Effect of thickening outermost layers in Al/Ni multilayer film on thermal resistance of reactively bonded solder joints

Abstract

To reduce the thermal resistance of solder joints obtained by Al/Ni self-propagating exothermic reaction, we focus on the interface between the solder and the reactive Al/Ni multilayer before the reaction. We fabricate an Al/Ni film in which the outermost Al or Ni layer is deposited more thickly. By the laser flash method with response function analysis, the thermal resistance of the bonded specimens was measured in the case of thickening and non-thickening of the outermost layer. It is found that the resistance decreases with the thickness of the outermost layers, particularly the Ni layer. To investigate the influence of the amount of void on the resistance, scanning acoustic tomography (SAT), scanning electron microscope (SEM) observation, and field-emission electron probe micro-analysis (FE-EPMA) were carried out. Differences in the interfacial state between Ni-thickening and Al-thickening specimens are discussed in light of solder wettability, diffusion into the solder, and the type of intermetallic compound at the solder interface. It can be considered that thickening the outermost Ni layer has a great impact on decreasing the number of voids generated at the AlNi–solder interface even in low-pressure bonding, leading to reduction in the thermal resistance of solder joints.

Effect of Free-standing Al/Ni Exothermic Film on Thermal Resistance of Reactively Bonded Solder Joint

“…Voids are traced and binarized for SEM images using the same analysis method as in our previous work. (18) Three types of void fraction were quantitatively analyzed. The white, blue, and red colors indicate the void area fractions in the solder, at the top-side solder interface, and at the bottom-side solder interface, respectively.…”

“…This would be related with the formation of a SnNi intermetallic compound. (18) Note that, however, almost no voids were found at the other interface regardless of the combination of Al and Sn (although some sub-microsized voids were newly produced); that is, the number of voids was less in the newly bonded interface regardless of the combination of atoms. Consequently, we experimentally confirmed that the solder joints with the voidless interface had a low-thermal-resistance bonded section.…”

“…The thermal resistance was measured by the laser flash method with response function analysis. (19) All the experiments were performed at room temperature in air using the ULVAC TC7000 system by the same procedure as described in our previous work, (15,16,18) and three fabricated specimens each were used for the as-sputtered and free-standing Al/Ni joint. When a pulsed laser is incident on the planar surface of a specimen, one-dimensional thermal diffusion occurs.…”

“…Many voids and cracks produced in the joint made the thermal resistance high. (16,18) By using the free-standing Al/Ni film, the thermal resistance decreased to 2.0 × 10 −6 m 2 KW −1 , a 29% reduction from that of the assputtered Al/Ni joint. In the 20 MPa bonding, the thermal resistance decreased further to around 1.7 × 10 −6 m 2 KW −1 , which is almost one-half that of the as-sputtered Al/Ni joint at 3 MPa, regardless of the Al/Ni film preparation method.…”

“…(17) By applying a Ni layer onto the solder on both sides, the number of voids at the interface could be reduced. (18) This would be related to the formation of a SnNi intermetallic compound at the interface.…”