2

RF Application Information

Freescale Semiconductor

AN1955

Die

Package

Buried in Heatsink 0.010

″ Below Mounting Interface

Heatsink

with IR Microscope

#4−40 Stainless Cap Screws

DuPont Delrin Clamp

Package

Figure 3. Heatsink Temperature Measurement

Figure 4. Exploded View of Clamping Scheme

for Metal Ceramic Devices

mounting interface of the RFPA to the circuit heatsink

(Figure 3) must be measured in certain situations. In these

cases, the 0.032″ diameter hole drilled for thermocouple

passage stops 0.010″ from the circuit heatsink surface. This

method of heatsink temperature measurement is particularly

useful in the following cases:

• If the RFPA is soldered into place where the

spring -loaded TC cannot be used.

• For thermal resistance measurements that include

various interface materials, such as thermal greases

or thermal pads, to determine their performance in the

thermal resistance stack-up.

THERMAL MEASUREMENT SEQUENCE

Before inserting each boltdown metal-ceramic part into the

RF test fixture, a layer of thermal grease (Dow Corningr

340-heatsink compound) is applied to the bottom of the flange

by a roller. A DuPonttDelrinr material clamp is used to apply

downward force to the ears and leads of the package

(Figure 4). This clamp fastens the device to the heatsink using

two #4-40 stainless steel cap screws, each tightened to 5

lb.-in. of torque.

With boltdown overmolded plastic devices, removing the

mold compound in the center portion of the device

compromises the mechanical rigidity of the part. This in turn

affects the flatness of the unit, leading to poor thermal contact

between the package and the heatsink. To correct this, a

solder that is liquid at room temperature (Indalloyr 51 from

Indium Corporationr) is used instead of thermal grease as the

interface material.

The stage on which the RF circuit is secured has the ability

to be electrically heated and cooled by liquid. The temperature

of this stage is adjusted so that the desired case temperature

(usually between 70_C and 90_C) for the part is achieved

during power testing. When the device is secured into the test

circuit, the IR scan is initiated and the desired RF signal and

power are applied. Once the desired case temperature is

reached for the part and is stable, the IR scan image is

captured along with all corresponding electrical data. This

data is recorded, and the corresponding thermal resistance

value is calculated.

The method for determining junction-to-case thermal

for both multi-die RFPA transistor products and multi-stage

RFIC products. For a multi-die RFPA transistor product for a

specified RF test condition, a single value is reported for the

junction -to-case thermal resistance. For a multi-stage RFIC

is reported for each stage.

For a multi-die RFPA transistor product, the highest die

surface temperature (“hot spot”) measured by the IR scan is

dissipated in the product is calculated as

– (RF output power + RF reflected power)

Junction -to-case thermal resistance is calculated as

For a multi-stage RFIC product, the highest die surface

temperature for each stage is measured by the IR scan and