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Anisotropic conductive film for fine-pitch interconnects
Keywords: ACF, Reliability, Miniturization
Anisotropic conductive films (ACFs) have proven valuable in the semiconductor industry to enable a simple, low cost and fine-pitch compatible bonding process [Paik 2012]. ACF has been demonstrated as a reliable process for flip-chip assembly of components down to 3.1 mm x 3.1 mm, with 80 µm diameter pads and 110 µm pitch [Larsson 2011]. In this work we have taken this miniaturization a step further and explored the use and reliability of ACF for flip-chip assembly of two silicon components with a size of 1.4 mm x 1.4 mm, 50 µm diameter pads and 85 µm pitch, onto one silicon interposer. The small silicon components and the interposer received an under bump metallisation (UBM) of thicknesses 3.4 µm and 13.4 µm, respectively (Ni/Pd/Au). The inter-pad distance was about 10 µm after UBM. An ACF (25 µm thick, containing 5 µm diameter metal coated polymer spheres (MPS)) was selected due to the fine-pitch of the design and because a similar ACF had previously been shown to be highly reliable [Larsson 2001, Nguyen 2012]. Two different MPS concentrations were tested: 10000 MPS/mm2 and 20000 MPS/mm2. In order for the two components to be assembled onto the same interposer sequentially with a limited spacing, the ACF was applied to the components at wafer level followed by dicing. However, the small size of our components augmented adverse edge effects on ACF transparency observed after dicing, which made alignment during flip-chip bonding challenging. These effects were alleviated by optimising the dicing procedure. Test samples replicating the two components with respect to pad layout were designed for assembly tests. The test samples contained Kelvin structures, daisy chains and open circuit structures. The optimal parameters for flip-chip bonding were found to be 1 MPa and 190 °C for 40 seconds. Both MPS concentrations gave an acceptable average interconnect resistance of about 3.3 Ω. The alignment was analysed by x-ray microscopy, considered as a viable inspection method as poorly performing interconnects were mostly recognized to correlate with misaligned pads . Even with a pad-to-pad distance only twice the diameter of the conductive MPS particles, we did not observe any short circuits. The fillet that formed due to ACF squeeze-out was confined within a 100 µm perimeter around the component, enabling a design where other components and contact pads can be placed in close proximity without risk of obstruction. In situ resistance measurements during humidity and temperature cycling indicated that some structures were more severely affected by hygrothermal aging than others. We postulate that this has to do with the number of trapped MPS and to which extent they have been squeezed between the interconnects. Another observation was a decrease in resistance of the open circuit structure when exposed to humidity and the decrease was apparently smaller for higher temperatures. Thermal shock cycling between -55 °C and +125 °C gave slightly ambiguous results where some structures survived whereas others broke down. Failure analysis is ongoing and results will be presented at conference.
Daniel Nilsen Wright, Research Scientist
Oslo, Oslo

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