Effect of microwave operating power and reflow time on the microstructure and tensile properties of Sn–3.0Ag–0.5Cu/Cu solder joints

Purpose This paper aims to investigate the morphology and tensile properties of SAC305 solder alloy under the influence of microwave hybrid heating (MHH) for soldering at different microwave parameters. Design/methodology/approach Si wafer was used as susceptor in MHH for solder reflow. Microwave operating power for medium and high ranging from 40 to 140 s reflow time was used to investigate their effect on the microstructure and strength of SAC305/Cu solder joints. The morphology and elemental composition of the intermetallic compound (IMC) joint were evaluated on the top surface and cross-sectional view. Findings IMC formation transformed from scallop-like to elongated scallop-like structure for medium operating power and scallop-like to planar-like structure for high operating power when exposed to longer reflow time. Compositional and phase analysis confirmed that the observed IMCs consist of Cu6Sn5, Cu3Sn and Ag3Sn. A thinner IMC layer was formed at medium operating power, 80 s (2.4 µm), and high operating power, 40 s (2.5 µm). The ultimate tensile strength at high operating power, 40 s (45.5 MPa), was 44.9% greater than that at medium operating power, 80 s (31.4 MPa). Originality/value Microwave parameters with the influence of Si wafer in MHH in soldering have been developed and optimized. A microwave temperature profile was established to select the appropriate parameter for solder reflow. For this MHH soldering method, the higher operating power and shorter reflow time are preferable.

Effect of Intermetallic Compound Bridging on the Cracking Resistance of Sn2.3Ag Microbumps with Different UBM Structures under Thermal Cycling

In this study, the effect of intermetallic compound (IMC) bridging on the cracking resistance of microbumps with two different under bump metallization (UBM) systems, Cu/solder/Cu and Cu/solder/Ni, under a thermal cycling test (TCT) is investigated. The height of the Sn2.3Ag solders was ~10 µm, which resembles that of the most commonly used microbumps. We adjusted the reflow time to control the IMC bridging level. The samples with different bridging levels were tested under a TCT (−55–125 °C). After 1000 and 2000 TCT cycles (30 min/cycle), the samples were then polished and characterized using a scanning electron microscope (SEM). Before IMC bridging, various cracks in both systems were observed at the IMC/solder interfaces after the 1000-cycle tests. The cracks propagated as cyclic shapes from the sides to the center and became more severe as the thermal cycle was increased. With IMC bridging, we could not observe any further failure in all the samples even when the thermal cycle was up to 2000. We discovered that IMC bridging effectively suppressed crack formation in microbumps under TCTs.

Transient Liquid Phase Bonding of Copper Using Sn Coated Cu MWCNT Composite Powders for Power Electronics

In this paper, a novel transient liquid phase bonding material was fabricated by consequent electroless plating of Cu and Sn on a multi-walled carbon nanotube (MWCNT). The resulting Sn-Cu-MWCNT composites were used to join the Cu interconnects at 260°C. After 8 min of reflow time, a complete transformation of Cu3Sn intermetallic compound (IMC) occurred, leaving a Cu/MWCNT-Cu3Sn /Cu joint capable of withstanding the high operating temperature. Due to flake-like morphology, the Sn-Cu-MWCNT composite particles were well packed with lesser voids. The shear strength of the Cu/Cu3Sn-MWCNT/Cu joint was measured as 35.3 MPa, thus exhibiting the scope for replacing conventional transient liquid phase (TLP) powders in the future.

