Electronic Packaging Science and Technology

11Thermomigration

11.1 Introduction

In Chapter 9, we have mentioned that when an inhomogeneous binary solid solution or alloy is annealed at constant temperature and constant pressure, it will become homogeneous. On the other hand, when a homogeneous binary alloy is annealed at constant pressure but under a temperature gradient, the alloy will become inhomogeneous. This de‐alloying phenomenon is called the Soret effect [1].

On the basis of the Soret effect, we can appreciate it why the study of thermomigration has to wait until the reliability of C‐4 solder joint becomes an issue [2–7]. This is because solder is an alloy or a two‐phase eutectic alloy. On the other hand, while electromigration is significant in Al and Cu interconnects, we have had very little discussion of thermomigration in them.

In 3D IC packaging, because of dense packing, not only Joule heating is serious but also heat dissipation is poor. To enhance heat dissipation, we need to have a high temperature gradient. [8] Unfortunately, a high temperature gradient can lead to fast thermomigration. This is especially serious in a small structure. For example, we consider a micro‐bump of solder joint of 10 μm in diameter. If there is a 1°C temperature difference across the bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration in the micro‐bump.

11.2 Driving Force of Thermomigration

In thermoelectric effect, a temperature gradient can move electrons. Similarly, a temperature gradient ...