength, and electrical and thermal conductivities) [10]. As can be seen in the figure, the metallurgical conduction path is formed by the flow-coalescence-wetting behaviors of molten LMPAs between the corresponding electrodes, and the simultaneous movement of the polymer composite outside of the conduction path region. At this time, the covalently bonded CNTs, with epoxides of polymer composites, moved outside of the conduction path via the polymer flow. The interconnection properties of SICAs can be enhanced compared to SICAs without CNTs due to the outstanding physical properties of CNTs within the polymer composite region.

When developing an interconnection material to apply to an electronic package device, the thermo-mechanical reliability properties of the interconnection material should be carefully considered to ensure reliable performance of the electronic packages. Electronic packages are composed out of various package materials with dissimilar coefficients of thermal expansion (CTEs). When the electronic packages are exposed to various environments, such as repetitive temperature change, high humidity by power consumption, or the external environment, high interfacial stresses can be generated in the package joints due to the CTE mismatch between the package materials [11]. Most fatigue failures can be attributed to thermo-mechanical stresses in the package joints [12]. If these stresses exceed the critical value, interfacial delamination and fracture will occur, and the functionality of the system will be destroyed [13].

In this study, to examine the influence of multi-walled carbon nanotubes (MWCNTs) on the thermo-mechanical reliability properties of SICAs, two types of SICAs (with and without MWCNTs) were prepared. Thermal shock (TS) and high-temperature and high-humidity (HTHH) tests were performed. The electrical and mechanical reliability properties of SICA assemblies were investigated and compared during reliability testing. In addition, the interfacial microstructure and fracture surface were analyzed.