Steady state and transient thermal analysis of chip scale packages

This paper summarizes a study of chip scale packages (CSPs) to determine their maximum allowable power dissipation within typical system level environments. These results can be used to determine the applicability of utilizing CSPs from the standpoint of die power dissipation. Both steady state and transient thermal performance is covered in this study. The steady state portion used in-house software, while closed-form solutions were used for the transient analysis. The steady state power limit, while governed by a number of parameters, is dependent mainly upon system level parameters (heat sinking, cooling mode, i.e. natural or forced convection, and PCB power loading). Thermal enhancement features (e.g. thermal vias and bumps) are not generally effective in increasing the maximum power that can be dissipated by the package in the end use environment. The variables investigated in the steady state study included die size, thermal vias and bumps, the addition of a heat sink, natural/forced convection boundary conditions, printed circuit board (PCB) heat loading, and PCB thermal conductivity. The transient portion considered die size, pulse shape and duration, and the addition of a heat sink. For relatively short duration transients (e.g. switching an inductive load), the power limit is governed by the die geometry; magnitude, shape and duration of the heating pulse; and the starting and maximum allowable temperatures of the junction.