Chunjin Hang

Coupling effects of mechanical vibrations and thermal cycling on reliability of CCGA solder joints

Abstract The microstructures and crack propagation behavior of CCGA (ceramic column grid array) solder joints after sinusoidal vibration loading, random vibration loading, and thermal cycling test have been discussed in this study. The failure mechanism of solder joints was analyzed using an experimental method and finite element analysis. It was found that the failed solder joints mainly distributed at the peripheral area in the solder column arrays and the crack initiation was mainly caused by mechanical vibrations. The deformation of PCB (printed circuit board) introduced by mechanical vibrations brought the outermost solder columns in CCGA devices with significant stress concentration and induced the initiation of cracks. Furthermore, cracks propagated during the process of mechanical vibrations and thermal cycling. The cracks propagated rapidly and the solder joints finally failed. The structure of the PCB holder was improved to relieve the vibration response from the peripheral joints. No visible crack was found in the solder joints after the same mechanical vibrations and thermal cycling test. The reliability of solder joints have been greatly improved with the new PCB holder.

Evolution of Cu/Al Intermetallic Compounds in the Copper Bump bonds during Aging Process

Growth of Cu/Al intermetallic compounds (IMCs) at the interface between the copper bond and the Al metallization pad would increase the contact resistance and degrade the bond reliabilities. In this study, the thermosonic copper bump bonds were aged at 250degC for 1 to 196 hours. Then the evolution of Cu/Al IMCs was investigated. Three IMC layers with different colors were found at the Cu/Al interface using the optical microscopy on the xy plane cross-section of the ball bond. The main phases in the Cu/Al IMC are confirmed to be Cu9Al4, and CuAl2. CuAl is believed to be another phase in the copper bond. During the aging process, Cu/Al IMC grew from the periphery of the bond initially and propagated inward to the centre area later. Moreover, some cavities appeared between the IMC layer and bond bottom from the bond periphery and developed into the centre area.

Secondary optical design of LED lamps with high CRI and adjustable CCT

A novel secondary optical design for LED lamps with high color rendering indices (CRI) and adjustable correlated color temperature (CCT) is proposed. RGB LED cells are assembled in a three dimensional manner together with a set of beam mixers. Perfect light mixing is resulted by optimizing the structural parameters of the reflectors and beam mixers. A supplement amber LED is used for light compensation. Illuminance and color of the luminaire can both be adjusted. CRI larger than 90 is maintained for a very wide temperature range from 4000K to 6500K. The largest Ra valued up to 93 is obtained at 6276K. Except for its outstanding optical characteristics, the present design is also featured by its advantages in simplifying the heat dissipation design. More LED cells could then be included in a lamp of relative small volume to get higher power.

Three-dimensional modelling of solder droplet impact onto a groove

Solder jet technology is widely applied in MEMS packaging. To optimize and control the packaging process, a three-dimensional model was developed to investigate the process of solder droplet impact on a groove with two rectangular pads. In this model, a volume-of-fluid (VOF) method was explored to track the droplet surface and to account for the presence of inertia, viscosity, gravitation, surface tension and wetting effects. The phenomenon of bubble formation was investigated. The numerical model was validated with visualizations of the transient impact processes. The results show that the deformation along the groove is greater than that along the pads. The interactions of the solder gas and solder pad have a significant influence on the droplet shape. The kinetic energy, surface energy and viscous dissipation are the main energy components that influence the impact dynamics, whereas the gravitational potential energy can be neglected. The influences of the impact velocity, droplet size and groove angle on the droplet impingement were quantitatively evaluated.

Cohesively enhanced electrical conductivity and thermal stability of silver nanowire networks by nickel ion bridge joining

A facile method for producing high-performance nickel enhanced silver nanowire (Ag NW) transparent electrodes on a flexible substrate is reported. The modified electroplating method called enhanced nickel ion bridge joining of Ag NWs, which provides a new route for improving the loose junctions in bare Ag NW networks. The sheet resistance of Ag NW electrode drops from over 2000 Ω sq−1 to 9.4 Ω sq−1 with excellent thermal uniformity after the electroplating process within 10 s. Nickel enhanced Ag NW transparent films are applied on flexible heaters with good thermal stability (165 °C for 2 h) and mechanical flexibility (3500 cycles under 2.5 mm bending radius) after mechanical bending process. Moreover, the mechanism of nickel growth is also confirmed that the nickel electroplating of the Ag NWs obeyed Faraday’s Laws.

