AN1955 1 rf application information freescale semiconductor thermal measurement methodology of rf power amplifiers by: mali mahalingam and edward mares introduction this document explains the methodology used by freescale for thermal measurement of high power rf (radio frequency) power amplifiers (rfpa). semiconductor device reliability heavily depends on device operating temperature so the accurate thermal characterization of these high power devices is crucial in establishing the reliability of the systems that use such devices. die surface temperature (t j ) measurement infrared (ir) microscopy is used to determine the die surface temperature (t j ) during amplifier operation. because this ir measurement method requires a direct view of the die, the protective ceramic lid is removed and replaced with a modified lid that has an opening to view the die. in the case of overmolded plastic packages, the center portion of the mold compound is etched away until the die is suf ficiently exposed. because the heat flow from the device to the heatsink is dominated by conduction, the measurement error caused by the removal of the lid or removal of the mold compound around the die surface is negligible. the exposed die is coated with a high emissivity coating (see appendix) to obtain a fixed emissivity value for ir thermal measurement. this coating greatly improves the accuracy of the ir measurement because it eliminates the need for any emissivity correction procedure used by the ir microscope. the emissivity correction procedure recommended by ir microscope manufacturers is ineffective at compensating for the translucent nature of silicon [1]. with the ir microscopy procedure, the maximum die surface temperature (?hot spot?) in the measurement field can be located. the hot spot temperature is selected as the die temperature (t j ) for thermal resistance ( jc ). the thermal resistance calculations are described later. case temperature (t c ) measurement the case temperature (t c ) of the package is measured by a 0.020 diameter stainless steel sheath thermocouple (type j; omega part # jmqss - 020g - 12) that is mounted within the heatsink of the rf circuit. it is mounted from the bottom and protrudes through the mounting interface to contact with the bottom surface of the package (figure 1). a 0.032 diameter hole is drilled through the circuit heatsink to permit thermocouple passage. this small hole provides minimal disturbance to the heat flow path and interface integrity. the thermocouple model is selected based on its sensitivity combined with excellent durability. a spring mechanism is added to the thermocouple to guarantee constant mechanical contact with the bottom side of the flange. the placement for this thermocouple is centered relative to the centermost active transistor in the package (figure 2). ????????????? ????????????? ???? die package temperature of case (t c ) measured with spring? loaded thermocouple making direct contact heatsink die temperature (t j ) measured with ir microscope 4 active die thermocouple centered relative to centermost active die in package figure 1. case temperature measurement figure 2. positioning of thermocouple AN1955 rev. 0, 1/2004 freescale semiconductor application note ? freescale semiconductor, inc., 2004, 2006. all rights reserved.
2 rf application information freescale semiconductor AN1955 ????????????? ????????????? ???? ???? die package heatsink temperature (t h ) measured with thermocouple buried in heatsink 0.010 below mounting interface heatsink die temperature (t j ) 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 (t h ) measurement the heatsink temperature (t h ) 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 corning 340 - heatsink compound) is applied to the bottom of the flange by a roller. a dupont delrin 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 (indalloy 51 from indium corporation ) 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, jc , calculation the method for determining junction - to - case thermal resistance ( jc ) 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 ( jc- 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 t j in the thermal resistance calculation. total power dissipated in the product is calculated as p diss = (rf input power + dc power (i d * v d )) ? (rf output power + rf reflected power) junction - to - case thermal resistance is calculated as jc = (t j - t c ) / p diss for a multi - stage rfic product, the highest die surface temperature for each stage is measured by the ir scan and
AN1955 3 rf application information freescale semiconductor used in the thermal resistance ( jc - stage ) calculation for that stage. power dissipated in each stage is determined and used in the thermal resistance calculation for that stage. data sheet jc value thermal resistance data reported on a freescale rfpa technical data sheet is based on performance tests done on a sample size of ten parts, taken from dif ferent manufacturing lots. each part is powered to the desired rf condition and measured. the mean thermal resistance value of that group is then used for the data sheet. confidence in jc data a gauge r&r (reproducibility and repeatability) assessment was used to demonstrate the methodology employed in measuring and reporting accurate thermal characterizations of freescale high power rf power amplifiers. this assessment showed that the measured standard deviation (part - to - part variation plus measurement variation) expressed as a percentage of the measured mean is around 5%. summary thermal measurement methodology has been developed and implemented to accurately characterize high power rf power amplifiers. integral to this measurement methodology are: ? using infrared microscopy to accurately determine the die temperature (t j ) at high frequency (~2 ghz) under rf test conditions ? using thermocouple measurements to accurately determine the case temperature (t case ) ? using the maximum die temperature of the device to calculate the junction - to - case thermal resistance ( jc ) ? establishing confidence level in the measured jc value ? implementing this methodology to determine the jc data for the freescale rfpa technical data sheet. this thermal measurement methodology is applied to both multi - die rf power transistor products and multi - stage power rfic products. references 1. m. mahalingam and e. mares, ?infrared temperature characterization of high power rf devices,? proceedings of ieee mtt - s international microwave symposium, may 2001.
