Comprehensive Study of Tin-Silver-Copper Lead-Free Alloys on Various Bond Pad Metallisation

Abstract

SnAgCu305 (SAC305) solder alloy is extensively used as solder bump interconnects in industry due to their high strength and good reliability performance. SAC305 solder bump is applicable both to 1st and 2nd level solder interconnects. It connects the chip and board together to provide the electrical connection, heat dissipation and mechanical strength to the whole package [1], [2]. Wettability of the molten solder ball onto the bonding pad is crucial. The formation of the solder ball to the bond pad will determine the electrical connections and impact the reliability results. Conventional reflow soldering process is a commonly used in the assembly packaging. It is a process which involved the flux being apply onto the metal bond pads first. Solder ball is then being place onto the metal bond pad by using stencils. Excess residues from the flux after reflow are then remove via cleaning. Hence, creating solder bumps and interconnections in the assembly packaging. This standard process which included the cleaning step may not be applicable to Optoelectronics and MEMS packaging [3]. The cleaning process can cause damage to the sensitive structures. Laser solder ball jetting technology which used a flux-less method is introduce to mitigate the handling of the fragile structures. Gold metallisation surface is an excellent bonding material for the bond pad to ensure good solder wettability. Due to cost consideration, Palladium, Platinum and Copper materials are explored as an alternative to expensive gold finishing. We will cover and study the laser solder ball jetting process comparing SAC305 solder ball onto Gold, Palladium, Platinum and Copper surface finishing. Initial results have shown that the average ball height for SAC305 with Au surface is 42.46um. The ball height for the Pd and Pt are comparable with measurement of 57.33um and 56.12 um. The ball height of SAC305 with Cu surface is the highest, with measurement of 60.28um as shown in Table 1. These prove that the Au have the best wettability as compared to the rest as shown in the Figure 7. Ball shear strength, failure mode and intermetallic growth of the SAC305 onto the Gold(Au), Palladium(Pd), Platinum(Pt) and Copper(Cu) surface finishing will be further study and discuss in the following sections.