IEEE Power Electronics Magazine - December 2019 - 25

Table 1. Examples of commercially available TIMs.
Thermal Resistance for
10-mm2 Area, 1-mm Thickness
Part Number

Form Factor

Thermal
Conductivity

Uncompressed

50-psi
Compression

Electrical Volume
Resistivity

Dielectric
Strength

Bergquist
Sil-pad 400 [12]

Silicone pad

0.9 W/m·K

Not specified

409 °C/W

1011 X·m

20 kV/mm

Wakefield-Vette
120 [13]

Silicone grease

0.735 W/m·K

136 °C/W

N/A

5 × 1012 X·m

9 kV/mm

Bergquist
GF4000 [14]

Liquid gap filler

4 W/m·K

25 °C/W

N/A

1010 X·m

18 kV/mm

t-Global
TG-X [15]

Ultrasoft silicone
pad

12 W/m·K

19 °C/W

8 °C/W

109 X·m

12 kV/mm

t-Global
TG-S606P [16]

Silicone grease

8 W/m·K

12.5 °C/W

N/A

1012 X·m

Not specified

Laird
TPCM-580 [17]

Phase-change
material

3.8 W/m·K

Not specified

0.8 °C/W

3 × 1010 X·m

Not specified

Heatsink Attachment for Bottom-Side Cooling
For traditional bottom-cooled [e.g., power quad flat no-lead
(PQFN)] devices, the thermal resistance down through the
PCB is the main heat flux path. This typically requires multiple thermal vias or a copper inlay, allowing improved heat
flow through the PCB [18]. On the opposite side, a heatsink
is often added to reduce PCB-to-ambient thermal resistance,
as shown in Figure 4.
Bottom-side cooling presents several design challenges.
First, the thermal vias or copper inlay may block the return
path for the optimum vertical power loop layout [19]. Second, both thermal vias and copper inlays may require a
costlier PCB. Third, the bottom side of the board is typically
left unpopulated so that the heatsink can be mounted flush
against the PCB. If components on the bottom side of the
board interfere with heatsink attachment, the heatsink can
be milled to accommodate them, or a thicker layer of TIM
can be used. Either of these options will impact the cost
and performance of the thermal design.

heatsink and the GaN devices. The spacer height should be
kept to a minimum, but it must be tall enough to account
for variation in the height and tilt of the transistors, spacers, and other nearby components, such as gate drivers
and capacitors. Either a soft highly compressible thermal

Thermal Vias

PQFN
PCB
Heatsink

FIG 4 A cross-sectional diagram of a PQFN packaged device
mounted on a PCB with a heatsink and thermal vias for bottom-side cooling. (Source: EPC; used with permission.)

Heatsink Attachment for Multisided Cooling
For a chip-scale GaN transistor, heat can be extracted
from the top, bottom, and even the sides of the device. Significant heat can still be extracted through the solder bars
or solder bumps to the connected copper area and adjacent layers of the PCB. But in this scenario, the heatsink
and TIM can be attached directly to the top side of the
transistor case, as shown in Figure 5, providing a more
direct path to remove the heat that avoids the added thermal resistance of PCB vias.
Typically, a top-mounted heatsink utilizes some form
of spacer or shim between the heatsink and PCB, along
with a soft TIM. These mechanical considerations are
important to limit excessive force being transferred to
the device, especially during the assembly process. The
spacer determines the minimum distance between the

Heatsink
LGA

Spacer
PCB

TIM

Copper Heat
Spreading

FIG 5 A cross-sectional diagram of a chip-scale LGA device
mounted on a PCB with a heatsink attached for top-side or sixsided cooling. (Source: EPC; used with permission.)

December 2019

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IEEE Power Electronics Magazine - December 2019

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