We investigated the influences of Pd film thickness of electroless Ni/Pd/Au plating on the solder ball joint reliability during reflow cycles and thermal aging, using high-speed solder ball shear test. As the solder ball, Sn-3.0Ag-0.5Cu solder ball was studied. After multiple reflow cycles and thermal aging, we found that the optimum thickness of Pd film with Sn-3.0Ag-0.5Cu solder ball was 0.05-0.2 μm.

View full abstract

Wafer Level Package (WLP) is applied to many mobile and electronics devices. However, bonding reliability of WLP is apparently different compared with Flip Chip Ball Grid allay (FCBGA) even if same solder alloy material is used. The paper will provide information related to the basic strategy for bulk alloy joint sphere structure through solid solution strengthening and shows effect of solder alloy material and design of WLP to bonding reliability

View full abstract

This paper describes a new cooling solution for the thermal management of mobile electronics. A thin loop heat pipe (LHP) with a thickness of 0.6 mm for the evaporator was fabricated by using a chemical-etching and diffusion-bonding process for thin copper plates, and it was designed in a size of 107 mm×58 mm to be embedded in a smartphone. The thermal performance of the thin LHP was evaluated under various operating conditions by measuring the temperature, and its effect on the smartphone was demonstrated. The thermal resistance of 0.8 K/W between the thin LHP’s evaporator and condenser was achieved at 5 W, at an evaporator temperature of 50 ℃ in the horizontal operation of the thin LHP, and minimum resistance was 0.12 K/W at 20 W. We confirmed that the performance of the thin LHP slightly depends on its operating orientation.

View full abstract

We have developed a new wafer stacking technology with solder-base through silicon vias fabricated by ViaS (vertical integration after stacking) process. The technology will provide a low cost solution of 3D integration which can fit to high volume and cost sensitive products such as IoT applications. The structure consists of organic insulator by vapor deposition polymerization and solder as conductor formed by molten solder filling technique. In this paper, we report the process flow, key technologies, a demonstration result, mechanical modeling, and analysis results of the interface interaction between the polymer insulator and the solder via metal.

View full abstract

Cu dissolution of electrodes on printed circuit boards (PCB) and Fe erosion of solder baths have been still the problems in wave soldering by Sn-Ag-Cu lead-free solder, because Sn-Ag-Cu alloy is highly reactive to metals compared with Sn-Pb eutectic alloy. The countermeasure to control dissolution rate of Cu is not only increasing Cu concentration, but also increasing Ni concentration in the molten lead-free solder. Although Ni addition is certainly effective to inhibit Cu dissolution in Sn-Ag-Cu solder, liquidus temperature of the solder rises according to Ni concentration. Therefore, it is concerned that solidification of Cu-Ni-Sn intermetallic compound (IMC) phase may occur in molten solder maintained at soldering temperature. In this study, the influence of Ni concentration of Sn-3.0Ag-0.5Cu-xNi alloy on solderability in wave soldering process is investigated by the observation of separating solder joint on PCB from the surface of molten solder with the high-speed VTR. As a result, it is clarified that the partial solidification slightly influences on fluidity of molten solder, but the wave soldering by Sn-3.0Ag-0.5Cu-xNi alloy will be practical even around the liquidus temperature.

View full abstract

A highly reliable joint structure for a power-semiconductor device was developed. In this structure, a die has a nano-spring layer on its surface and this layer is bonded to a metallic circuit by soldering. The nano-spring layer is a structure composed of multiple nanoscale springs made by the glancing angle deposition method. Thermal stress in the structure is reduced because the nano-spring layer absorbs the difference of the thermal deformations between the die and the metallic circuit. In this study, a manufacturing method for a nano-spring layer made of copper and aluminum was developed and the mechanical characteristics and thermal characteristics of the nano-spring were measured. In addition, a manufacturing process for a power semiconductor device by using a die with the nano-spring layer was demonstrated. This demonstration confirmed that the manufacturing process can be carried out in the same manner as that using a conventional die and successfully manufactures a joint structure composed of a nano-spring layer.

View full abstract

A tensile test and a stress relaxation test were carried out using nano-sized Ag particles pressureless sintered at two levels of temperature to investigate the effects of sintering temperatures on the creep mechanism of sintered nano-sized particles. Inverse analysis using micro scale FEM model revealed that the creep stress exponent of the compact portion in the sintered nano-sized Ag particles was close to the value of dislocation creep. However, the sintered nano-sized Ag particles have a different type of creep mechanism from that of bulk Ag, regardless of sintering temperature levels. This creep is considered to be resulted from the low quality grain boundary structure. And the grain boundary quality does not improve with an increase in sintering temperature. Since the deformation in the low quality grain boundary is considered to be dominant on microscopic behavior of sintered body, sintered nano-sized Ag particles exhibit extremely brittle behavior compared to pure metals.

View full abstract

It is important that evaluations of thermal fatigue life of lead-free solder joints under cyclic temperature changes are performed more than before exactly. We have to conduct FE analysis using a constitutive model in order to estimate the fatigue life of solder joints. There, classical isotropic hardening elastic-plastic-creep model, classical kinematic hardening elastic-plastic-creep mode and Anand model are often used because these models are implemented into commercial FEA soft. In this study, as such mechanical properties of the lead-free solder joints, stress-strain curves, stress-strain hystereses and stress-relaxation curves with these temperature dependencies and these rate dependencies were regarded. And, how accuracy simulation results by these model accord with these experimental test data were investigated. Next, using these models, FE structural analysis of solder joints under cyclic temperature change on actual equipment were conducted and the accumulated inelastic strain behavior were gotten. Finally, the characteristics of each constitutive model were connected with these behavior.

View full abstract