The effect of formic acid surface modification on the bond strength of the solid state bonded interface of tin has been investigated by SEM observations of the interfacial microstructures and fractured surfaces. Formic acid surface modification was carried out by boiling a tin surface (finished by electrolytic polishing) in formic acid for 750 s. Solid state bonding was carried out in a vacuum chamber at a bonding temperature T j of 383 453 K and a bonding pressure P j of 7 MPa (bonding time=1.8 ks). Because of surface modification, bonded joints were obtained at a bonding temperature that was ~50 K lesser than the typical temperature required, and the bond strength was comparable to that of the base metal. When the joint strength was much lower than the base metal strength, a number of fine inclusions were found to be distributed in the interfacial region. As the joint strength increased with bonding temperature, these inclusions were coarsened, and their distribution density decreased. The increase in joint strength with bonding temperature corresponded well with the observed change in the size and density of the inclusions.
To examine the effect of Bi ller metal on bond strength of the Sn/Sn bonded joint interface, the interfacial microstructures and fractured surfaces of joint were observed by using SEM. After Bi ller metal had been deposited to the surface, the diffusion bonding was carried out in a vacuum chamber at bonding temperature of 413~463 K. The application of ller has decreased bonding temperature by 20 K or more which the bonded joints obtained bond strength comparable to the base metal. As the bonding temperature increases, the thickness of the Bi diffusion layer increases as well. Moreover, the interfacial hardness has decreased with a rise in bonding temperature, and the failure mode changes from brittle to ductile. The changes in the interfacial eutectic reaction layer between Sn and Bi accompanied by the expansion of the contact area between Sn metal surfaces are considered as the contributing factor to the increase in the bond strength.
The effect of acetic acid surface modification on the superficial oxide film that covers the solid state bonded joint of tin has been investigated by conducting SEM and TEM observations of the interfacial microstructures and fractured surfaces. Acetic acid surface modification was carried out by boiling a tin surface in acetic acid for 900 s. Solid state bonding was carried out in a vacuum chamber at a bonding temperature T of 403 483 K and a bonding pressure P of 7 MPa (bonding time=1800 s). It was observed that the bond strength increased with the bonding temperature, regardless of the acetic acid surface modification. Because of surface modification, bonded joints were obtained at a bonding temperature that was 40 K lesser than the typical temperature required, and the bond strength was comparable to that of the base metal. When the joint strength was much lower than the base metal strength, belt like inclusions were found to be distributed in the interfacial region. As the joint strength increased with bonding temperature, these inclusions were coarsened, and their distribution density decreased. The increase in bond strength with bonding temperature corresponded well with the observed change in the size and density of the inclusions. This tendency was observed at a bonding temperature that was 40 K lesser than that of the joint that did not undergo surface modification. Therefore, it is believed that the joint that underwent surface modification possessed a high tensile strength despite having a low bonding temperature; this is because tin () acetate coheres at a temperature lower than required for the coherence of an oxide film, and the area of the metal to metal contact increased at a low temperature.
1. AbstractEquipment for evaluating the wettability of solder paste has been developed using a laser displacement method. Using the equipment, the descent behavior of an electronic part in reflow soldering can be measured with a laser displacement meter. From the measured height displacement curve, the wetting time and spread rate of the solder paste onto the electrodes of an electronic part and a substrate can be investigated. The developed equipment can detect the difference in the wettability of solder paste which depends on the solder paste type, the electrode type, and the degradation of the solder paste over time.
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