Abstract

In this paper, in order to assemble electronic components onto substrates, a local rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. Among others, the main interest of this technique is that it can allow the use of high temperature melting solders, without the need to heat the whole assembly above this melting temperature. The reactive foil is commercially available and is formed from alternatively stacked nanolayers of Ni and Al until it reaches the total film thickness. Once the film is activated by using an external power source, a reaction takes place and releases such an amount of energy that is transferred to the solder preforms. If this amount of energy is high enough, solder preforms melt and insure the adhesion between the materials of the assembly. The process was evaluated using a standard SAC305 and a high temperature Au80Sn20 preforms. The influences of the applied pressure, the reactive film thickness as well as the solder and the attached materials nature and thicknesses were investigated. The initial joint quality was evaluated using scanning acoustic microscopy, scanning electron microscopy, and shear strength measurements. It was shown that the applied pressure during the process has a strong effect on the joint initial quality. The voids ratio between metallized diode dice and an Active Metal Braze (AMB) substrate decreases from 64% to 26% for pressure values between 0.5kPa and 100kPa respectively. Otherwise, under a constant low pressure of 13kPa, reducing the substrate metal thickness on a low thermal conductivity insulator allows the improvement of the initial joint quality and a voids ratio of about 15% was reached when using 35μm of copper on FR4 substrate. The use of aluminum instead of copper as a metal for the ceramic metallized substrate (with the same gold finishing layer) led to a reduction in the void ratio in the joint. The microstructure of the AuSn joint achieved using the reactive films shows very fine phase distribution compared to the one obtained using conventional solder reflow process in the oven. The mechanical properties of the joint were evaluated using shear tests performed on 350μm thick silicon diodes assembled on AMB substrates under a pressure of 100kPa. The reactive films were 60μm thick and were sandwiched between two 25μm thick SAC preforms. The void ratio was about 37% for the tested samples and shear strength values above 9.5MPa were achieved which remains largely higher than MIL-STD-883H requirements. Finally, the process impact on the electrical properties of the assembled diodes was compared with a commonly used solder reflow assembly and results show a negligible variation.

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