Furukawa Electric Develops Low-Young’s-Modulus, Heat-Resistant Oxygen-Free Copper “TOFC™”

In recent years, the use of power semiconductors for power conversion and control has been rapidly expanding, driven by the adoption of renewable energy sources including solar and wind power, as well as in applications such as xEVs and data centers. In particular, next-generation power semiconductor modules using SiC (silicon carbide) chips generate more heat as output power and performance increase. Oxygen-free copper is widely used as heat-dissipation substrates and terminals in these modules due to its excellent thermal conductivity.
However, exposure to elevated temperatures during soldering or resin bonding processes causes softening and warpage, leading to reduced bonding reliability. Furthermore, differences in the coefficients of thermal expansion between copper and other materials, such as semiconductor chips, ceramics and resins in the module generate thermal stresses during operation, which can cause delamination or cracking at the bonded interfaces between materials, ultimately leading to malfunction.

In addition to conventional oxygen-free copper (C1020), Furukawa Electric has been mass-producing GOFC®, a heat-resistant oxygen-free copper developed using proprietary materials design and manufacturing technologies that suppress crystal grain coarsening during high-temperature bonding. Furukawa Electric has now expanded its product lineup with the addition of TOFC™, a low-Young’s-modulus, heat-resistant oxygen-free copper. While maintaining high thermal conductivity, TOFC™ achieves excellent heat resistance without softening at high temperatures (Figure 1). As a result, TOFC™ maintains its stiffness even at temperatures above 300 °C, where soldering or resin bonding is performed, which is expected to reduce warpage and deformation. Additionally, TOFC™ exhibits a lower Young’s modulus than conventional oxygen-free copper at temperatures up to 600 °C (Figure 2, Table). This property helps suppress delamination at the bonded interfaces between materials with differing coefficients of thermal expansion, significantly enhancing connection reliability.

With these features, TOFC™ is well suited for applications such as heat spreaders and terminals in power semiconductor modules, as illustrated in the schematic cross-section (Figure 3). Furthermore, when applied to high-current busbars requiring high heat resistance, TOFC™ can suppress strength degradation during high-temperature operation compared with conventional oxygen-free copper and tough-pitch copper. These advantages make TOFC™ suitable for broader applications of TOFC™ in areas such as renewable energy inverters and xEV power control units.

Mass production and sales of TOFC™ are scheduled to begin within the current fiscal year.
Furukawa Electric will continue to supply high-performance oxygen-free copper strip products for a wide range of applications, contributing to the advancement and performance enhancement of power semiconductor modules.