Development of a thermal resistance model for chip-on-board packaging of high power LED arrays

Thin film coolers can provide large cooling power densities compared to bulk thermoelectrics due to the close spacing of hot and cold junctions. Important parameters in the design of such coolers are investigated theoretically and experimentally. A three-dimensional (3D) finite element simulator (ANSYS) is used to model self-consistently thermal and electrical properties of a complete device structure. The dominant three-dimensional thermal and electrical spreading resistances acquired from the 3D simulation are also used in a one-dimensional model (MATLAB) to obtain faster, less rigorous results. Heat conduction, Joule heating, thermoelectric and thermionic cooling are included in these models as well as nonideal effects such as contact resistance, finite thermal resistance of the substrate and the heat sink, and heat generation in the wire bonds. Simulations exhibit good agreement with experimental results from InGaAsP-based thin film thermionic emission coolers which have demonstrated maximum cooling of 1.15 °C at room temperature. With the nonideal effects minimized, simulations predict that single stage thin film coolers can provide up to 20–30 °C degrees centigrade cooling with cooling power densities of several 1000 W/cm2.

Design and characterization of thin film microcoolers

The accurate determination of the channel temperature in field-effect transistors (FETs) and monolithic microwave integrated circuits is critical for reliability. An original accurate closed-form expression is presented for the thermal resistance of multifinger FET structures. The model is based on the solution of Laplace's equations in prolate spheroidal coordinates and elliptical cylinder coordinates. The model's validity is verified by comparing the results with finite-element simulations, and experimental observations from liquid-crystal measurements and spatially resolved photoluminescence measurements. Very close agreement is observed in all cases.

Accurate determination of thermal resistance of FETs

A new approach to the dynamic thermal modelling of semiconductor packages

In this paper, a new and more general solution for thermal spreading resistance in compound, orthotropic systems with interfacial resistance is considered. This new solution, which extends beyond previously published results, is obtained for a finite rectangular heat source of uniform strength arbitrarily located on a rectangular substrate. By means of superposition, one can obtain the temperature field in the source plane for multiple heat sources as well as the source mean and centroid temperatures. By means of orthotropic transformations, systems containing orthotropic materials can be easily modeled. Extension of the present solutions using a computationally efficient influence coefficient method is also given, such that the effects of large numbers of heat sources are superimposed. The application of these closed-form expressions for the temperature rise is demonstrated with calculations for Gallium nitride (GaN) high electron mobility transistors (HEMTs). These solutions are shown to be more flexible than previously reported analytical expressions and much more computationally efficient than 3-D finite element analysis, especially for a large number of discrete heat sources associated with multifinger GaN HEMTs.

Thermal Spreading Resistance and Heat Source Temperature in Compound Orthotropic Systems With Interfacial Resistance

The model presented in this work is based on an extension of the constant angle heat spreading, taking into account chip and substrate dimensions, and changing the spreading angle accordingly. This yields to closed form expressions accurate enough for most practical applications. Circular, square, and rectangular geometries have been analyzed and the results compared with Bessel and Fourier series-like solutions.

F.N. Masana

Papers

A closed form solution of junction to substrate thermal resistance in semiconductor chips