Goro Izuta

Dissolution of copper on Sn‐Ag‐Cu system lead free solder

Purpose – The papers purpose is to provide a solution to a problem on dissolution and disappearance of copper electrodes in solder bath in lead free soldering on printed circuit board (PCB).Design/methodology/approach – The influence of the copper concentration, temperature, and flowing velocity of molten solder on the copper dissolution have been estimated, and it has been found that the dissolution rate of copper electrodes in Sn‐3.0Ag‐xCu solder alloys is defined by temperature and copper concentration in solder.Findings – It was found that increasing the copper concentration to 1.5 mass% in Sn‐3.0Ag‐xCu solder could lower the rate of copper dissolution to the equivalent level as that of the conventional Sn‐Pb eutectic alloy at the temperature of 560 K.Research implications/implications – In this paper, a dissolution phenomenon has been studied on Sn‐Ag‐Cu system alloys. It is interesting about the effect of other elements for controlling the dissolution.Practical implications – The method to control ...

Improvement of fatigue life of solder joints by thickness control of solder with wire bump technique [power modules]

The solder joint between the ceramic substrate and the copper base plate is of critical importance to the reliability of the power module. An improvement of the fatigue life of the solder joint is essential to achieve a reliable module. The development of a new assembly technology for the power module improves the fatigue life of solder joints between the ceramic substrate and the copper base plate. The effect of the solder thickness on the fatigue life has been investigated with the elasto-plastic finite element analyses and experiments. The principal factor was the minimum thickness of solder, not the average thickness. The tilt of the substrate caused the solder thickness to become small partially. So the key technology to improve the fatigue life of the solder joint is to prevent the tilt of the substrate, and to control the solder thickness in high-precision level. The technique of setting spacers on the soldering area of the substrate has been examined, and the wire bump technique has been newly developed. In this technique, some small spacers called the wire bump are stitch-bonded near the comers using the aluminum bonding wire.

Development of ball bump forming technology using solder paste and new simplified CSP

We have investigated a new bump forming process and developed a machine using solder paste. The ball bumps formed by the machine satisfied the planned EIAJ standards. The process is as follows. First, cavities are formed by superimposing a stencil onto a base plate. Secondly, they are filled with solder paste using a screening operation and a printed circuit board (PCB) is aligned face down on the stencil. Finally, they are heated together and solder paste is reflowed to be transferred onto the pads. The separable transfer jig is suitable for mass-production compared with block-type one, reducing the flux residue cleaning time, and realizing bump forming with a 0.3 mm pitch. The bump forming machine will be applied to the mass-production of chip scale packages (CSP) with a 0.8 mm pitch at the rate of 100,000 packages a month. We have also developed a new CSP with a unique structure as an application of the bump forming process.

Development of transient liquid phase soldering process for LSI die-bonding

For high-power semiconductor devices with Cu lead-frames in plastic packages, die-bonding process that can obtain a heat-resistant bond (with high melting point) without large residual stress has been developed by applying transient liquid-phase bonding to the soldering process. A three-layered solder sheet the inner layer of which is 95Pb-5Sn solder and the surface layers of which are Pb-Sn eutectic solder is used as the bonding material. The bonding temperature is about 460 K so as to melt only eutectic layers. The eutectic layers are isothermally solidified and the melting point of the bond rises to over 520 K by a two-hour heat treatment at the bonding temperature. As a result, a heat-resistant bond can be obtained whose residual stress can be reduced by more than 1/2 by a low-temperature process.<<ETX>>

Simplified Measuring Method of Copper Concentration in Solder Bath Utilizing Copper Dissolution

In lead-free soldering on printed circuit boards (PCBs), the dissolution and disappearance of PCB copper electrodes in solder bath has been a problem for the dip soldering method, such as wave soldering, in which PCBs are dipped in molten solder. In the former report, it has been estimated the influence of solder composition, temperature, and flowing velocity on the dissolution of copper electrodes by the experiment of the dip soldering process and clarified the dissolution rate for Sn-3.0Ag-xCu alloys, which is defined by solder temperature and copper concentration. Based on above study, the simplified measuring method of copper concentration in the solder bath has been developed, which uses the dissolution of wedge-shaped copper pattern formed on FR-4 PCB. As a result of the experiment on dipping the PCB in solder of Sn-3.0Ag-xCu (x = 0.5, 0.9, 1.2 and 1.5) alloys, it has been confirmed that the method enables the measuring copper concentration in the solder bath easily.

Reliability of die attached AlN-DBC module using Zn-Sn high temperature lead-free solders

Interfacial reaction and reliability of Si die attached joints with Zn-xSn (chi= 20, 30, 40 wt%) were investigated, and Pb-5wt%Sn, and Au-20wt% were compared as a typical high temperature solder alloy. The joining strength of Cu/solder/Cu joints was investigated using the as soldered joints and thermal cycled joints up to 2000 cycles. At the interface with Si die coated with Au/TiN thin layers, the TiN layer did not react with the solder and worked as a good protective layer. At the interface with Cu, for Zn-Sn solders, CuZn5 and Cu5Zn8 IMC layers was confirmed. Au-20Sn and Pb-5Sn solder forms zeta-Au5Sn and Cu3Sn IMC layers at the interface with Cu, respectively. For improving the joint reliability, TiN layer was proposed as a protective layer and was applied to the DBC substrates. For the die attached joints on DBC substrates coated with Au/TiN thin layers, no fatigue crack has been found until 1000 cycles of thermal cycling test. The joining strength of the Cu/solder/Cu joints with Zn-Sn solder was about 30~34 MPa, which was higher than that of Pb-5Sn solder (26.2 MPa). For the thermal cycled Cu/solder/Cu joints up to 2000cycles, Zn-Sn solder also shows the higher joining strength than that of Pb-5Sn solder.

IC fillet-lifting mechanism on wave soldering after reflow soldering

Solder joint separation of IC leads is caused by partial melting and displacement of a lead-free solder joint having Pb contamination in process of wave soldering after reflow soldering. An evaluation method of solder joint separation was developed by focusing on the vertical displacement of the joint, which is defined by IC package size and curvature of a printed circuit board (PCB). It has been clarified that the critical displacement causing the solder joint separation is 15 micrometers or less by the model experiment of wave soldering process. Based on the above data, a new in-process measurement tool of PCBs curvature in wave soldering has been developed. The results of practical wave soldering agree well with the model experiment values