Development of flip chip attach technology using Ag paste bump which formed on printed wiring board electrodes

We have developed original flip chip attach (FCA) technology using anisotropic conductive film (ACF), which can perform electrical connection and encapsulation simultaneously. The advantages are that all bumps on the substrate are formed simultaneously and that ordinary bare chips without bumps on electrodes can be used. This paper presents the results of the FCA process for a 150 μm pad pitch sample chip and of the reliability tests. Ag paste bumps were formed by Ag paste transfer and curing on the electrode pads of a PWB by screen-printing methods, and printing conditions were evaluated for good bump formation for this FCA process. Three kinds of ACFs with different physical properties were used for the experiment, and the FCA conditions were evaluated for each ACF. Moreover, the effect of the surface treatment on Ag paste bumps was also evaluated with regard to FCA connection resistance. Assuming that passive component assembly is done by soldering, reliability tests of the FCA samples were carried out after two additional heat treatments at reflow temperature. High-temperature and high-humidity storage tests (THS) and temperature cycling tests (TCT) were performed as reliability tests. The connection resistance between electrodes on bare chip and PWB was measured by a four-probe method. Although connection resistance increased after THS and TCT, it was saturated at under 500 μΩ per bump in the sample with surface treated Ag bumps