Measurement of thermal resistance of first-level Cu substrate used in high-power multi-chips LED package

The thermal resistance of the first-level Cu dissipation substrate (RCu) with different Cu thickness is investigated in this work. Using the “constant-forward-voltage” method, the thermal resistances of the first-level Cu dissipation substrates (RCu) were measured against different Cu thickness. In the initial increase in the Cu thickness (up to 0.6 mm), RCu decreases with the Cu thickness. As the Cu thickness over 0.6 mm, RCu starts to slightly increase with the Cu thickness. The thermal resistance (RCu) of the Cu substrate is composed of the z-direction thermal resistance (Rz) and the two-dimensional horizontal spreading resistance (Rs). The initial decrease in RCu should attribute to the decrease in Rs with the Cu thickness. After the initial increase in RCu, the RCu would increase and be dominated by the Rz increase with the Cu thickness. Intriguingly, a minimum RCu value occurs at the Cu thickness of about 0.6 mm. Also, in this paper, we discuss the possible inaccuracy factors of the “constant-forward-voltage” method.