In the convective cooling processes of electronic components with metal-foam heat sinks, the waste heat generated by an electronic component passes through the heat sink wall (abbreviated as HSWALL) in contact with the electronic component and then into the metal foam. In this study, we increased the surface area of the HSWALL by vertically placing a solid aluminum cylinder in the center (i.e., core) of an aluminum-foam heat sink to enhance the cooling performance of the heat sink. The contact ratio (a), defined as the ratio of the surface area of the extended HSWALL to the solid-fluid interface area inside the aluminum-foam heat sink, was varied systematically to experimentally investigate the heat transfer characteristics under impinging-jet flow conditions. The experimental results showed the dominant heat-transfer mechanism changed when the contact ratio increased. When the contact ratio increased from 0 to 0.013, the Nusselt number first increased and then decreased. When the contact ratio was equal to 0.00676, the Nusselt number reached a maximum of approximately 2.2 times that of the sample without a core. The increase in the Nusselt number when alpha < 0.00676 was caused by the increased surface area of the HSWALL and that the core allowed the cooling air to enter immediately the effective heat dissipation region near the surface of the core. On the other hand, the decrease in the Nusselt number when alpha > 0.00676 was due to the fact that the area of the gas-solid interface of the aluminum foam was not sufficiently large to effectively transfer the waste heat from the core to the cooling air. The measured temperature distributions showed the existence of a local thermal equilibrium when alpha was greater than 0.00676, and a non-local thermal equilibrium when a was less than 0.00676. This change from a local to a non-local thermal equilibrium was caused by the reduced solid-fluid interface area resulting from the reduced aluminum-foam volume at larger contact ratios. The measured Nusselt number was found to increase with increases in both the overall height and the outlet height of the heat sink. (C) 2016 Elsevier Ltd. All rights reserved.