Heat Transfer in a Surface-Mount Package for a Silicon Chip

PDF

Heat Transfer in a Surface-Mount Package for a Silicon Chip

All integrated circuits — especially high-speed devices — produce heat. In today’s dense electronic system layouts heat sources are many times placed close to heat-sensitive ICs. Designers of printed-circuit boards often need to consider the relative placement of heat-sensitive and heat-producing devices, so that the sensitive ones do not overheat.

One type of heat-generating device is a voltage regulator, which can produce several watts of heat and reach a temperature higher than 70°C. If the board design places such a device close to a surface-mounted package that contains a sensitive silicon chip, the regulator’s heat could cause reliability problems and failure due to overheating.

Figure 1: Layout of the simulated silicon device, its package, and a voltage regulator. The chip and the voltage regulator are connected through a ground plate, a pin, and the interconnect.

This simulation investigates the thermal situation for a silicon chip in a surface-mount package placed on a circuit board close to a hot voltage regulator. The chip is subjected to heat from the regulator and from internally generated heat.

Model Definition

The model is based on a SMD IC and voltage regulator layout as in Figure 1. The silicon chip sits in the center of the package and dissipates its heat to the surrounding environments. The chip also connects to a ground plane through an interconnect and one of the pins. A heat generating voltage regulator is placed on the same ground plane. This means that the voltage regulator may affect the silicon chip by the conducted heat and this may lead to overheating of the chip.

Heat transfers through the mount package to the surroundings through conduction according to:

The heat source, Q, is negligible in the circuit board, pins and package, while in the chip, this model sets that parameter to a value equivalent to 20 mW. The conductivities of the components are chosen to be similar to:

•

silicon, for the chip

•

aluminum, for the pins

•

FR4, for the PC board

•

copper, for the ground plane and interconnect

•

an arbitrary plastic, for the chip package

Heat dissipates from all air-exposed surfaces through forced heat convection, which is modeled using a heat transfer coefficient, h:

The voltage regulator is simulated by setting a fixed temperature at that surface. The thin conducting layers of the ground plane and interconnect within the package is modeled using a 2D shell approximation, according to:

where ds is the layer’s thickness, and ∇t represents the nabla operator projected onto the direction of the plane. The model uses a Heat Transfer interface to describe the 3D heat transfer as well as the 2D shell heat transfer.

Results and Discussions

Figure 2 illustrates the temperature distribution through the thickness. Being a good conductor, the interconnect delivers heat to the outer edge of the package, which gives the fairly constant temperature distribution around the interconnect.

Figure 2: Slice plot of the temperature through the circuit board, interconnect, chip, and package. The effect of the interconnect is evident by its ability to conduct heat from the chip to the outer parts of the package.

An alternative view is achieved by using the transparency feature in the visualization tools of COMSOL Multiphysics. This results in a transparent 3D view of the temperature distribution, as shown in Figure 3. In that figure you can see the temperature distribution around the chip and along the interconnect.

Figure 3: Boundary plot of the temperature created with the assistance of the transparency tool in COMSOL Multiphysics. This view also gives the temperature distribution on the chip and along the interconnect.

To get a closer look at the stationary temperature of the silicon chip, plot the temperature at the bottom boundary of the chip.

Figure 4: Temperature distribution on the bottom surface of the silicon chip.

The simulation predicts a maximum temperature of the silicon device of 47.7°C. This means that the device does not overheat in the present configuration.

Heat_Transfer_Module/Power_Electronics_and_Electronic_Cooling/surface_mount_package

Notes About the COMSOL Implementation

This tutorial uses the Heat Transfer interface from the Heat Transfer Module and, in particular, its Thin Layer feature. The thin layers of the modeled device are good heat conductor, so the Thin Layer feature is well-adapted for their representation. The two layers that have this definition are:

•

The interconnect between the chip and the grounded pin.

•

The ground plate that is also thermally connected to the temperature constraint coming from the voltage regulator.

While the numerical method considers these two modeling domains as interior boundaries, the model still includes a thickness to take the 3D heat flux into account.