Solder paste wicking in socketable BGAs

The trend of the electronics industry to miniaturize package design has caused the need to adopt BGA packages for a variety of applications. OEM microprocessors have conventionally used LGA designs press fitted into sockets for ease of reworkability. However, BGA packages, which have been widely used for surface mount (SMT) applications, face challenges when they are used in sockets. One of the major challenges faced is the formation of intermetallics between the bare solder ball and the gold paddle of the socket. To address these challenges, Georgia Tech is developing a universal solution for socketing and SMT applications by surface modification of solder spheres. In this new approach, the solder spheres have an outermost noble metal layer which prevents any damage to the BGA and doesn't react with the gold paddle, when placed in a socket. The surface coating would also collapse with the solder ball under specific conditions during reflow when used in SMT applications. This paper focuses on the process of attaching these modified solder spheres on the package and subsequent assembly of the package on the board. We evaluated high and low melting solder pastes to attach the coated spheres to the package. A major challenge observed was the wicking of the solder paste to the entire surface of the solder sphere during reflow. This was addressed by studying the wetting characteristics of the solder paste on different metal surfaces and controlling the volume of solder paste required. Optimization of solder paste volume was done to control wicking and at the same time, achieve a strong and stable joint. Microstructure of the solder joint was analyzed to determine its effect on the joint stability. It was found that the amount of wicking is a strong function of the composition of the solder paste, the reflow time, and the material in surface with which the solder paste is in contact with. This work advances the assembly requirements of socketable BGAs.

Ripening Growth Kinetics of Cu6Sn5 Grains in Sn-3.0Ag-0.5Cu-xTiO2/Cu Solder Joints During the Reflow Process

The ripening growth kinetics of interfacial Cu6Sn5 grains between Cu substrates and Sn-3.0Ag-0.5Cu-xTiO2 (x = 0, 0.02, 0.05, 0.1, 0.3, and 0.6 wt %) (SAC305-xTiO2) solders were investigated. The results show that the Cu6Sn5 grain morphology is affected by the solder composition and the reflow time. The Cu6Sn5 grain size decreases upon addition of TiO2 and shows a significant decrease when the TiO2 nanoparticle fraction is increased to 0.1 wt %. At higher TiO2 nanoparticle fractions, the Cu6Sn5 grain size increases slightly. The growth of the Cu6Sn5 grains is mainly supplied by the flux of the interfacial reaction and the flux of ripening; the ripening flux plays a dominant role because it is approximately one order of magnitude greater than the interfacial reaction flux. The ripening growth of the Cu6Sn5 grains in the TiO2-containing solder joints is reduced more effectively than that of the Cu6Sn5 grains in the TiO2-free joint. For the SAC305/Cu and SAC305-0.6TiO2/Cu solder joints, the particle size distribution (PSD) of the Cu6Sn5 grains is well fit with the Marqusee and Ross (MR) model when the normalized size value of r/<r> is less than 1, and it is consistent with the flux-driven ripening (FDR) model when the value of r/<r> is greater than 1. On the other hand, for the SAC305-0.1TiO2/Cu solder joint, the Cu6Sn5 grains with a nearly hemispheric scallop shape and the PSD of the Cu6Sn5 grains show good agreement with the FDR model.

Effect of Indium Additions on the Formation of Interfacial Intermetallic Phases and the Wettability at Sn-Zn-In/Cu Interfaces

The wettability of copper substrates by Sn-Zn eutectic solder alloy doped with 0, 0.5, 1, and 1.5 at.% of indium was studied using the sessile drop method, with flux, in air, at 250°C and reflow time of 3, 8, 15, 30, and 60 min. Wetting tests were performed at 230, 250, 280, 320, and 370°C for an alloy containing 1.5 at.% of indium, in order to determine activation energy of diffusion. Solidified solder/substrate couples were studied using scanning electron microscopy (SEM), the intermetallic phases from Cu-Zn system which formed at the solder/substrate interface were identified, and their growth kinetics was investigated. The ε-CuZn4 was formed first, as a product of the reaction between liquid solder and the Cu substrate, whereas γ-Cu5Zn8 was formed as a product of the reaction between ε-CuZn4 and the Cu substrate. With increasing wetting time, the thickness of ε-CuZn4 increases, while the thickness of ε-CuZn4 does not change over time for indium-doped solders and gradually disappears over time for Sn-Zn eutectic solder.