Effects of Current Stressing on Formation and Evolution of Kirkendall Voids at Sn–3.5Ag/Cu Interface

Abstract

Kirkendall voids (KVs) are known to be formed at the Cu/Cu 3 Sn interface, which can remarkably weaken solder joints. In this paper, the formation and evolution processes of KVs at Sn-3.5Ag/Cu joints were systematically investigated under isothermal aging and current stressing. It was found that the processes develop faster when joints are subjected to current stressing as opposed to thermal aging. This can be illuminated by the high KV densities caused by current stressing at both cathode and anode Cu/Cu 3 Sn interfaces. Moreover, KVs formed under current stressing showed some polarity characteristics, namely that higher KV density was observed on the anode side compared with the cathode side. The interfacial reaction generated at the Cu 3 Sn/Cu 6 Sn 5 interface, which was partly affected by current stressing, contributed to this polarity effect. As the holding time was prolonged, microvoids coalesced into larger porosities and microcracks. These defects will greatly threaten the reliability of the interface.

Nucleation and Location of Kirkendall Voids at the Tin‐Based Solder/Copper Joint: A Review

“…The interdiffusion in the reaction area and development of Cu [63] Copyright 2010, Springer. Reproduced with permission.…”

“…[62] Normally, the driving force is assisted by diffusion and it is developed by a chemical potential or concentration gradient. [63] The higher temperature shows the fast dissolution of Cu atoms in the solder and the substrate during the creation of the IMCs (Cu 6 Sn 5 and Cu 3 Sn). Cu 6 Sn 5 IMC formed near the Cu 3 Sn and bulk solder.…”

“…Another option to form the Cu 6 Sn 5 IMCs is due to the chemical reaction of solder and Cu 3 Sn IMC which is found in the bulk area in the solder. [63] Figure 4 demonstrates the schematic Cu concentration profile of Cu-solder diffusion at the interface and the interdiffusion of Cu and Sn arises within each phase. First, atoms of Cu are diffused from the substrate to the Cu/Cu 3 Sn surface due to the concentration gradient, and then the rough Cu 3 Sn layer diffused to the Cu 3 Sn/Cu 6 Sn 5 interface, and finally Cu 6 Sn 5 is established at the Cu 6 Sn 5 /solder through the Cu 6 Sn 5 layer, where reaction with Sn occurs to form new Cu 6 Sn 5 IMC.…”

“…IMCs formation during chemical reactions. [63,68,70,77,81,86,87] Layer vacancy flux. Spherical voids are formed in the reflow process due to dewetting with the substrate from the flux.…”

Intermetallic Compound Layer Growth at the Interface between Sn-Ag and ENImAg Surface Finish

“…This is because each surface finish produced different IMC growth during soldering. Besides, temperature and soldering conditions such as solid diffusion during multiple reflows [16], [26] or isothermal aging also influence the reaction of IMC growth [42], [43].…”