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.
•  

Solution procedure:

• 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.

Results

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

Conclusions:

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.