AN-9079 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – SPM 2 Series Thermal Performance by Mounting Torque

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AN-9079
SPM^® 2 Series Thermal Performance by Mounting Torque
Overview
Semiconductor devices are very sensitive to junction
temperature. As the junction temperature increases, the
operating characteristics of a device alter and the failure rate
increases exponentially. This makes the thermal design of
the package very important in the device development stage
and in applications. In particular, contact pressure or
mounting torque can affect thermal performance. This
application note shows a correlation between the mounting
torque and the thermal resistance.
To gain insight into the device’s thermal performance, it is
common to introduce thermal resistance, which is defined as
the difference in temperature between two adjacent
isothermal surfaces divided by the total power flow between
them. For semiconductor devices, junction temperature, T_J,
and reference temperature, T_x, are typically used. The
amount of power flow is equal to the power dissipation of a
device during operation. The selection of a reference point
is arbitrary, but the hottest spot on the back of a device on
which a heat sink is attached is usually chosen. This is
called junction-to-case thermal resistance, R_JC. When the
reference point is an ambient temperature, it is called
junction-to-ambient thermal resistance, R_JA. Both are used
for characterization of a device’s thermal performance. R_JC
is usually used for a device mounted on a heat-sink, while
R_JA is for a device used without a heat sink. Figure 1 shows
a thermal network of heat flow from junction-to-ambient for
the motion SPM, including a heat sink. The dotted
component of R_CA can be ignored due to its large value.
T_J
T_C
T_H
T_A
RθJC
RθCH
RθHA
P_D
C_JC R_θCA
C_CH
C_HA
Being ignored
Transient impedance
of each section
Figure 1. Transient Thermal Equivalent Circuit with Heat
Sink
The thermal resistance of motion SPM is defined as:
R_{J C}

T_J  T_C
P_D
(1)
where R_JC (^oC/W) is the junction-to-case thermal
resistance and P_D (W), T_J (^oC), and T_C (^°C) are power
dissipation per device, junction temperature, and case
reference temperature, respectively. By replacing T_C with
ambient temperature (T_A), the junction-to-ambient
thermal resistance R_JA can be obtained as:
R_{J A}

T_J  T_A
P_D
(2)
where R_JA indicates the total thermal performance of the
SPM, including the heat sink. R_JA is basically a
summation of thermal resistances; R_JC, R_CH and R_HA:
R_JA  R_JC  R_CH  R_HA
(3)
where R_CH is contact thermal resistance between the
package case and the heat sink, where the gap is filled with
thermal grease, and R_HA is heat sink thermal resistance.
From Equation (3), it is clear that minimizing not only R_JC,
but also R_CH and R_HA, is essential to maximize the power
capability of the SPM. An infinite heat sink would result if
R_CH and R_HA are assumed to be zero and the case
temperature, T_C, would be locked at the fixed ambient
temperature, T_A. Usually, the value of R_CH is proportional
to the thermal grease thickness and governed by the skills
at the assembly site, while R_HA can be adjusted slihgtly by
selecting an appropriate heat sink.
In practical operations, the power loss, P_D, is not a constant
DC value, but rather an AC value. Therefore, the transient
RC equivalent circuit shown in Figure 1 should be
considered. For pulsed power loss, the thermal capacitance
delays the rise in junction temperature and thus permits a
heavier loading of the 1200 V Motion SPM 2 Series.
© 2014 Fairchild Semiconductor Corporation
Rev. 1.2 • 3/23/15
www.fairchildsemi.com