Modeling Instructions

From the menu, choose .

In the window, click

.

1

In the window, click

.

2

In the tree, select .

3

4

5

In the tree, select .

6

The geometry sequence for the model is available in a file. If you want to create it from scratch yourself, you can follow the instructions in the Geometry Modeling Instructions section. Otherwise, insert the geometry sequence as follows:

1

In the toolbar, click and choose .

2

Browse to the model’s Application Libraries folder and double-click the file surface_mount_package_geom_sequence.mph.

3

In the toolbar, click

.

4

button in the toolbar.

You should now see the geometry shown above.

The geometry includes the interconnect between the pin and the chip as well as the ground plate and the temperature surface resulting from the voltage regulator. To see the interconnect and the chip, you need to turn on transparency.

5

button in the toolbar.

6

button in the toolbar again to remove the transparency.

Now, define the domain settings including material properties, element order, heat source, and initial values.

To define material properties for the model domains, use four predefined materials from the and one custom material.

1

In the toolbar, click

to open the window.

2

Go to the window.

3

In the tree, select .

4

Click in the window toolbar.

5

In the tree, select .

6

Click in the window toolbar.

Aluminum (mat1)

1

In the window, under click .

2

In the window for , locate the section.

3

From the list, choose .

FR4 (Circuit Board) (mat2)

1

In the window, click .

2

In the window for , locate the section.

3

From the list, choose .

1

In the toolbar, click

.

2

In the window for , type Plastic in the text field.

3

Locate the section. From the list, choose .

4

In the window, expand the node, then click .

5

In the window for , locate the section.

6

.

7

In the dialog box, type density in the text field.

8

9

In the tree, select .

10

11

In the window for , locate the section.

12

.

13

In the dialog box, type heatcapacity in the text field.

14

15

In the tree, select .

16

17

In the window for , locate the section.

18

.

19

In the dialog box, type thermalconductivity in the text field.

20

21

In the tree, select .

22

23

In the window for , locate the section.

24

In the table, enter the following settings:


Property	Variable	Expression	Unit	Size
Density	rho	2700	kg/m³	1x1
Heat capacity at constant pressure	Cp	900	J/(kg·K)	1x1
Thermal conductivity	k_iso ; kii = k_iso, kij = 0	0.2	W/(m·K)	3x3

1

Go to the window.

2

In the tree, select .

3

Click in the window toolbar.

4

In the tree, select .

5

Click in the window toolbar.

6

In the toolbar, click

to close the window.

1

In the window, under click .

2

In the window for , locate the section.

3

From the list, choose .

1

In the window, click .

2

In the window for , locate the section.

3

From the list, choose .

4

From the list, choose .

This completes the materials settings.

Heat Transfer in Solids (ht)

1

In the window, under right-click and choose .

2

Select Domain 11 only.

3

In the window for , locate the section.

4

In the Q0 text field, type 2e8.

This completes the domain settings. Now, set up the boundary conditions.

1

In the toolbar, click

and choose .

2

In the window for , locate the section.

3

From the list, choose .

4

Locate the section. From the list, choose .

5

In the h text field, type 50.

6

In the Text text field, type 30[degC].

1

In the toolbar, click

and choose .

2

Select Boundary 4 only.

3

In the window for , locate the section.

4

In the T0 text field, type 50[degC].

1

In the toolbar, click

and choose .

2

Select Boundary 7 only.

3

In the window for , locate the section.

4

From the list, choose . In the Lth text field, type 1e-4[m].

5

Locate the section. From the list, choose .

1

In the toolbar, click

and choose .

2

In the window for , locate the section.

3

From the list, choose .

4

Locate the section. From the list, choose . In the Lth text field, type 5e-6[m].

5

Locate the section. From the list, choose .

1

In the window, under right-click and choose .

2

In the window for , locate the section.

3

From the list, choose .

4

Select Boundaries 4 and 7 only.

5

Locate the section. From the list, choose .

Free Tetrahedral 1

In the toolbar, click

.

1

In the window, click .

2

In the window for , locate the section.

3

From the list, choose .

