EFFECT OF COPPER SUBSTRATE GRAIN SIZE ON THE FORMATION KINETICS AND MORPHOLOGY OF INTERMETALLIC PHASES IN THE CU–SN SYSTEM

Abstract

Joints in the Cu–Sn system are a key element in the fabrication technology of modern microelectronic integrated circuits. At the Cu/Sn interface, during soldering and subsequent service, the intermetallic phases Cu₃Sn and Cu₆Sn₅ are formed. The formation of the Cu₆Sn₅ compound represents the first stage in establishing a metallurgical bond between copper and the solder. Its formation is thermodynamically and kinetically more favorable at the initial stages of reactive diffusion. However, as a result of prolonged thermal exposure, this phase transforms into needle-like structures, which induce brittleness and reduce the mechanical strength of the joint. Further evolution of the interface leads to the formation of the Cu₃Sn phase, which exhibits a higher tendency toward fracture than Cu₆Sn₅. Porosity develops within its structure and microcracks nucleate, ultimately causing degradation and failure of soldered joints.

The present study analyzes how the initial microstructure of the copper substrate affects the growth rate of Cu₃Sn and Cu₆Sn₅ intermetallics during aging at 250°C. For the experiment, copper substrates with average grain sizes of 68.6 μm and 121.3 μm were fabricated, and pure liquid tin was deposited onto their surfaces. Isothermal annealing was then carried out for 15, 30, 45, and 60 minutes. The thickness of the diffusion layers was determined by metallographic analysis. The results show that the initial copper grain size significantly influences the growth rate of the Cu₃Sn + Cu₆Sn₅ intermetallic phases: an increase in grain size leads to a decrease in the intensity of their formation. In addition, the time-dependent evolution of the grain morphology of the intermetallic phases was analyzed, allowing the identification of regularities in their structural transformations.

Soldering is one of the key technological processes widely used to form electrical interconnections in microelectronics. Joints based on the Cu–Sn system play a crucial role in the fabrication of modern microelectronic devices. Due to its relatively low melting temperature, tin serves as the main component of most solders, while copper, characterized by high electrical conductivity, is used as a material for printed circuit boards, electric motors, and transformers.

When the Cu–Sn system is heated, reactive diffusion occurs between the copper substrate and the tin solder, accompanied by the formation of the Cu₆Sn₅ and Cu₃Sn intermetallic phases [1–3]. The Cu₆Sn₅ phase forms first and provides the metallurgical bonding between the solder and the substrate. However, during subsequent aging its morphology changes to a needle-like form, which reduces the mechanical strength of the joint. Thereafter, the growth of the more brittle Cu₃Sn phase begins. This phase is prone to the formation of pores and microcracks, significantly increasing the risk of premature failure of soldered contacts.

In the present work, the influence of the initial morphology of the copper substrate on the growth kinetics of the Cu₃Sn and Cu₆Sn₅ intermetallic phases during isothermal annealing at 250°C was investigated. Copper substrates with different average grain sizes – 68.6 μm and 121.3 μm – were prepared. Pure liquid tin was deposited onto the substrate surfaces, followed by isothermal annealing for durations ranging from 15 to 60 minutes, with a 15-minute time step. The thickness of the formed intermetallic layers was determined by metallographic analysis.

As a result of the experimental investigation of reactive diffusion during isothermal annealing (T = 250°C) in the Cu–Sn system with copper substrates of different grain sizes (68.6 μm and 121.3 μm): It was established that the initial grain size of the copper substrate affects the formation rate of the Cu₃Sn + Cu₆Sn₅ intermetallic phases. As shown in the graph in Figure 5a, an increase in copper grain size slows down the growth of the Cu₃Sn and Cu₆Sn₅ phases. The time dependence of the phase thickness growth on both copper substrates in the Cu–Sn system at the given annealing temperature (T = 250°C) follows a parabolic relationship (see Fig. 5b). The growth rate constant k, as determined from the graph and calculations, is higher for the sample with a grain size of 68.6 μm than for the sample with a grain size of 121.3 μm. This further confirms the influence of copper grain diameter on the rate of the solid–solid reaction and on the growth kinetics of the Cu₃Sn + Cu₆Sn₅ phases.

It is shown that the change in block morphology in the Cu₃Sn and Cu₆Sn₅ phases, at least for the case considered, can be quantitatively described by a parameter that follows a second-order polynomial dependence.