Introduction Characteristics Model The thermal performance of semiconductor packages is a very important consideration for the board designer. The reliability and functional life of the device is directly related to its junction operating temperature. As the temperature of the device increases, the stability of its junctions decline, as does its reliable life. The thermal performance is also important for the board design, because it may limit the board density, or dictate the board location of high power dissipating devices, or require expensive cooling method for the system. As devices have become more complex and boards have become denser, the need to account for the thermal characteristics of packages has shifted from being a minor consideration to being a necessary consideration. The thermal performance of a package is measured by its ability to dissipate the power required by the device into its surroundings. The electrical power drawn by the device generates heat on the top surface of the die. This heat is conducted through the package to the surface and then transferred to the surrounding air by convection. Each heat transfer step has corresponding resistance to the heat flow, which is given the value R, the thermal resistance coefficient. Subscripts are added to the coefficient to specify the two points that the heat is transferred between. Commonly used coefficients are Rja (junction to ambient air), Rjc (junction to case) and Rca (case to ambient). Package Thermal Characteristics Technical Note An electrical analogy can be made, as shown in the figure below, to illustrate the heat flow of a package. The heat transfer can be characterized mathematically by the following equation: Tj-Ta=P x Rja Where P = Device operating power (Watts) Tj = Temperature of a junction on the device (C) Ta = Temperature of the surrounding ambient air (C) Rja = Rjc + Rca in C/W Rja Rca Heat Flow cavity Die Rjc Package Rev. B - 22-Aug-01 1 Packages Graphs The graphs show typical values of Rja versus pin count for few dice dimensions at 3 different dissipation power values, in still air; because of the difficulty to perform accurate measurements, some points may diverge from the natural trend, as in the next graph where the natural trend should be that the Rthj-A should asymptote to 30-35C/W rather than seeming to be diving to 0. R T h eta ja in S till Air D ie siz e 56 m m 70 60 R Theta ja 50 40 0,5 W 30 1,5 W 20 3 W 10 0 0 50 100 150 200 250 P in Co un t R T heta ja in Still Air Die size 76 mm 70 60 R Theta ja 50 0.5 W 40 1.5 W 30 3W 20 10 0 0 50 100 150 200 250 300 Pin Count 2 Rev. B - 22-Aug-01 R T heta ja in Still Air Die size 121 mm 60 50 R Theta ja 40 1W 2,5 W 30 4W 20 10 0 0 50 100 150 200 250 300 Pin Count R Theta ja in Still Air D ie siz e 169 mm 50 45 40 R Theta ja 35 1W 30 25 2,5 W 20 15 4W 10 5 0 0 50 100 150 200 250 300 350 400 Pin Count 3 Packages Rev. B - 22-Aug-01 Packages This graph shows typical values of Rja versus pin count for few dice dimensions, in 1m/s air flow. R Theta ja for 1m/s Air Flow 40 35 30 25 R Theta ja 57mm 76mm 20 121mm 15 169mm 10 5 0 0 50 100 150 200 250 300 Pin Count Rjc is not reported because of secondary effect in the total R value. In any case it is below 1C/W. 4 Rev. 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