Effect of bump characteristics and temperature variation on the online contact resistance of anisotropic conductive joints

Abstract

Anisotropic conductive film (ACF) suffers a major drawback in regard to reliability even though it has merits, such as reduction in interconnection distance, high performance, and environmental friendliness. The factor of thermal warpage may lead to a highly unreliable electrical connection in the assembly. The work presented in this paper focuses on the online contact-resistance behavior of the ACF joint during thermal shock and compares the results of two different types of dies (Au/Ni bump and bumpless). For this work, we used a flip chip of 11 ϫ 3 mm 2 in dimension. The flex substrate used was made of polyimide film with an Au/Ni/Cu electrode and daisy-chained circuit for a matching die-bump pattern. The ACF that was used is an epoxy resin in which nickel and gold-coated polymer balls are dispersed. Tests for three different thermal-cycling profiles (125°C to Ϫ55°C, 140°C to Ϫ40°C, and 150°C to Ϫ65°C) were carried out. The samples bonded at a temperature of 180°C, and a pressure of 80 N was used. The initial contact resistances of Au/Ni bump and bumpless samples were 0.25 Ω and 0.4 Ω, respectively. A comparative study was carried out from the results obtained. The results showed that for the flip-chip-on-flex (FCOF) packages having an Au/Ni bump, the increase in online contact resistance is higher than that of the FCOF packages having bumpless chips. For example, in the thermal-cycling profile of 140°C to Ϫ40°C, the online contact resistance for the Au/Ni bump raised to 4.6 Ω after 180 cycles, whereas it was only 1.3 Ω for the bumpless sample. The bump height and bump materials were found to be the main factor for such variation. Results show that, above the glass-transition temperature (T g ), the ACF matrix becomes less viscous, which reduces its adhesive strength and lets the higher bump height of the chip result in a higher standoff of the package and thus sliding is easier to take place. The responses by the assemblies in hot and cold conditions are examined, and in-chamber behavior of the assembly is studied and explained.

Anisotropic Conductive Adhesives for Flip-Chip Interconnects

“…Two types of bumps were used to test the effect of bump height on contact resistance: gold/nickel (Au/Ni), which is a 4 µm-thick nickel bump with gold coating, and a 1 µm-thick aluminum pad [14]. After bonding, the online contact resistance is measured for different thermal cycling profiles (−55 to 125 • C, −40 to 150 • C and −65 to 150 • C).…”