Design Tip
Predicting junction temperature
and MTTF for MMIC devices
By Radha P.N. Setty
T
he power rating of monolithic microwave integrated circuits
(MMIC) is continuously going up. And the life of an MMIC
component due to thermal related failure is directly related to the hot
spot temperature on the die. Hence, circuit designers need to carefully
design the interface between the MMIC component and the ambient
to minimize the thermal related failure or increase the mean-time to
failure (MTTF) of MMIC devices. In this article, we provide a simple
method for thermal design of a printed circuit board (PCB). More
accurate calculation requires performing thermal analysis using any of
the finite element software such as ANSYS and COSMOS.
Thermal resistance is defined as the ratio of temperature rise to the
power dissipated and is expressed in °C/W. Manufacturers of MMIC
components provide the thermal resistance from junction to case (θjc).
Figure 1 shows the cross section of an MMIC component, which
contributes to heat rise.
A packaged MMIC component consists of a semiconductor die (or
chip) mounted on a lead frame. The die is attached to the lead frame
using conductive epoxy or solder. Electrical connections from the die
to lead frame are done using wire bonding. To protect the die from the
harsh environment, it is covered with a plastic molding compound.
The thermal resistance of the hot spot on the die to ambient consists
of a series of thermal resistances:
■ Hot spot on the die to bottom of die ( die )
■ Conductive epoxy (ce )
■ Lead frame (lf )
■ Solder on PCB (sl )
■ Top of PCB to bottom surface (PCB )
■ PCB to ambient ( PCB -A)
Out of the six factors, the sum of the first three is popularly called θjc,
thermal resistance from junction to case, and supplied by the component
manufacturers. The contribution of solder (θsl)
is small and not covered here.
This design tip explains the method of
computing thermal resistance of the PCB
(PCB). For example, with Mini-Circuits’
monolithic amplifier ERA-5XSM, the PCB
layout for an amplifier must consider both
the electrical requirements of the parts (proper
line impedance, bypassing, etc.) and the
thermal requirements. Via holes play an
important part in providing good electrical
and thermal path to ground.
Thermal resistance is defined as (Eq. 1):
l
θ=
k*A
where:
l= length (m),
A= Area in m2
k= Thermal conductivity (W/m-K) .
For an unfilled via (Eq. 2):
Figure 1. Cross section of an MMIC component.
2
A=
2
π (d o − di )
4
where:
do= outer diameter of the via (m) and
di= inner diameter of the via (m).
Substituting Equation 2 into Equation 1, the thermal resistance of
a single copper via can be calculated as (Eq. 3):
4*l
θ=
2
2
k*π *(d o - d i )
Figure 2. Suggested PCB layout of ERA-5XSM amplifier.
68
From the above equation it is seen that, thermal resistance will
decrease as the
■ length of the via decreases
■ via diameter (do) increases
■ copper thickness in the via increases
For ERA-5XSM, the data sheet provides a suggested PCB
layout as reproduced in Figure 2. It consists of 12 via holes
of 0.02-inch diameter (d o ) under DUT. Using 0.002-inch
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August 2005
Figure 3. Thermal simulation of PCB layout of Figure 2.
Figure 4. MTTF vs. junction temperature.
copper thickness, the inside diameter of the via hole is 0.016
inches (di). Also assuming 0.002-inch thickness of copper on
top and bottom of the PCB, the length of the via
is 0.034 inches (which is “l” in Eq. 1 and Eq. 3).
Thermal conductivity of copper is 384 W/m-K.
Substituting these parameters in Equation 3, thermal
resistance of a 20-mil diameter unfilled via hole is
30.8 C/W. For 12 via holes, the thermal resistance
is calculated as if they are in parallel, which is
2.6 C/W. This is a first-order approximation,
as it does not consider the spatial distribution of
the vias.
Figure 3 shows thermal simulation using ANSYS
power dissipation of 1 W. Hence the temperature
rise indicated in Figure 3 is also the thermal resistance, which reads as 2.35 C/W, which is close to
2.6 C/W calculated.
Hence manual calculation of the thermal resistance
as shown can be used as a first-order approximation.
Power dissipation in an ERA-5XSM amplifier is
0.3185 W (device voltage multiplied by operating
current).
There is an additional thermal resistance from PCB to ambient.
The measured temperature rise of ERA-5XSM on the test board
is around 11 C. So one can calculate the thermal resistance from
PCB to ambient as:
 PCB-A = (temperature rise)/Power dissipated = (11)/0.3185
= 34.5 C/W
Hence the total thermal resistance is:
jA = jc + sl + PCB + PCB-A = 133 C + 0 C + 2.6 C + 34.5 C =
170.1 C/W
Hence Tjmax = Junction temperature at the die hot spot
= Max ambient temperature +jA * Pd (Power dissipated)
Assuming a max ambient temperature of 85 C, for ERA-5XSM, the
example under consideration,
Tjmax = 85 C + 170.1 C * 0.3185 = 139.2 C.
Figure 4 shows the MTTF vs. junction temperature for ERA-5XSM. At the
calculated junction temperature, the MTTF
is around 200 years.
References
1. Radha Krishna Setty, Kelvin Kiew and
Harvey Kaylie, “Commercial-off-the-shelf
MMIC components offer high reliability,” RF
Design, February 2005, pp. 12-16.
2. www.minicircuits.com/applications.
html, “Hela-10: High Ip3, Wide Band, Linear
Power Amplifier.”
3. D.R. Pitts and L.E. Sissom, “Heat Transfer,” Schaum’s Outline series, McGraw-Hill
Publications.
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ABOUT THE AUTHOR
Radha P.N. Setty is director of engineering
at Mini-Circuits, Brooklyn, N.Y.
find the best product to suit your needs at: w w w. r a k o n . c o m
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70
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August 2005