4

.

The mesh should consist of around 30,000 elements.

In the toolbar, click

.

By default, you get surface and isosurface plots for the temperature. Note that the temperature is displayed in kelvin, which is the default temperature unit in the SI system.

To get a surface temperature plot in degrees Celsius, simply change the unit for the first default plot group.

1

In the window, expand the node, then click .

2

In the window for , locate the section.

3

From the list, choose .

1

In the window, click .

2

In the window for , locate the section.

3

From the list, choose .

4

Click to expand the section. From the list, choose .

1

In the window, click .

2

In the window for , locate the section.

3

From the list, choose .

4

Locate the section. From the list, choose .

5

In the toolbar, click

.

6

button in the toolbar.

To see the chip as well, turn on transparency.

7

button in the toolbar.

Compare the resulting plot to that in Figure 3.

8

button in the toolbar again to remove the transparency.

Reproduce the plot in Figure 2 with the following steps.

Temperature, Slices

1

In the toolbar, click

and choose .

2

In the window for , type Temperature, Slices in the text field.

1

In the toolbar, click

.

2

In the window for , locate the section.

3

From the list, choose .

4

Locate the section. From the list, choose .

5

Locate the section. From the list, choose .

6

In the toolbar, click

.

Compare the result to that in Figure 2.

To visualize the temperature distribution on the silicon chip’s bottom surface, follow the steps given below.

Temperature, Chip Surface

1

In the toolbar, click

and choose .

2

In the window for , type Temperature, Chip Surface in the text field.

3

Locate the section. Clear the check box.

1

In the toolbar, click

.

2

In the window for , locate the section.

3

From the list, choose .

4

Locate the section. From the list, choose .

1

In the toolbar, click

.

2

Select Boundary 195 only.

3

In the toolbar, click

.

4

button in the toolbar.

Compare the resulting plot to that in Figure 4.

Geometry Modeling Instructions

If you want to create the geometry yourself, follow these steps.

1

In the window, under click .

2

In the window for , locate the section.

3

From the list, choose .

1

In the toolbar, click

.

2

In the window for , type PC Board in the text field.

3

Locate the section. In the text field, type 20.

4

In the text field, type 10.

5

Locate the section. In the text field, type -10.

6

In the text field, type -5.

7

In the text field, type -1.9.

8

Locate the section. Select the check box.

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type 9.9.

4

In the text field, type 3.9.

5

In the text field, type 0.2.

6

Locate the section. From the list, choose .

Hexahedron 1 (hex1)

1

In the toolbar, click

and choose .

2

In the window for , locate the section.

3

In row , set to -4.95.

4

In row , set to 4.95.

5

In row , set to 4.95.

6

In row , set to -4.95.

7

In row , set to -4.95.

8

In row , set to 4.95.

9

In row , set to 4.95.

10

In row , set to -4.95.

11

In row , set to -1.95.

12

In row , set to -1.95.

13

In row , set to 1.95.

14

In row , set to 1.95.

15

In row , set to -1.713419348.

16

In row , set to -1.713419348.

17

In row , set to 1.713419348.

18

In row , set to 1.713419348.

19

In row , set to 0.1.

20

In row , set to 0.1.

21

In row , set to 0.1.

22

In row , set to 0.1.

23

In row , set to 0.75.

24

In row , set to 0.75.

25

In row , set to 0.75.

26

In row , set to 0.75.

27

In row , set to -1.72.

28

In row , set to -1.72.

29

In row , set to 1.72.

30

In row , set to 1.72.

Mirror 1 (mir1)

1

In the toolbar, click

and choose .

2

Select the object only.

3

In the window for , locate the section.

4

Select the check box.

1

In the toolbar, click

and choose .

2

In the window for , type Chip Package in the text field.

3

Select the objects , , and only.

4

Locate the section. Clear the check box.

5

Locate the section. Select the check box.

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type 0.4.

4

In the text field, type 0.26.

5

In the text field, type 0.2.

6

Locate the section. In the text field, type -4.645.

7

In the text field, type -2.21.

