Thermal Considerations for PAs
Thermal Considerations for Power Amplifiers
Proper heatsinking to control junction temperature is an extremely important consideration for use of all
power amplifiers. Gallium Arsenide (GaAs) devices can tolerate considerably higher junction
temperatures than Silicon (Si), but due to its lower thermal conductivity, it requires more consideration
than Si to remove heat. The thermal conductivity of GaAs is only about one third that of Si, and it has a
nonlinear relationship with temperature (the conductivity worsens with increasing temperature).
Although many factors inside and outside the package influence the junction temperature, the end user of
the power amplifier can control the implementation of the connection and layout on the printed circuit
board (PCB). The choice of the board material and thickness, the number of thermal vias placed beneath
the part and the design of the heatsink are all important factors in properly using the part under difficult
thermal requirements.
Application Requirements
ANADIGICS power amplifiers are typically packaged in LPCC (Leadless Plastic Chip Carrier) packages.
These packages offer excellent thermal characteristics due to the thin copper paddle used for mounting
the chip. The part itself (bare die) is also very thin and enables very good heat dissipation. A cross
section of the package is shown in Figure 1.
Mold Compound
Gold Wire
Solder Plating
Copper Lead Frame
Die Attach Epoxy
Figure 1. Cross section of an LPCC package.
The thermal design of such a part involves maintaining the junction temperature below a certain level,
defined as Tmax. The junction temperature depends directly on the case temperature, defined as the
temperature at the bottom of the copper lead frame. Although each application is different, junction
temperatures of 150°C are considered desirable with an almost infinite device life. Junction temperatures
of 180 °C are typical in many applications and yield device MTTFs (Median-Time-To-Failures) better than
1 million hours (114 years). Figure 2 indicates the MTTF curves1 for the fabrication processes used in the
ANADIGICS MESFET power amplifiers. Various applications may require different maximum junction
temperatures depending upon the MTTF requirements.
Figure 2. Typical MTTF data for TriQuint MESFET, HFET, and PHEMT.
Dissipated pow er
In some applications, the device performance needs to be de-rated to ensure that the maximum
acceptable junction temperature (Tmax) is not exceeded. Figure 3 shows a generic de-rating curve. The
device can dissipate the maximum power for case temperatures less than Td. For case temperatures
greater than Td, the dissipated power is limited as indicated in Figure 3 in order to keep the junction
temperature below Tmax. ANADIGICS performs detailed thermal analysis of the junction to case
temperature using finite difference software. These analysis results have been verified through the use of
infrared microscope measurements, and are used to develop the de-rating curves.
T ma x
Case te
mp erature
Figure 3. Generic de-rating curve.
ANADIGICS has developed a generic thermal analysis for power amplifiers. Although each design has
unique implementation requirements, the typical dimensions of a 4x4 mm part are presented in Figure 4.
0.1 mm
0.01 mm
0.2 mm
0.9 mm
Thermal vias
2.8 mm
4 mm
2.8 mm
4 mm
Figure 4. Typical dimensions of a 4x4 mm part.
PCB Thermal Capability
In typical applications the user should place a large number of thermal vias beneath the mounting pad for
the paddle. For a 4x4 mm part mounted on a 0.032" (0.8 mm) PC board, it is possible to place
approximately 16 vias under the part with each fabricated using a 0.014" (0.35 mm) drill. This drill size is
typically used for holes of 0.010" (0.25 mm) finished size. Typically the plating thickness will be 0.001"
(0.025mm). Thicker copper plating will lower the thermal resistance, helping to reduce the junction
The thermal resistance of a single copper via can be calculated as:
σ π (RO − RI )
Where RTH is the thermal resistance in °C/W, σ is the thermal conductivity in W/m*K, and the radii (R O
and RI) and length (L) of each via are in meters. For a 0.032" thick board, with 0.001" plating, and using a
thermal conductivity of 390 W/m*K, the thermal resistance of a single via is on the order of 80°C/ W. The
total thermal resistance is simply the result of Equation 1 divided by the total number of thermal vias
beneath the part. For 16 thermal vias this yields approximately 5 °C/W.
