Ultrasonic bonding of flexible PCB to rigid PCB using an Sn interlayer

KyooSeok Kim, JaePil Jung, Y. Norman Zhou

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


Purpose – The aim of the paper is to study the feasibility of direct ultrasonic bonding between contact pad arrays on flexible printed circuit boards (FPCB) and rigid printed circuit boards (RPCB) at ambient temperature. Design/methodology/approach Metallization layers on the RPCB comprised Sn on Cu while the pads on the FPCB consisted of Au/Ni/Cu. Prepared RPCB and FPCB were bonded by ultrasound at ambient temperature using an ultrasonic frequency of 20 kHz, a power of 1,400 W, and 0.62 MPa of bonding pressure. The bonded samples were crosssectioned and the joints and microstructures were observed by Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS). The soundness of the joints was evaluated by pull testing. Findings Robust bonding between FPCB and RPCB was obtained by bonding for 1.0 and 1.5 s. This result has confirmed that direct room temperature ultrasonic bonding of Au and Sn is feasible. At a longer bonding time of 3.0 s, cracks and voids were found in the joints due to excessive ultrasonic energy. The IMC (intermetallic compound) between the Sn layer and pads of the RPCB was confirmed as Cu6Sn5. On the FPCB side, Cu6Sn5 and Ni3Sn4 were formed by contact with the facing Sn coating, and mechanically alloyed Cu0.81Ni0.19 was found within the pads. Meanwhile, the strength of bonded joints between FPCB and RPCB increased with bonding time up to 1.5 s and the maximum value reached 12.48 N. At 3.0 s bonding time, the strength decreased drastically, and showed 5.75 N. Footprints from the fracture surfaces showed that bonding started from the edges of the metal pads, and extended to the pad centers as ultrasonic bonding time was increased. Originality/value Direct ultrasonic bonding with transverse vibration at ambient temperature between the surface layers of the pads of FPCB and RPCB has been confirmed to be feasible.

Original languageEnglish
Pages (from-to)4-10
Number of pages7
JournalSoldering and Surface Mount Technology
Issue number1
StatePublished - 6 Feb 2009


  • Bonding
  • Joining processes
  • Printed circuits


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