Pulsed Laser micro welding of Si to Cu for die attachment

Abstract

A die attach material joins a semiconductor die (typically Si) to a metallic substrate (typically Cu) for mechanical support and heat dissipation, in microelectronic circuits. In aerospace applications, electrical vehicles (EV), there is increasing demand for die attach technologies that can withstand operating temperatures in excess of 200 0C and sustain high mechanical stresses. Use of lead-based solders is prohibited as per regulations. Implementation of other methods of die attachment such as soldering, eutectic bonding, sintering and epoxy bonding, encounter challenges related to die attachment failure on multiple fronts. This thesis studies the feasibility of high power short pulse laser spot micro-welding process for die attach alternative. Practical concern to reduce energy requirements leads to the enhancement in absorptivity of Cu substrate by application of black marking on its surface exposed to laser. This also reduces the possibility of thermal damage to the joint. The detailed laser parameters, specifically laser peak power and pulse duration, required for damage-free joining, are discussed in Chapter 3, which highlights the effectiveness of the black marking and Ag interlayer in reducing the peak power for joining. Laser spot welds with high peak power and short pulse duration prove to be ideal for micro-joining. The corresponding temperature characteristics and the microstructure evolution have been discussed in Chapter 4. While maximum temperatures attained indicate the potential for phase change, cooling rates can influence the mechanical properties of the joint. Some trials were conducted to evaluate the shear load for failure of the bonds, as specified by MIL-STD-833, for various spot-welding patterns and reported in Chapter 5. This test successfully proves that the utilization of a laser for spot welding is feasible for obtaining a joint with satisfactory mechanical load-bearing capacity.