Electronic Packaging Science and Technology

5Solid‐State Reactions Between Copper and Solder

5.1 Introduction

The solder joint after SLID reaction will be annealed around 150 °C up to 1000 hours for reliability consideration. Thus, we need to study solid‐state interfacial reaction between Cu and solder. In the binary Cu‐Sn system, there are two IMCs: Sn/Cu₆Sn₅/Cu₃Sn/Cu. Thus, Cu₆Sn₅ is unstable on Cu, and the reaction between them will form Cu₃Sn. For comparison, in SLID, we have molten solder/Cu₆Sn₅/Cu. If the reflow time is over several minutes, we will have a thin layer of Cu₃Sn between the Cu₆Sn₅ and Cu. In solid‐state reactions, Cu₃Sn forms quickly, and often it is accompanied by Kirkendall voids. Furthermore, Sn whiskers can form on the surface of the solder. This is because the diffusion of Cu into Sn will generate compressive stress in Sn to grow whiskers. A detailed discussion on Sn whisker growth will be presented in Chapter 12 on stress‐migration. We shall discuss Kirkendall void formation in a later section in this chapter.

It is worth noting that in wetting reaction, the time is about 1 min, which is much shorter than the time of 1000 hours in solid‐state reaction. However, it has been found that the amount of IMC formation in these two reactions is about the same, 5–10 μm thick. We recall that in wetting reaction, the IMC is scallop‐type and the diffusion of Cu is via the channels between scallops. The channel has molten solder and the diffusivity of Cu through the molten solder in the channel is about ...