Impact of electromigration and isothermal ageing on lead-free solder joints of chip-sized SMD components

Abstract

This paper presents the effect of electromigration and isothermal ageing in lead-free SAC305 (Sn3Ag0.5Cu) solder joints of chip-size surface mounted (SMD) components, where different track-to-pad joining angles define the current density in the joint. The samples were based on 0402 and 0603 size components and were loaded for an extensive period of 4000 h with 2 and 2.5 A, respectively, in an increased temperature environment (40 °C). The solder joint reliability was assessed at selected times during the loading, with shear tests and cross-section analysis on metallurgic structures. Optical and scanning electron microscopy were used to analyse the microstructure of the solder joints. The shear strength of the solder joints was found to decrease with the increasing loading times. Intermetallic compound (IMC) was found on the 4000 h current stressed samples fracture surface, contrary to the other samples. The IMC layer thickness increased with the loading times and was slightly higher for the anode side of the current stressed components. The current stressing resulted in extensive copper dissolution from the pad and IMC layer accumulation at the component metallisation at the cathode side, affecting the reliability of the joints and pointing to electromigration.

Keywords

• Electromigration
• Microstructure
• Reflow soldering
• Passive chip components