In Case You Missed It

Lead-free Solder

“The Impact of Intermetallic Compound on Microstructure, Mechanical Characteristics, and Thermal Behavior of the Melt-Spun Bi-Ag High-Temperature Lead-Free Solder”

Authors: Rizk Mostafa Shalaby and Musaeed Allzeleh

Abstracts: This study aims to study the impact of intermetallic compound on microstructure, mechanical characteristics and thermal behavior of the melt-spun Bi-Ag high-temperature lead-free solder. In this paper, a new group of lead-free high-temperature Pb-free solder bearing alloys with five weight percentages of different silver additions, Bi-Agx (x = 3.0, 3.5, 4.0, 4.5 and 5.0 Wt.%) have been developed by rapidly solidification processing (RSP) using melt-spun technique as a promising candidate for replacement of conventional Sn-37Pb common solder. The effect of adding a small amount of Ag on the structure, microstructure, and thermal properties of Bi-Ag solder was analyzed by means of x-ray diffractometer, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Vickers hardness technique. Applying RSP commonly results in departures from conventional microstructures, giving improved grain refinement. Further, the grain size of rhombohedral hexagonal phase Bi solid solution and cubic IMC Bi0.97Ag0.03 phase is refined by the addition of Ag. Microstructure analysis of the as-soldered revealed that relatively uniform distribution, equiaxed refined grains of secondary IMC Bi0.97Ag0.03 particles about 10µm for Bi-Ag4.5 dispersed in a Bi matrix. Addition of trace Ag led to a decrease in the solidus and liquidus temperatures of solder, meanwhile; the mushy zone is about 11.4°C and the melting of Sn-Ag4.5 solder was found to be 261.42°C, lower compared with Sn-Ag3 solder (263.6°C). This means silver added to Bi enhances the melting point. The results indicate that an obvious change in electrical resistivity at room temperature was noticed by the Ag addition. Also observed was that the Vickers microhardness (Hv) increased with Ag, increasing from 118 to 152 MPa. This study recommends use of Bi-Ag lead-free solder alloys for higher temperature applications. (Soldering & Surface Mount Technology, Feb. 28, 2023, https://doi.org/10.1108/SSMT-03-2022-0015)

OECTs

“Vertical Organic Electrochemical Transistors for Complementary Circuits”

Authors: Wei Huang, et al.

Abstract: Organic electrochemical transistors (OECTs) and OECT-based circuitry offer great potential in bioelectronics, wearable electronics and artificial neuromorphic electronics because of their exceptionally low driving voltages (<1V), low power consumption (<1µW), high transconductances (>10mS) and biocompatibility. Successful realization of critical complementary logic OECTs is currently limited, however, by temporal or operational instability, slow redox processes and/or switching, incompatibility with high-density monolithic integration and inferior n-type OECT performance. Here, the authors demonstrate p- and n-type vertical OECTs with balanced and ultra-high performance by blending redox-active semiconducting polymers with a redox-inactive photocurable and/or photopatternable polymer to form an ion-permeable semiconducting channel, implemented in a simple, scalable vertical architecture that has a dense, impermeable top contact. Footprint current densities exceeding 1kAcm-2 at less than ±0.7V, transconductances of 0.2-0.4S, short transient times of less than 1ms and ultra-stable switching (>50,000 cycles) are achieved in, to the authors’ knowledge, the first vertically stacked complementary vertical OECT logic circuits. This architecture opens many possibilities for fundamental studies of organic semiconductor redox chemistry and physics in nanoscopically confined spaces, without macroscopic electrolyte contact, as well as wearable and implantable device applications. (Nature, Jan. 18, 2023, https://doi.org/10.1038/s41586-022-05592-2)

PCBA Reliability

“Effect of the IMC Layer Geometry on a Solder Joint Thermomechanical Behavior”

Authors: Paulina Araújo Capela, et al.

Abstract: In a printed circuit board assembly (PCBA), the coefficient of thermal expansion (CTE) mismatch among solder joint materials has a detrimental impact on reliability. Mechanical stresses caused by thermal changes of the assembly lead to fatigue and sometimes the failure of the solder joints. The purpose of this study is to propose a novel pad design to obtain an interrupted solder/substrate interface, to improve PCBA reliability. An interruption in the continuous intermetallic compound (IMC) layer of a solder joint was implemented, by the deposition of a silicone film in the pad, changing its geometry. That change permits a redistribution of stresses in the most ductile zone of the solder joint, the solder. The stress concentration at the solder/substrate interface is reduced, as well as the general state of stress at the solder joint. A new way was developed to reduce stress on the solder joints caused by thermal variations because of the different components’ CTE mismatch. This new method consists of strategically interrupting the IMC layers of the solder joint, redirecting the usual stresses to a more ductile area of the joint, the solder. This innovative method increases the lifetime of PCBAs. (Soldering & Surface Mount Technology, Feb. 28, 2023, https://doi.org/10.1108/SSMT-04-2022-0035)

“Effects of Concentration of Adipic Acid on the Electrochemical Migration of Tin for Printed Circuit Board Assembly”

Authors: Yi Sing Goh, et al.

Abstract: Closer interconnection spacing and higher electric field density increases the risk of electrochemical migration (ECM)-related failures. The ECM of tin (Sn) attracts great interest due to the wide use of Sn on the surface of the printed circuit board assembly. In this work, the authors investigated the effects of adipic acid (1ppm–saturated concentration) on the ECM of Sn using the water drop test (WDT) at 5V. In situ observation and ex situ characterization of ECM products were carried out using optical and electrochemical techniques. Results show that ECM failure probability is higher at intermediate adipic acid concentrations (10ppm, 100ppm and 1000ppm). The major ECM reactions include anodic corrosion and the formation of dendrites, precipitates and gas bubbles. ECM failure does not occur at higher adipic acid concentrations (≥5000ppm) although the anodic corrosion becomes more severe. The complexation of Sn with adipic acid to form Sn adipate complex is suggested as the main factor suppressing ECM failure at higher concentrations (≥5000ppm) by retarding ion transport. The electrochemical parameters (Ecorr and Icorr) do not correlate with the ECM failure probability. They affect the anodic dissolution stage, but not the subsequent stages in the ECM mechanism. In this study, the ion transport stage plays a more significant role in determining ECM failure probability. (Journal of Electronic Materials, Jan. 12, 2023, https://doi.org/10.1007/s11664-022-10155-2)