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AN1955 Datasheet(PDF) 2 Page - Freescale Semiconductor, Inc |
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AN1955 Datasheet(HTML) 2 Page - Freescale Semiconductor, Inc |
2 / 5 page 2 RF Application Information Freescale Semiconductor AN1955 Die Package Heatsink Temperature (TH) Measured with Thermocouple Buried in Heatsink 0.010 ″ Below Mounting Interface Heatsink Die Temperature (TJ) Measured 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 HEATSINK TEMPERATURE (TH) MEASUREMENT The heatsink temperature (TH) directly beneath the 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. THERMAL RESISTANCE, qJC, CALCULATION The method for determining junction-to-case thermal resistance (θJC) under a chosen RF test condition is described 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 product, the junction-to-case thermal resistance (θJC-stage) 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 used as TJ in the thermal resistance calculation. Total power dissipated in the product is calculated as Pdiss = (RF input power + DC power (ID * VD)) – (RF output power + RF reflected power) Junction -to-case thermal resistance is calculated as θJC = (TJ - TC) / Pdiss For a multi-stage RFIC product, the highest die surface temperature for each stage is measured by the IR scan and |
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