Erja Jokinen & Professor Eero Ristolainen

The chips become bendable when they are thin enough, e.g. less than 100 um. Furthermore, when the joining is done using anisotropic conductive adhesives (ACA) even the interconnection area is bendable. The result is a fully flexible electronics module for multi-chip module (MCM) applications. In addition, if needed, the electronics can be bent to fit into cases of different shapes and sizes.

In this study, flip chip interconnections were made on very flexible polyethylene naphthalate (PEN) substrates using anisotropic conductive film (ACF). The PEN substrate had copper traces. The chips were very thin, only 50 um thick. Due to the thinness of the chips, they were flexible and the entire joint was bendable. The pitch of the test vehicles was 250 um, the chips had 25 um high gold bumps.

For resistance analysis there were two four-point measurement positions in each test vehicle. For finding the optimal bonding pressure, the bonding was done using two different bonding pressures, the better one of which was chosen for the final tests. Furthermore, the test vehicles were subjected to thermal cycling tests between -40 and +125°C (half an hour cycle) and humidity test (85% RH /85°C). Some of the test vehicles were bent during the tests. The effect of bending on the reliability of the joints was studied. Finally, the structures of the joints were studied using scanning electron microscopy (SEM).

More details of the results will be presented.