TITLE : Thermal conduction analysis of a Plastic Quad Flat Pack (PQFP)
BY : CHAM Development Team - F Liu DATE : 1999 FOR : Demonstration Case PHOENICS Version : HOTBOX VR
NOTES : Three-dimensional, conduction only, Cartesian computational grid
Description of the problem:
- This case is to simulate a situation that heat is dissipated by conduction only in the absence of air movement, which is commonly encountered when plastic packaging techniques are used for electronic components;
- For example, the requirement of thermal design of a PQFP is to reduce temperature difference between the component and the landing PCB as small as possible;
- The PQFP studied here is a compact CFD model sitting on a PCB, comprising the die, the die attach, the lead frame, the bond wire, the plastic encapsulant and the thermal interface;
- The lead frame consists of a die paddle and lead fingers;
- The thermal interface is made up of three layers: solder, copper and thermal paste, click here to see Overall view of the PQFP, the view without encapsulant and the cross-sectional view;
- The die contains 1W heat output; and
- The temperature are maintained at 85°C at the edges of the PCB.
- The solution for velocity and pressure are switched off; thus only conduction is calcualted.
The geometrical dimensions and thermal conductivities used in the CFD model are listed in the following table
Part Dimension (side*side*thickness) (mm) Model thermal conductivity (W/mK) Object type in HOTBOX Package size 28*28*3 - - Die 9.525*9.525*0.5 117.5 Blockage Die attch 9.525*9.525*0.025 2.5 ThinPlate Bond wire 11*11*0.1524 11.7 Blockage Lead frame 30*30*1.068 116.3 Blockage Thermal paste 30*30*0.1524 0.48 Blockage Copper 30*30*0.254 391.0 Blockage Solder 30*30*0.1016 35.0 Blockage PCB 60*60*3 15 Blockage Encapsulant 28*28*1.424 0.67 Blockage
- To set up the correct package structure, the encapsulant block has to be entered before the die, the die attach and the bond wire, since the latter entered objects overwrite the properties of the previously entered blocks over the region in which they are intersect; and
- The die attach is very thin. In oder to avoid allocating grid cells to it so to prevent convergence difficulty due to large differences in cell sizes, HOTBOX uses an object called 'ThinPlate' that has zero thickness in numerical grid and a thermal resistance calculated from the real conductivity and thickness.
The following figures show the temperature distributions of the PQFP dissipating 1W with the thermal paste/copper/solder interface at several layer heights.
Temperature contours above the component
Temperature contours across the component
Temperature contours across the lead frame
Temperature contours across the thermal interface
Temperature contours across the PCB
This case demonstrates how HOTBOX-VR handles
- pure conduction in the event of no air movement;
- large disparity in component dimension and thermal conductivity; and
- the use of ThinPlate, which is an important feature to mimic thermal reality with moderate numeric effort.