Mechanical properties of Sn-Bi bumps on flexible substrate

Abstract

The future electronics will be flexible, bendable, and wearable device. The damages like thermal stress and warpage occurred generally during bonding process between flexible substrate and chip used Pb-free solder for wearable electronic device. Though the wearable electronic devices have been researched, there is few study on bonding technology and reliability of joint between chip with solder bump and flexible substrate. In this study, we investigated joint properties of eutectic Sn-58Bi solder bump formed on flexible substrate finished with ENIG by flip chip process. The Si chip with Sn-58Bi solder bumps and Cu pillar bumps for interconnection were formed by electroplating method. The diameter and height of Sn-58Bi bumps were 25um and 16um, respectively. The Sn-58Bi solder bumps were bonded on flexible substrate at the conditions of bonding temperature of 170°C, bonding time for 15sec, and bonding pressure of 1N, 2N, and 3N. The Si chip and flexible substrate have been designed to daisy chain for measurement of electric resistance. After flip chip bonding, we observed crosssectioned images by SEM. The Cu 6 Sn 5 IMC was formed at the Cu pillar bump side and Ni 3 Sn 4 IMC was formed at the ENIG substrate side. And then, the thermal reliability of joints was evaluated by thermal shock test. The test conditions were in the range or -40°C to 105°C and dwell time for 15min. After thermal shock test, we observe that crack initiated between Cu 6 Sn 5 IMCs and Sn-Bi solder and then propagated within Sn-Bi solder and/or in interface between IMCs and solder. And we also evaluated reliability of bending fatigue. We observed that failure occurred at Ni 3 Sn 4 IMCs/solder and in solder matrix after bending test

Low Melting Temperature Sn-Bi Solder: Effect of Alloying and Nanoparticle Addition on the Microstructural, Thermal, Interfacial Bonding, and Mechanical Characteristics

“…On the other hand, when 0.5 wt% In was added, the ductile fracture surface with dimples were observed. Chen et al [17] reported that tensile strength changed slightly with increases in addition to Sn-(53-58)%Bi, which means tensile strength decreased from approximately 72 MPa to 68 MPa up to 3% In, and increased to 69 MPa at 5% In. Meanwhile the elongation increased remarkably from 20% to 45% by adding 2.5% In, but decreased to 15% by adding 5% In.…”

“…Sn-Bi solder is cost effective as compared to Sn-In and has better thermal reliability due to its relatively higher melting temperature. The low melting point solder is suitable for temperature-sensitive fabrics [16] and flexible substrates [17]. For example, Sn-Bi solder melting point solder is suitable for temperature-sensitive fabrics [16] and flexible substrates [17].…”

“…The low melting point solder is suitable for temperature-sensitive fabrics [16] and flexible substrates [17]. For example, Sn-Bi solder melting point solder is suitable for temperature-sensitive fabrics [16] and flexible substrates [17]. For example, Sn-Bi solder can be used for the bonding of electronic devices on a wearable or flexible substrate, like a fabric, as shown in Figure 1 for smart fabrics [16].…”

Sn–Bi–Ag Solder Enriched with Ta2O5 Nanoparticles for Flexible Mini-LED Microelectronic Packaging

“…6,10,12 The mechanical characteristics of Sn−Bi alloy in polymer matrices have also been investigated. 13,14 Notably, Sn−Bi alloys could be reasonably used to bond electronic components onto smart flexible or wearable fabrics at 180 °C for 60 s 15,16 eutectic solder does, however, have some shortcomings, including poor fatigue and low ductility due to the brittle Birich phase. Brittle Bi phase coupled with intermetallic compounds (IMCs) at the solder/substrate interface raises reliability questions because the bending of flexible printed circuit boards (FPCBs) could result in mechanical shock.…”

“…A minor Ag addition to the Sn–Bi alloy improves the matrix wetting properties onto the conducting substrates. Additionally useful for improving shear strength and drop reliability are the epoxy-decorated solders that create an epoxy layer over the matrix layer. ,− , …”