Study of Thermal Stress Fluctuations at the Die-Attach Solder Interface Using the Finite Element Method

Solder joints in electronic packages are frequently exposed to thermal cycling in both real-life applications and accelerated thermal cycling tests. Cyclic temperature leads the solder joints to be subjected to cyclic mechanical loading and often accelerates the cracking failure of the solder joints. The cause of stress generated in thermal cycling is usually attributed to the coefficients of thermal expansion (CTE) mismatch of the assembly materials. In a die-attach structure consisting of multiple layers of materials, the effect of their CTE mismatch on the thermal stress at a critical location can be very complex. In this study, we investigated the influence of different materials in a die-attach structure on the stress at the chip–solder interface with the finite element method. The die-attach structure included a SiC chip, a SAC solder layer and a DBC substrate. Three models covering different modeling scopes (i.e., model I, chip–solder layer; model II, chip–solder layer and copper layer; and model III, chip–solder layer and DBC substrate) were developed. The 25–150 °C cyclic temperature loading was applied to the die-attach structure, and the change of stress at the chip–solder interface was calculated. The results of model I showed that the chip–solder CTE mismatch, as the only stress source, led to a periodic and monotonic stress change in the temperature cycling. Compared to the stress curve of model I, an extra stress recovery peak appeared in both model II and model III during the ramp-up of temperature. It was demonstrated that the CTE mismatch between the solder and copper layer (or DBC substrate) not only affected the maximum stress at the chip–solder interface, but also caused the stress recovery peak. Thus, the combined effect of assembly materials in the die-attach structure should be considered when exploring the joint thermal stresses.

Optimized Design of Various Ag Decorated Sn-xAg-Cu0.7 Solder Bump on Cycling Fatigue Reliability for Wafer-Level Chip Scale Packaging

Effects of Ag content (0 ~ 3 wt.%) in Sn-xAgCu0.7 solders on microstructure characteristics and low cycling fatigue at different temperature conditions are overall investigated. To increase Ag content, the solidus point 228.8 ? of Sn-Cu0.7 gradually decreases to 218.5 ? and temperature range of solid-liquid coexistence phase is also decrease. The Sn-Cu0.7 matrix consisted of small particles of Cu6Sn5 within ß-Sn equiaxial grains and did not significantly influence solder hardness. Moreover, much intermetallic compound of plate-like Ag3Sn and rod-like Cu6Sn5 existed in Sn-xAgCu0.7 solders enables to enhance the hardness due to dense network of Ag3Sn precipitation and near eutectic point. As a result of plastic displacement decreases with higher Ag additions, better fatigue lifetime could be achieved at Ag content to 1.5 wt.%. Besides, crack stemmed from thicker IMC layer in Sn-3.0Ag-Cu0.7 solder interface will decrease fatigue performance especially for 80 ? and 120 ?.

Joining of SiO2 glass and 316L stainless steel using Bi–Ag-based active solders

Active brazing is a commonly used method for joining dissimilar materials with at least one non-metallic component. In the present study, joining of SiO2 glass to 316L stainless steel was performed utilizing Bi–Ag-based solders. Ti up to a concentration of 4 and Mg up to 1 wt.% were added as active elements. Microstructures of the solder alloys in the as-cast state and of cross sections of the joined compounds were analysed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. In the as-cast state of the solder, Ti is found in Bi–Ti intermetallic phases; Mg is partially dissolved in the fcc-(Ag) phase and additionally contained in a ternary Ag-Bi-Mg phase. After soldering, a tight joint was generated using several alloy compositions. Ti leads to the formation of reaction products at the steel/solder and glass/solder interfaces, and Mg is exclusively accumulated at the glass/solder interface.

Low Conductivity Decay of Sn–0.7Cu–0.2Zn Photovoltaic Ribbons for Solar Cell Application

: The present study applied Sn–0.7Cu–0.2Zn alloy solders to a photovoltaic ribbon. Intermetallic compounds of Cu6Sn5 and Ag3Sn formed at the Cu/solder/Ag interfaces of the module after reflow. Electron probe microanalyzer images showed that a Cu–Zn solid-solution layer (Zn accumulation layer) existed at the Cu/solder interface. After a 72 h current stress, no detectable amounts of Cu6Sn5 were found. However, a small increase in Ag3Sn was found. Compared with a Sn–0.7Cu photovoltaic module, the increase of the intermetallic compounds thickness in the Sn–0.7Cu–0.2Zn photovoltaic module was much smaller. A retard in the growth of the intermetallic compounds caused the series resistance of the module to slightly increase by 9%. A Zn accumulation layer formed at the module interfaces by adding trace Zn to the Sn–0.7Cu solder, retarding the growth of the intermetallic compounds and thus enhancing the lifetime of the photovoltaic module.

Microstructure and Grain Orientation Evolution in SnPb/SnAgCu Interconnects Under Electrical Current Stressing at Cryogenic Temperature

Electromigration was characterized at the cathode Cu/solder interface—without the effect of Joule heating—by employing scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses. Rapid (Cux,Ni1−x)6Sn5 intermetallic compound (IMC) growth was observed at the anomalous region at the cathode end due to the effect of current crowding. The abnormal isotropic diffusion and parallel distribution of Pb were characterized in an ultra-low temperature environment in a monocrystalline structure stressed at −196 °C. The interesting results were attributed to crystallographic transformation due to the simultaneous effect of cryogenic and electrical stressing. The diffusion behavior of Pb atoms in face-centered cubic lattices performed isomorphism. As a result, Pb atoms of the bump gathered at the high-energy grain boundaries by diffusing through the face-centered cubic lattices around the long grain boundary, eventually forming a long-range distribution and accumulation of Pb elements. Our study may provide understanding of cryogenic electromigration evolution of the Cu/solder interface and provide visual data for abnormal lattice transformation at the current stressing.

Fine Pitch Paste for System-in-Package Applications

Water soluble solder paste developed using T7 powder particles revealed good solderability when printed on copper, tin, gold flash nickel plated surfaces and on reflow as well. Its cross-section showed absence of voids, good wetting and soldering to the plated surfaces with angle of contact from 42° to 84° on reflow. All the solder interface are integral with pad/substrate surfaces and revealed formation of tin based intermetallics. T7 solder powder processed using Welco technology showed spherical, clean, smooth, un-agglomerated powder particles with the size range of 2 to 12μm. The developed solder paste is used for fine pitch applications.