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PurposeAnisotropic conductive film (ACF) offers miniaturization of package size, reduction in interconnection distance and high performance, cost‐competitive packaging and improved environmental impact. However, a major limitation for ACF is the instability caused by thermal warpage. The purpose of this paper is to study the effects of thermal warpage on contact resistance in real time i.e. make online measurements of contact resistance fluctuations while the assembly undergoes thermal shock.Design/methodology/approachThe ACF assemblies are subjected to thermal cycling with different temperature profiles that have peak temperatures either below or above the glass transition temperature (Tg) of the ACF. The flex substrate used was made of polyimide film, with Au/Ni/Cu electrodes and a daisy‐chained circuit matched to the die bump pattern. The ACF used was based on epoxy resin in which nickel and gold‐coated polymer particles are dispersed. A comparative study was carried out on the results obtained.FindingsThe results showed that the glass transition temperature (Tg) of the ACF material plays an important role in the high temperature contact resistance. Above Tg, the ACF matrix becomes less viscous, which reduces its adhesive strength and allows the bumps on the chip to slide away from the pads on the substrate. Even though a flex substrate was used in this study, the sliding effect is severe at the corner bumps of the chip, where cumulative forces are generated due to the thermal expansion mismatch. For every thermal cycling profile, there is an incubation period encountered from this work that would have a significant impact in the application of ACF. After the incubation period the contact resistance increased rapidly and the assemblies were therefore no longer reliable.Originality/valueThe work in this paper focuses on contact resistance changes during thermal shock. The paper discusses the reliability issue of ACF during thermal warpage, which is useful to industries using ACF for flip‐chip assemblies.

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The reliability of ACF (Anisotropic conductive film) interconnection is a serious concern especially under thermal loading condition. This paper focuses on the online contact resistance behavior of the ACF joint for bumpless flip-chip on flex packages during different thermal cycling conditions. In this work, flip chips of 11×3 mm2 having bare aluminum pad were used. Real time contact resistance (i.e. live measurement contact resistance variation with temperature) was measured by four points probe method when the packages were inside thermal shock chamber. Tests for three different thermal cycling profiles (125°C to −55°C, 140°C to −40°C and 150°C to −65°C) were carried out. The samples bonded at temperature 180°C and pressure of 2.42Mpa was used. The initial contact resistance of the bumpless samples was 0.4Ω. Contact resistance increased with the number of thermal cycles, however the effect was severe when the temperature variation was above the glass transition temperature (Tg) of the ACF matrix (131°C). Differences in co-efficient of thermal expansion (CTE) between the chip and the substrate generated thermal stresses during temperature fluctuation, which caused the pad of the substrate to slide over the Al pad of the chip. Thus variation of the contact resistance was also observed along the interconnection position in the package, i.e. corner joint showed higher value of increase in contact resistance than the middle position. Even though flex substrate was used in this study; the sliding effect was severe at the corner Al pads of the chip, where cumulative forces generated due to the thermal stress. Results show that for thermal cycling profile 140°C to −40°C, online contact resistance increased to 1.2 Ω in corner joint, whereas for the middle joints the contact resistance just increased to 0.5 Ω. Glass transition temperature (Tg) of the ACF material plays an important role on the high temperature contact resistance. For every thermal cycling profile, there is an incubation period that would have significant impact in the application of ACF. After the incubation period the contact resistance increases rapidly and the joints are no longer reliable.

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.