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Longitudinal Ultrasonic Bonding of Electrodes between Rigid and Flexible Printed Circuit Boards

In this study, two electrodes between a flexible printed circuit board (FPCB) and a rigid PCB (RPCB) are bonded using longitudinal ultrasonic vibration. The surface finish of the electrode of the RPCB is fixed as an electroless Ni/immersion Au (ENIG), whereas two different surface finishes are employed for the FPCB: immersion Sn and ENIG. Electrodes of the RPCB and FPCB are successfully bonded using longitudinal ultrasonic vibration. While a continuous intermetallic compound (IMC) of (CuxAu1-x)6Sn5 is formed at the ENIG/Sn interface, a Au–Ni–P reaction layer is formed with an un-bonded region at the ENIG/ENIG interface. The peel strength of the ENIG/Sn joint is higher than that of the ENIG/ENIG joint. Results of peel tests show that the bonding conditions have a significant effect on joint integrity.