Rapid sintering of copper nanopaste by pulse current for power electronics packaging

Sintering of metal nanopaste (NP) has been proposed as an alternative in die-attach bonding for the soldering to overcome such high operation temperature in wide-gap semiconductor power devices. Silver (Ag) NP as one of the candidates has some obstacles for mass production due to the high cost of Ag and vulnerability to the electrochemical migration. In the present study, the authors developed a simple solution method to synthesize copper (Cu) nanoparticles with the mean diameter of 57.5 nm. Cu NP was prepared by dispensing as-prepared Cu nanoparticles into deionized water with the assistance of ultrasonic. Pulsed Electric Current Sintering (PECS) was used to join two pieces of bare copper with Cu NP to achieve strong joints in a short time less than 200 ms. Die-shear testing was used to evaluate the bonding strength. The bonding strength increased with the increasing current and reached the highest value of 58.5 MPa at the current of 1.0 kA. Microstructural analysis of the sintered joints showed that the sintered Cu NP layer had a uniform microstructure with micrometer-sized porosity with the potential for high reliability under high-temperature applications. The PECS was recommended to bond two pieces of bare copper with Cu NP in power electronic applications without any protective atmosphere.

Thermal characterization of high power LED array in aluminum base copper clad laminate package

With the increasing power of a single LED chip and the requirement of more LED chips in one package for high illumination, the conventional LED lamps with the surface mound device-printed circuit board (SMD-PCB) package have met some troubles. In the SMD-PCB package, there has too many thermal interfaces and hence has very high thermal resistance from chip to heat sink, which bring the high junction temperature in the LED chips. In this paper, we have proposed a new chip-on-board package structure which has used the aluminum base copper clad laminate as the package substrate. The LED chips are directly bonded on the aluminum based and there has no insulation layer between the LED chips and the metal base. The forward voltage method is used to measure the junction temperature (Tj) of the active LED chips. The Tj of the chips in the new package is found to be 21°C lower than that in the traditional package. The proposed package of chip-on-board LED structure has showed excellent thermal properties.

Growth kinetics of Cu 6 Sn 5 intermetallic compound in Cu-liquid Sn interfacial reaction enhanced by electric current

In this paper, electric currents with the densities of 1.0 × 102 A/cm2 and 2.0 × 102 A/cm2 were imposed to the Cu-liquid Sn interfacial reaction at 260 °C and 300 °C with the bonding times from 15 min to 960 min. Unlike the symmetrical growth following a cubic root dependence on time during reflowing, the Cu6Sn5 growth enhanced by solid-liquid electromigration followed a linear relationship with time. The elevated electric current density and reaction temperature could greatly accelerate the growth of Cu6Sn5, and could induce the formation of cellular structures on the surfaces because of the constitutional supercooling effect. A growth kinetics model of Cu6Sn5 based on Cu concentration gradient was presented, in which the dissolution of cathode was proved to be the controlling step. This model indicates that higher current density, higher temperature and larger joint width were in favor of the dissolution of Cu. Finally, the shear strengths of joints consisted of different intermetallic compound microstructures were evaluated. The results showed that the Cu6Sn5-based joint could achieve comparable shear strength with Sn-based joint.

Reliability prediction of different size solder bumps in thermal shock test using FEM

Three-dimensional structure has been achieved using interlayer connecting technology with double-side solder bump and the lifetime of the solder joints under thermal shock test has been forecasted by finite element method. The simulation result presents that the stress at the interface between 0.76mm solder bump and PCB is evidently higher than that in other places. The interface between 0.76mm solder bump and PCB A is the fracture position in experimental test which indicates that the stress distribution of 0.76mm solder bump is accurate and the finite element model is reasonable. The simulation results have indicated that the size of solder bump has a significant influence on the reliability of Sn63Pb37 solder interconnections under thermal shock tests. The predicted lifetime of 0.76mm diameter solder bump is 259 cycles while that of 2mm diameter solder bump is up to 3605 cycles.

Microstructure evolution and mechanical property of Sn-37Pb and Sn-3.0Ag-0.5Cu BGA solder joints under extreme temperature environment

In this study, the microstructure evolution, mechanical property and fracture behavior of ball grid array (BGA) solder joints under extreme temperature environment (−196 °C to 150 °C) were investigated. Two different kinds of solders, Sn-37Pb and Sn-3.0Ag-0.5Cu (SAC0305) (all wt.%) were employed in this work. A continuous Cu6Sn5 intermetallic compound (IMC) layer was formed at the interfaces between both the Sn-37Pb and SAC0305 solders and the substrate during reflow. After thermal shock, Cu3Sn was formed between Cu6Sn5 layer and Cu pad. The thickness of interfacial IMC layer increased and the morphology of IMC layer transferred from scallop-like to planar-like during extreme temperature thermal shock. With increasing number of extreme temperature thermal shock cycles, the shear forces of Sn-37Pb and SAC0305 solder joints both decreased, and the fracture mode of SAC0305 solder joints transformed from ductile fracture to mixed fracture of ductile fracture and interfacial fracture, while the fracture mode of Sn-37Pb solder joints changed from ductile fracture to a brittle fracture with partial ductile fracture, because of the thickening of IMC layer.