CYTOP for bonding highly oriented pyrolytic graphite to be used as heat spreader in mobile devices

Thin heat spreaders made of materials with high in-plane thermal conductivity are envisioned for the mitigation of hot spots in mobile devices. Among layered materials, highly oriented pyrolytic graphite (HOPG) is promising for hot-spot thermal management. Depending on its thickness, the thermal conductivity of HOPG varies in the range of 700 to 1,600 W/m-K. The superior thermal properties of HOPG cannot be fully exploited if the thermal interface material (TIM) used to bond the HOPG to a silicon chip significantly hinders the heat flow. In this work, we used CYTOP®, a polymer with good flowability, but capable of producing bond lines (BLs) thinner than 2 µm. To compare the mechanical performance of CYTOP®, we used gold-to-gold and copper-to-copper metallurgical bonding as benchmark. The bonding of HOPG with 10 x 10 mm
²
silicon substrates using CYTOP® was performed using a vacuum laminator at a temperature of 180 °C, whereas the bonding with gold or copper was completed using a chip bonder at temperatures of 300 and 400 °C. The mechanical performance of the bonded interfaces was tested using a standard shear test. This test revealed that fractures occur in the HOPG itself by delamination, and not in the BL. Furthermore, after the shear test, microscopies of the fractured joint revealed possible bonding defects that will affect the thermal performance. For the thermal performance, we simulated the effectiveness of HOPG as a heat spreader, as well as the thermal performance of the different bonding materials. With these results, we expect to shed light on the use of materials that can improve the thermal performance of HOPG as a heat spreader, and its compatibility with the manufacturing processes, which will help HOPG to be incorporated in industrial applications.