The contact characteristics and NMOS performance of ACF-bonded Ultra-Thin Chip-on-Flex (UTCOF) interconnects for bendable electronics

In this study, our main goal is to evaluate the effects of residual stress between devices and interconnections after various processes and being applied a bending load on the electrical performances. Moreover, both the Contact Characteristics and metal-oxide-semiconductor field-effect transistor (NMOSFET) NMOS Performance of ACF-Bonded ultra-thin chip-on-olex (UTCOF) Interconnects under bending load are also investigated, a chip with n-type MOSFET was applied to UTCOF package. Firstly, gold stud bumping process was carried out on the NMOS wafer. Then, this wafer was thinning to about 50 μm in thickness. Finally, thin chip was flip- chip bonded onto PI substrate with ACF. Direct current (DC) characteristics of devices were measured after following processes: before bumping, after bumping, after DBG, after bonding, after bending and 85 °C / 85 % RH thermal humidity storage test (THST). The electrical performances of ACF joints were also monitored by measuring contact resistance. The effect of various processes could be quantified from the value of NMOS and contact resistance of ACF joints. Compared with UTCOF samples, a face-up ultra thin chip (UTC) was also laminated onto PET films. After package and lamination process, the THST test was performed under bending. The electrical performance of ACF joints and functionality of NMOS in the UTCOF interconnects have been measured before and after THST reliability test. Based on the results, process-induced residual stress effect is confirmed. The electrical characteristics and failure sequences between devices and joints in the UTCOF during bending and reliability test are understood.