The effect of curing on the performance of ACF bonded chip-on-flex assemblies after thermal ageing

Purpose – Anisotropic conductive film (ACF) is now an attractive technology for direct mounting of chips onto the substrate as an alternative to lead-free solders. However, despite its various advantages over other technologies, it also has many unresolved reliability issues. For instance, the performance of ACF assembly in high temperature applications is questionable. The purpose of this paper is to study the effect of bonding temperatures on the curing of ACFs, and their mechanical and electrical performance after high temperature ageing. Design/methodology/approach – In the work presented in this paper, the curing degree of an ACF at different bonding temperatures was measured using a differential scanning calorimeter. The adhesion strength and the contact resistance of ACF bonded chip-on-flex assembly were measured before and after thermal ageing and the results were correlated with the curing degree of ACF. The ACF was an epoxy-based adhesive in which Au-Ni coated polymer particles were randomly dispersed. Findings – The results showed that higher bonding temperatures had resulted in better ACF curing and stronger adhesion. After ageing, the adhesion strength increased for the samples bonded at lower temperatures and decreased for the samples bonded at higher temperatures. ACF assemblies with higher degrees of curing showed smaller increases in contact resistance after ageing. Conduction gaps at the bump-particle and/or particle-pad interfaces were found with the help of scanning electron microscopy and are thought to be the root cause of the increase in contact resistance. Originality/value – The present study focuses on the effect of bonding temperatures on the curing of ACFs, and their adhesion strength and electrical performances after high temperature ageing. The results of this study may help the development of ACFs with higher heat resistance, so that ACFs can be considered as an alternative to lead-free solders.

Modelling the Performance of Flexible Substrates for Lead-Free Applications

In this paper, the performance of flexible substrates for lead-free applications was studied using finite element method (FEM). Firstly, the thermal induced stress in the flex substrate during the lead free solder reflow process was predicted. The shear stress at the interface between the copper track and flex was plotted. This shear stress increases with the thickness of the copper track. Secondly, an ACF flip chip was taken as a typical lead-free application of the flex substrate. The reflow effect on the reliability of ACF interconnections was analyzed. Higher stress was identified along the interface between the conductive particle and the metallization, and the interfacial stress increases with the reflow peak temperature and the coefficient of thermal expansion (CTE) of the adhesive. The moisture effect on the reliability of ACF joints were studied using a macro-micro modeling technique, the predominantly tensile stress found at the interface between the conductive particle and metallization could reduce the contact area and even cause the electrical failure. Modeling results are consistent with the findings in the experimental work

Effects of thermal cycling profiles on the performance of chip-on-flex assembly using anisotropic conductive films

Anisotropic conductive films (ACFs) are widely used in the electronic packaging industries because of their fine pitch potential and the assembly process is simpler compared to the soldering process. However, there are still unsolved issues in the volume productions using ACFs. The main reason is that the effects of many factors on the interconnects are not well understood. This work focuses on the performance of ACF-bonded chip-on-flex assemblies subjected to a range of thermal cycling test conditions. Both experimental and three-dimensional finite element computer modelling methods are used. It has been revealed that greater temperature ranges and longer dwell-times give rise to higher stresses in the ACF interconnects. Higher stresses are concentrated along the edges of the chip-ACF interfaces. In the experiments, the results show that higher temperature ranges and prolonged dwell times increase contact resistance values. Close examination of the microstructures along the bond-line through the scanning electron microscope (SEM) indicates that cyclic thermal loads disjoint the conductive particles from the bump of the chip and/or pad of the substrate and this is thought to be related to the increase of the contact resistance value and the failure of the ACF joints.

Analyzing the performance of flexible substrates for lead-free applications

The performance of flexible substrates for lead-free applications was studied using finite element method (FEM). Firstly, the thermal induced stress in the flex substrate during the lead free solder reflow process was predicted. The shear stress at the interface between the copper track and flex was plotted. This shear stress increases with the thickness of the copper track and the thickness of the flex. Secondly, an anisotropic conductive film (ACF) flip chip was taken as a typical lead-free application of the flex substrate and the moisture effect on the reliability of ACF joints were studied using a 3D macro-micro modeling technique. It is found that the time to be saturated of an ACF flip chip is much dependent on the moisture diffusion rate in the polyimide substrate. The majority moisture diffuses into the ACF layer from the substrate side rather than the periphery of the ACF. The moisture induced stress was predicted and the predominant tensile stress was found at the interface between the conductive particle and metallization which could reduce the contact area and even cause the electrical failure