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

Article Information:

Title:

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

Author(s):

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

Journal:

Soldering & Surface Mount Technology

Year:

2009

Volume:

21

Issue:

1

Page:

4 - 10


ISSN:

0954-0911


DOI:

10.1108/09540910910928256

Publisher:

Emerald Group Publishing Limited


Acknowledgements:

This study was carried out with the financial support of Seoul R&BD project of 2008.

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Abstract:

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.

Keywords:

Bonding, Joining processes, Printed circuits


Article Type:

Research paper


Article URL:

http://www.emeraldinsight.com/10.1108/09540910910928256

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