Electronic Packaging Science and Technology

4Solid–Liquid Interfacial Diffusion Reaction (SLID) Between Copper and Solder

4.1 Introduction

Solder has been used to join copper wires to form electrical circuitry connections in every electronic product, as well as to join copper pipes for plumbing in every modern house. The basic process in solder joint bonding is the chemical reaction between molten solder (typically the eutectic SnPb alloy or Pb‐free SnAg alloy) and copper to form intermetallic compounds (IMC) of Cu‐Sn, which have strong metallic bonds. [1, 2] The most important feature in solder joint bonding is the use of flux, which is a chemical reagent to remove the oxide on the Cu surface so that the molten solder can wet the clean Cu surface. The wetting angle, the smaller the better, is a direct indication of the success of a solder joint.

Often, people wonder that in today’s big data era, why do we still study Cu‐Sn, which is a bronze‐age material and has been used for a long time? This is because first we cannot find a better replacement. Nevertheless, there have been very active research on Cu‐to‐Cu direct bonding to replace solder joints as discussed in Chapters 2 and 3. Second, solder joint has become extremely small in size in microelectronic packaging technology for consumer electronic products. Now it is less than 10 μm in diameter, so many unexpected problems in solder joint yield and reliability keep coming up due to the fact that the operation temperature of electronic products is about 100 °C, and ...