Processing and reliability of bare die LED chip bonding on flexible plastic substrate

Printed hybrid electronics combining traditional semiconductor devices and polymer substrates allows augmenting the performance of printed electronics. In this work, chip bonding on plastic substrate, i.e. chip on flex (COF), using anisotropically conductive adhesive and LED chips was investigated. Unpackaged semiconductor chips were bonded to PET foil with printed silver conductors. The yield and reliability of the bonding and the effect of several bonding parameters were studied. The experiments were done using automatic equipment to manufacture LED ribbons. Reliability of the ribbons was investigated by subjecting them to temperature and humidity of 85 °C and 85 % respectively and by bending tests. It was found that with correct choice of manufacturing parameters, it is possible to attain 100 % yield and the samples produced with optimal parameters can withstand the 85/85 environment for period over 100 days. Furthermore, samples produced this way are not damaged by bending them around cylinders with radiuses of 20 mm and 15 mm, for 10 000 and 27 000 cycles respectively. The good tolerance for bending can mostly be attributed to the small size of the chips.