Thermal Considerations for PAs
AN-0003
Thermal Considerations for Power Amplifiers
Overview
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.
Gold Wire
Mold Compound
DIE
0.9mm
Lead
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 T
max
. 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 curves
1
for the fabrication processes used in the
ANADIGICS
MESFET power amplifiers. Various applications may require different maximum junction
temperatures depending upon the MTTF requirements.
05/2003
ANADIGICS [ ANADIGICS, INC ]
