The Detection of Minimum BLT Condition With Electrical Capacitance Measurement

Recently, the increase in performance of semiconductor devices has been accompanied by a simultaneous rise in the amount of heat they emit. Thermal interface materials (TIMs) play an important role in connecting different surfaces to ensure efficient transfer of heat [1]. TIMs can be classified into the following three types: Type 1 materials are greases and pastes, Type 2 materials consist of viscoelastic solids such as gap fillers (an example of which is silicone sheeting), and Type 3 materials are non-compressible solids. Typical Type 1 materials consist of liquid and solid fillers which have a uniform diameter [2]. When pressure is applied, they can easily fill the gap between a heat source and a heat sink, and the gap thickness (BLT: Bond Line Thickness) will eventually decrease to the diameter of the filler. It is well known that under these conditions, the total thermal resistance from a heat source to a heat sink can be minimized. In this study, we demonstrate that we can clearly detect whether the TIM reaches this minimum thickness or not, by measuring the electric capacitance with an LCR meter, instead of evaluating heat related parameters. We performed experiments for a Type 1 TIM which has a viscosity of 240 Pa·s. We hope this technology will be effective, especially in cases of mass-production, because electrical measurement is faster, more repeatable, and more accurate than thermal measurement [3].