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Parametric study of electronics components joining using reactive films for high temperature applications
Keywords: high temperature solder, reactive films, local heating
Reliable joint between electronics devices and substrates can be achieved by using the solders below 80% (temperature in Kelvin units) of their melting temperature (Tm) while the reflow temperature should be about 30 K higher than Tm. The maximal reached temperature during the reflow may become a limiting factor for the use of high temperature solders. In fact, high reflow temperature can induce the degradation of temperature sensitive materials as well as the substrates bowing. Those issues are commonly encountered in high temperature electronics packaging and can be bypassed when rapid and local heating techniques are used. In this paper, in order to assemble electronics components (power semiconductor dice, capacitors etc.) onto substrates, a local rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. The reactive foil is commercially available and is formed from alternatively stacked nanolayers of Ni and Al until reaching the total film thickness. Once the film is activated, 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 assembled materials. The process was evaluated using a standard SAC and a high temperature AuSn preforms. The influences of the applied pressure, the reactive film thickness and amount, the solder nature and thickness as well as the attached materials thicknesses and their physical properties were investigated. The initial joint quality was evaluated using scanning acoustic microscopy, scanning electron microscopy, and shear strength measurements. It was shown that under a constant low pressure of 13kPa, reducing both the substrate metal thickness and the thermal conductivity of the insulator allow the improvement of the initial joint quality and a void free surface ratio of about 85% was reached when using 35m 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. In addition, the applied pressure during the process has a strong effect on the joint initial quality. The void free attached surface between metallized die and an active metal braze (AMB) substrate (Si3N4/Cu) increases from 34% to 74% for pressure values between 0.5kPa and 100kPa respectively. All the aforementioned results can be explained by the fact that the flow ability of solders is improved by increasing the applied pressure and the solder melting duration, leading to a better joint quality. The solder melting duration can be longer if the thickness and the amount of the reactive film are increased, the die, the substrate metallization and the solder preforms thicknesses are decreased and if the thermal conductivity and the product of the density by the specific capacity of the assembled elements are reduced. The mechanical properties of the joint were evaluated using shear tests performed on 300m thick Si diodes assembled on AMB substrates (Si3N4/Cu) under a pressure of 13kPa by the use of two reactive films of 60m thick each in sandwich between three 25m thick preforms. The void free surface ratio was about 55% for the tested samples and shear strength values above 9.5MPa were achieved which remain largely higher than MIL-STD- 883H standard 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. The microstructure of the AuSn joint achieved using the reactive films shows very fine structure compared to the one obtained using conventional solder reflow process in the oven. This can induce a positive impact on the joint mechanical properties. The results show that joining electronics components using reactive films and solder preforms can be a promising and economically viable technology for high temperature electronics applications. Moreover, the process can allow to attach the electronics packages on non-planar surface and/or on massive structures which remain time consuming and very complex to achieve by using the commonly used solder reflow thermal profile in a furnace.
Rabih KHAZAKA, Research Engineer
Safran SA, Safran Tech, Department of electrical and electronic systems,
Magny-Les-Hameaux, Yvelines

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