Thermal simulations are also used in ANADIGICS IC design process. Figure 5 shows the results from
an FR4 board simulation and the temperature distribution through the 16 thermal vias. The simulation
included a 3.4W power amplifier die, which is not shown. The expected temperature rise through the
board from previous calculations is 5 °C/W x 3.4W =17°C, in agreement with the plotted data.
As an example, a device with junction to case thermal resistance of 30°C/W, combined with a PCB layout
configuration of 5 °C/W, will show a junction temperature rise of 35°C/W. For a device operating at 7V,
400 mA and delivering no RF power, all of the 2.8 Watts of power must be dissipated as heat. The part
will operate at a lower temperature when it is delivering RF power to the load. This can sometimes
contribute to “gain-expansion” when evaluating output power versus input power. In this example the
junction temperature would rise by 98 °C. For typical operation where the case temperature reaches
85°C, the corresponding junction temperature would be 183 °C.
Figure 5. Thermal simulation of 16 thermal vias.
Manufacturing Suggestions
To properly use the parts in a manufacturing environment, it is recommended that an additional reflow
process is used to ensure solder has filled the thermal vias beneath the part. The steps are as follows:
1.) Apply solder paste to area containing thermal vias; 2.) Reflow the board and the solder paste will wick
through the via holes; 3.) Cover the backside of the board beneath the power amplifier with Kapton tape
and reflow the part to the board. The solder will remain in the via holes and the power amplifier will be
properly connected to the thermal/ground pad below it.
In some applications it will be possible to mount the PCB directly to the chassis at least in the critical area
beneath the power amplifier. This is highly recommended as it provides the shortest path to the large
surface area of the chassis. If this is not possible, then a heatsink should be attached directly beneath
the amplifier. Heatsinks will be specified in degrees Celsius of temperature rise per Watt of dissipated
power. Since the amplifiers are in small packages, the heatsink must be located directly beneath the
amplifier and maintain good thermal contact with the board.
In order to maintain good thermal contact between the heatsink and the backside of the PCB, an interface
material should be used to fill any voids. Many heatsink compounds are not very good thermal
conductors but are inert, low cost and fill in the air gaps. Typical thermal conductivities of heatsink
compounds are on the order of 0.6 W/m*K. These compounds should be used sparingly, and applied in a
very thin layer. Alternative materials have been developed with better thermal properties such as
Chomerics T725 THERMFLOW ™ or Thermoset’s CoolPhase™ MPC-120. These materials are phase
change interface materials. At room temperature they are typically applied as thin sheets, and must be
heated slightly to cure and spread the material.
The Chomerics T725 exhibits a thermal impedance of 0.03 °C-in^2/W, and is available in 0.005" thick
sheets. Below is the contact information of the two companies mentioned above.
THERMFLOW is a trademark of Chomerics.
Div. of Parker Hannifin
77 Dragon Court
Woburn, MA 01888-4014
Ph: 1-781-935-4850
CoolPhase is a trademark of Thermoset, Lord Chemical Products.
Thermoset, Lord Chemical Products
5101 E. 65th Street
Indianapolis, Indiana 46220-0902
Ph: 1-800-746-8343
In general, it is important to keep the junction temperature of the devices low to extend the life of the part
and to improve the overall performance. Proper heat sinking of GaAs devices will ensure optimum
performance over the life of the part in your design.
TriQuint Semiconductor website,
141 Mount Bethel Road
Warren, New Jersey 07059,U.S.A.
Tel: +1(908)668-5000
Fax: +1(908)668-5132
E-mail: [email protected]
ANADIGICS, Inc. reserves the right to make changes to its products or to discontinue any product at any time without notice. The product
specifications contained in Advanced Product Information sheets and Preliminary Data Sheets are subject to change prior to a product’s formal
introduction. Information in Data Sheets have been carefully checked and are assumed to be reliable; however, ANADIGICS assumes no
responsibilities for inaccuracies. ANADIGICS strongly urges customers to verify that the information they are using is current before placing orders.
ANADIGICS products are not intended for use in life support appliances, devices or systems. Use of an ANADIGICS product in any such application
without written consent is prohibited.