8

In the text field, type -0.1.

Revolve 1 (rev1)

1

In the toolbar, click

.

2

On the object , select Boundary 3 only.

3

In the window for , locate the section.

4

Click the button.

5

In the text field, type 90.

6

Locate the section. From the list, choose .

7

Find the subsection. In the text field, type -2.211.

8

In the text field, type -0.24.

9

Find the subsection. In the text field, type 1.

10

In the text field, type 0.

Extrude 1 (ext1)

1

In the toolbar, click

.

2

On the object , select Boundary 2 only.

3

In the window for , locate the section.

4

In the table, enter the following settings:


Distances (mm)
0.322

Revolve 2 (rev2)

1

In the toolbar, click

.

2

On the object , select Boundary 3 only.

3

In the window for , locate the section.

4

Click the button.

5

In the text field, type -90.

6

Locate the section. From the list, choose .

7

Find the subsection. In the text field, type -2.69.

8

In the text field, type -0.561.

9

Find the subsection. In the text field, type 1.

10

In the text field, type 0.

Extrude 2 (ext2)

1

In the toolbar, click

.

2

On the object , select Boundary 2 only.

3

In the window for , locate the section.

4

In the table, enter the following settings:


Distances (mm)
0.16

1

In the toolbar, click

and choose .

2

Select the object only.

3

In the window for , locate the section.

4

Clear the check box.

1

In the toolbar, click

and choose .

2

Select the object only.

3

In the window for , locate the section.

4

In the text field, type 8.

5

Locate the section. In the text field, type 1.27.

6

Locate the section. Find the subsection. Click .

7

In the dialog box, type Pins in the text field.

8

Mirror 2 (mir2)

1

In the toolbar, click

and choose .

2

In the window for , locate the section.

3

Select the check box.

4

From the list, choose .

5

Locate the section. Find the subsection. From the list, choose .

6

Locate the section. In the text field, type 1.

7

In the text field, type 0.

Work Plane 1 (wp1)

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type -0.9.

Work Plane 1 (wp1)>Plane Geometry

In the window, click .

Work Plane 1 (wp1)>Rectangle 1 (r1)

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type 6.

4

In the text field, type 4.

5

Locate the section. In the text field, type -10.

6

In the text field, type -5.

7

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (mm)
Layer 1	2

8

Clear the check box.

9

Select the check box.

1

In the window, under right-click and choose .

2

In the window for , type Interconnect in the text field.

3

Locate the section. Select the check box.

Interconnect (wp2)>Plane Geometry

In the window, click .

Interconnect (wp2)>Rectangle 1 (r1)

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type 4.145.

4

In the text field, type 2.15.

5

Locate the section. In the text field, type -4.645.

6

In the text field, type -1.95.

Interconnect (wp2)>Rectangle 2 (r2)

1

In the toolbar, click

.

2

In the window for , locate the section.

3

In the text field, type 3.745.

4

In the text field, type 1.75.

5

Locate the section. In the text field, type -4.245.

6

In the text field, type -1.95.

Interconnect (wp2)>Difference 1 (dif1)

1

In the toolbar, click

and choose .

2

Select the object only.

3

In the window for , locate the section.

4

Find the subsection. Click to select the

toggle button.

5

Select the object only.

1

In the window, under right-click and choose .

2

In the window for , type Chip in the text field.

3

Locate the section. In the text field, type 0.1.

4

Locate the section. Select the check box.

5

Locate the section. From the list, choose .

Form Union (fin)

1

In the window, click .

2

In the window for , click

.

1

In the toolbar, click

and choose .

2

In the window for , type Geometry in the text field.

3

Locate the section. From the list, choose .

4

Select the object only.

Exterior Boundaries

1

In the toolbar, click

and choose .

2

In the window for , type Exterior Boundaries in the text field.

3

Locate the section. Click

.

4

In the dialog box, select in the list.

5

1

In the toolbar, click

and choose .

2

In the window for , locate the section.

3

From the list, choose .

4

On the object , select Boundaries 7 and 37 only.

5

In the text field, type Copper Layers.