4 rf application information freescale semiconductor AN1955 appendix: coating methodology for infrared thermal measurement of rf power amplifiers this appendix describes the coating recipe used to fix the emissivity value of target objects in the infrared (ir) thermal measurement metho dology of metal ceramic and overmolded plastic rf power amplifiers. we have assessed that an ir microscope?s emissivity correction procedure does not work well when applied to uncoated ir translucent targets, such as an si device [1]. in some cases, the nonactive regions of the die layout show up as higher temperature regions than the active regions. this issue is resolved once the device is coated with a high emissivity coating. another issue is that the temperature is measured lower for noncoated devices in comparison to the same devices when coated and measured under identical operating conditions. removal and replacement of protective lid for metal ceramic product measurement to allow viewing of the die for ir thermal measurement, the protective lid of the metal ceramic package must be removed. to do this, a metal ceramic package is placed on a hot plate at a temperature of ~ 280 c for about 45 seconds. it remains on the hot plate until the epoxy seal of the protective ceramic lid has suf ficiently melted to allow removal. the unit is relidded with a modified lid that has an opening. the part is then ready for application of paint. removal of mold compound for plastic product evaluation to permit thermal evaluation of an overmolded plastic package, the mold compound is etched away from the middle portion of the package without damaging the die and interconnects. selection of coating applied to device based on an internal study comparing six different coatings, the one with the l east impact on rf performance (gain, efficiency and intermodulation distortion at both 1 ghz and 2 ghz) was chosen. this coating is applied with an airbrush. application of coating to device using an airbrush the use of an airbrush to apply the coating permits accurate and even coverage of paint onto the device. an air pressure of about 20 - 25 psi is supplied to the airbrush for spraying. the airbrush is held about 1/2 away from the units during application. a few passes of paint application are made to all of the units. to expedite the drying process between application coats, the paint supply is shut off to allow only air to pass through. the units are then sprayed with air only until the paint is dry. the coating process is repeated until the active die has been adequately covered with paint. an emissivity measurement was run using this coating process and was determined to be 0.98. therefore, a constant emissivity value of 0.98 is input into the ir microscope when performing thermal measurements with coated devices. references 1. mahalingam and e. mares, ?infrared temperature characterization of high power rf devices,? proceedings of ieee mtt - s international microwave symposium, may 2001.
AN1955 5 rf application information freescale semiconductor information in this document is provided solely to enable system and software implementers to use freescale semiconductor products. there are no express or implied copyright licenses granted hereunder to design or fabricate any integrated circuits or integrated circuits based on the information in this document. freescale semiconductor reserves the right to make changes without further notice to any products herein. freescale semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does freescale semiconductor assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. ?typical? parameters that may be provided in freescale semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all operating parameters, including ?typicals?, must be validated for each customer application by customer?s technical experts. freescale semiconductor does not convey any license under its patent rights nor the rights of others. freescale semiconductor products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the freescale semiconductor product could create a situation where personal injury or death may occur. should buyer purchase or use freescale semiconductor products for any such unintended or unauthorized application, buyer shall indemnify and hold freescale semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that freescale semiconductor was negligent regarding the design or manufacture of the part. freescale and the freescale logo are trademarks of freescale semiconductor, inc. all other product or service names are the property of their respective owners. ? freescale semiconductor, inc. 2004, 2006. all rights reserved. how to reach us: home page: www.freescale.com e - mail: support@freescale.com usa/europe or locations not listed: freescale semiconductor technical information center, ch370 1300 n. alma school road chandler, arizona 85224 +1 - 800 - 521 - 6274 or +1 - 480 - 768 - 2130 support@freescale.com europe, middle east, and africa: freescale halbleiter deutschland gmbh technical information center schatzbogen 7 81829 muenchen, germany +44 1296 380 456 (english) +46 8 52200080 (english) +49 89 92103 559 (german) +33 1 69 35 48 48 (french) support@freescale.com japan: freescale semiconductor japan ltd. headquarters arco tower 15f 1 - 8 - 1, shimo - meguro, meguro - ku, tokyo 153 - 0064 japan 0120 191014 or +81 3 5437 9125 support.japan@freescale.com asia/pacific: freescale semiconductor hong kong ltd. technical information center 2 dai king street tai po industrial estate tai po, n.t., hong kong +800 2666 8080 support.asia@freescale.com for literature requests only: freescale semiconductor literature distribution center p.o. box 5405 denver, colorado 80217 1 - 800 - 441 - 2447 or 303 - 675 - 2140 fax: 303 - 675 - 2150 ldcforfreescalesemiconductor@hibbertgroup.com AN1955 rev. 0, 1/2004
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