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 Cu 6 Sn 5. On the FPCB side, Cu 6 Sn 5 and Ni 3 Sn 4 were formed by contact with the facing Sn coating, and mechanically alloyed Cu 0.81 Ni 0.19 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.
In order to evaluate the reliability of Sn-8 mass%Zn-3 mass%Bi (hereafter, Sn-8Zn-3Bi) solder, thermal shock tests (temperature range: from 233 to 353 K) and high temperature humidity tests (test condition: 353 K, 85% relative humidity) were conducted. The PCB (Printed Circuit Board) pads were finished by OSP (Organic Solderability Preservative), Sn and Ni/Au. The electric part for test was QFP (Quad Flat Package), and the lead was plated by Sn-3 mass%Bi. The joint strength was estimated by pull testing, and Zn-phase behavior in solder joint was examined.The pull strengths of Sn-8Zn-3Bi joints in the as-soldered state were measured to be about 16N, irrespective of the PCB pad coatings. After thermal shock testing (hereafter, TS test) up to 1000 cycles, the pull strengths decreased to around 14N. Meanwhile, after high temperature humidity testing (hereafter, HTH test) up to 1000 h, the pull strengths deceased to around 6N, irrespective of the PCB pad coatings. The reason for the drastic decrease of pull strength after the HTH test was proven to be a crack that was observed along the zinc oxide-phase. The Zn-phase in the Sn-8Zn-3Bi solder changed from a separate acicular shape to a lined-up branched arrangement after HTH tests of 1000 h.
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