Development, Fabrication, and Experimental Study of Flat Polymer Micro Heat Pipes

摘要

An issue of concern with the recent densification of electrical components in integrated circuits is heat removal to avoid damage to the semiconductor structure. Flat heat pipes have been seriously considered and studied over the past 20 years as a solution for thermal management of these devices. In this, flat polymer based micro heat pipes were designed, fabricated and assessed for thermal performance. Novel fabrication processes was developed that uses liquid crystal polymer (LCP) film with copper filled thermal vias and a micro-scale hybrid liquid wicking structure to construct a flat heat pipe suitable for thermal management of semiconductor devices. LCP was chosen for its high chemical resistance, reliability, flexibility, and its ability to be readily incorporated into current printed circuit board production technologies. The microfabrication techniques of photolithography and reactive ion etching were used to form copper filled thermal vias through the polymer to decrease thermal resistance of the casing. Photolithography, wet etching, and electroplating were used to form a hybrid wicking consisting of 200 µm copper pillar forming 31 µm grooves with a woven copper mesh bonded to the top surface. In addition, a novel method for bonding woven metallic mesh to liquid flow channels has been developed. A 250 Å thick layer of atomic layer deposited (ALD) TiO2 was coated on the hybrid wicking structure to enhance the evaporation and capillary force on the liquid in the device. The thermal resistance of the assembled and water charged thermal ground plane displayed a thermal resistance of 0.5 K/W with a power input of 40 W (63 W/cm2) with both adverse and favorable acceleration fields. The same device displayed an effective thermal conductivity of 1653 W/m*K at 0g and 541 W/m*K at 10g acceleration. This high performance suggests that excess capillary pumping pressure was achieved with the hybrid wick. Additionally, flexible thermal ground planes have been developed using multi-layer sintered wick structures and 130 µm thick PET casing material. These devices displayed a thermal resistance up to 4 times less than an equivalent copper reference sample and at a mass of up to 1/6th that of copper.