Filler size and content effects on the composite properties of anisotropic conductive films (ACFs) and reliability of flip chip assembly using ACFs

Abstract

Flip chip assembly on organic board using anisotropic conductive films (ACFs) is gained more attention because of its many advantages. But to obtain more reliable flip chip assembly, it is necessary to have low coefficient of thermal expansion (CTE) value of ACFs. To control the CTE of ACF materials, non-conductive silica fillers were incorporated into ACFs. The effect of non-conductive silica filler content and size on cure kinetics and thermo-mechanical properties of ACFs was studied. Furthermore, filler content and size effects on reliability of flip chip assembly using ACFs were also investigated. In accordance with increasing filler content, curing peak temperature and storage modulus (E′) increased. But CTE decreased as the filler content increased. The effect of filler size on composite properties and assembly reliability showed similar tendency with the filler content effect. The smaller filler size was applied, the better composite properties and reliability were obtained. Conclusively, incorporation of non-conductive fillers, particularly in case of smaller size and higher content, in ACFs improves the material properties significantly, and as a result, flip chip assembly using ACFs is resulted in better reliability.

Introduction

Flip chip assembly directly on organic board offers miniaturization of package size, as well as reduction of weight and interconnection distance, resulting in high performance of electrical and thermal properties. Furthermore, because of recent innovation of IC packaging technology, high density and high performance interconnection method has become essentially important. As a result, flip chip technology gains attractiveness as one of the best chip packaging candidates to meet recent technical trends [1], [2], [3].

Until now, solder balls are main material for flip chip technology, but flip chip assembly using anisotropic conductive films (ACFs) has been under development for its unique merits such as low processing temperature (less than 180 °C relative to 240 °C for solder reflowing temperature), finer pitch interconnects (able to ∼30 μm pitch), cost effective due to the simple process, and green process with no fluxes, lead and cleaning solvent [4].

Basically, ACFs are composite materials composed of an epoxy adhesive resin as a matrix and conductive particles such as metallic particles or metal-coated plastic particles. And the reliability of assembly using ACFs significantly depends on the properties of epoxy composite matrix [5], [6], [7]. Therefore, it is important to understand and to make an estimation of epoxy matrix properties of ACFs to obtain more reliable ACF assembly.

The most common failure trend of ACF assembly is observed during thermal cycling test, which is due to the thermal expansion mismatch between silicon chips and ACFs. (An unfilled epoxy matrix has a typical CTE value of 50 × 10⁻⁶ K⁻¹ or above, much greater than that of silicon, ∼3–5 × 10⁻⁶ K⁻¹). Therefore, in recent years, inorganic particle-filled matrix composites have received much attention, particularly because of their low coefficient of thermal expansion (CTE) for reducing the thermal mismatch with device materials such as silicon [8].

For purposes of obtaining optimum ACFs materials for reliable flip chip assembly, the focus of this study is to investigate the effect of added filler size and content on ACFs characteristics such as cure kinetics, dynamic mechanical and thermo-mechanical properties. In addition, the reliability of flip chip assemblies using ACFs, which are filled with different size and content, is also considered.