AN RFMD APPLICATION NOTE RFCM2680 PCB Thermal Design Requirements Product Description The RFCM2680 is a power doubler amplifier SMD Module. The part employs GaAs phemt die and GaN HEMT die, has high output capability, and is operated from 45MHz to 1003MHz. It provides excellent linearity and superior return loss performance with low noise and optimal reliability. DC current of the device can be externally adjusted for optimum distortion performance versus power consumption over a wide range of output level. For further information please refer to the RFCM2680 datasheet at: http://www.rfmd.com/cs/documents/rfcm2680ds.pdf Absolute Maximum Ratings Parameter Rating Unit Operating Mounting Base Temperature 110 C Maximum Module Molding Cap Temperature 115 C Maximum GaAs phemt Die Junction Temperature* 145 C *All R TH calculations are related to GaAs phemt die junction temperature Module Parameters Parameter Value Unit Description R THMOD 3.3 K/W Thermal resistance of RFCM2680 module (die junction to module backside) S 2.9 K/W Slope (see Figure 8) C 1.25 Constant factor for RFCM2680 module because of the non-uniform temperature distribution on the module backside. RF MICRO DEVICES, RFMD, Optimum Technology Matching, Enabling Wireless Connectivity, PowerStar, POLARIS TOTAL RADIO and UltimateBlue are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks and registered trademarks are the property of their respective owners. 2012, RF Micro Devices, Inc. 1 of 11
PCB Metal Land Pattern Figure 1 shows the recommended land pattern for RFCM2680. The thermal vias on the application PCB should be distributed equally underneath the RFCM2680 ground plane. Underneath this thermal via array a heat sink needs to be placed which is able to dissipate the complete module DC power (up to 10.4W). The interface between PCB and heat sink should be filled with thermal grease. A PCB =38.7mm 2 136 x 0.400mm VIA (0.035mm plating) 3.700 9x 0.600 12x 0.550 Pin1 2x 2.000 0.650 1.000 0.000 0.650 1.000 A 2x 2.000 A 3.700 4x 4.100 3.400 0.000 = 0.600 x 0.600 mm 3.400 5x 4.100 Figure 1. Proposed RFCM2680 Land Pattern (Dimensions in Millimeters) 2 of 11
Cross Section And Design Requirements Of RFCM2680 On Application PCB The cross section (Figure 2) shows a virtual cut through the RFCM2680 soldered on an application PCB with heat sink underneath. A-A application PCB metal plating thermal via thermal conductive zone solder joint die on heatspreader module mounting base molding cap module substrate thermal grease application PCB R thmod R thsn R thpcb R thtg R th heat sink R thhs Figure 2. Cross Section of RFCM2680 on Application PCB and R TH Layer Definition The application PCB and the heat sink should be fixed with screws right next to the module to ensure a proper thermal contact. Additionally a thermal contact material (thermal grease, for example) should be used between the heat sink and the customer PCB. Screws to fix application PCB to the heat sink Figure 3. Example of Screw Position to Fix Application PCB to the Heat Sink 3 of 11
Thermal Scan GaAs phemt die GaN die Figure 4. Thermal Scan of RFCM2680 on Application PCB The GaAs phemt die temperature is the limiting factor for the RFCM2680, even if the GaN dies are hotter. Due to this all maximum ratings and R TH calculations are related to GaAs phemt die junction temperature. Thermal Calculations Calculation Of Thermal Resistance Overall Thermal Resistance - R TH (1) R TH = R THMOD + c R THSN + R THPCB + R THTG + R THHS (2) T j = R TH P R TH R THMOD R THSN R THPCB R THTG R THHS c T j P = Overall thermal resistance in K/W = 3.3 K/W; Thermal resistance of RFCM2680 module in K/W (die junction to module backside) = Thermal resistance of solder joint in K/W = Thermal resistance of application PCB in K/W = Thermal resistance of thermal grease in K/W = Thermal resistance of heat sink in K/W = Constant factor of 1.25 for RFCM2680 module because of the non-uniform temperature distribution on the module backside. This constant factor is experimentally derived from thermal scan. = Temperature difference between die junction and ambient air in K = Module DC power in W (supply voltage module current) 4 of 11
Thermal Resistance of Solder Joint - R THSN (3) 2 A SN = x GND y GND n VIA -- d 4 i (4) R THSN = z ---------------------- SN A SN SN A SN = Effective area of solder joint underneath of RFCM2680 ground plane in m 2 x GND = Width of RFCM2680 ground plane in m y GND = Length of RFCM2680 ground plane in m n VIA = Number of vias underneath of RFCM2680 ground plane d i = Inner diameter of thermal via in m (final plated via hole diameter; d i = d o - 2 plating) z SN = Thickness of solder joint in m SN = Thermal conductivity of solder joint in W/(m K) Thermal Resistance of Application PCB - R THPCB (5) R THSPCB = z PCB ----------------------------------------------------------- 2 2 n VIA -- d 4 o d i CU z PCB = Thickness of application PCB in m d o = Outer diameter of thermal via in m (drilled via hole diameter ; d o = 2 plating + d i ) CU = Thermal conductivity of copper in W/(m K) Thermal Resistance of Thermal Grease - R THTG (6) A TG = A SN = x GND y GND n VIA 2 -- d 4 i (7) R THTG = z ----------------------- TG A TG TG A TG = Effective area of thermal grease in m 2 z TG = Thickness of thermal grease underneath of RFCM2680 ground plane in m TG = Thermal conductivity of thermal grease in W/(m K) Thermal Resistance of Heat Sink - R THHS Thermal resistance of heat sink in K/W is given by customer application. 5 of 11
Example of R TH Calculation (Die Junction to Heat Sink) Per equation (3) A SN 7.4 10 3 m 6.8 10 3 m 136 3 = -- 0.33 10 m 2 = 38.7 10 6 4 m 2 Per equation (4) 50 10 6 m R THSN = -------------------------------------------------------- 38.7 10 6 m 2 60 W = 0.02 K W ---- ------------ m K Per equation (5) 1.60 10 3 m R THSPCB = ---------------------------------------------------------------------------------------------------------------------------------- 136 3 -- 0.40 10 m 2 0.33 10 3 m 2 390 W = 0.75 K W ---- ------------ 4 m K Per equation (6) A TG = A SN = 38.7 10 6 m 2 Per equation (7) 50 10 6 m R THTG = ----------------------------------------------------- 38.7 10 6 m 2 5 W = 0.26 K W ---- ------------ m K Per equation (1) R TH = 3.30 K W ---- + 125 0.02 K W ---- + 0.75 K W ---- + 0.26 K W ---- + R THHS = 4.59 K W ---- + R THHS Per equation (2) T j = 4.59 K W ---- + R THHS 10.32W = 47.4K+ R THHS 10.32W The temperature rise from the backside of the application PCB to DIE junction is 47.4K in this example. Additionally to this temperature rise the impact of the heat sink design must be considered to calculate the final module die temperature. 6 of 11
Table 1. Values for R TH Calculation Symbol Value Unit R THMOD 3.30 K/W c 1.25 P 10.32 W x GND 7.4 10-3 m y GND 6.8 10-3 m n VIA 136 d i 0.33 10-3 m z SN 50 10-6 m λ SN 60 W/(m K) z PCB 1.60 10-3 m d o 0.40 10-3 m λ CU 390 W/(m K) z TG 50 10-6 m λ TG 5 W/(m K) How to Improve Thermal Design of Application PCB Increase number of thermal vias (n VIA ) underneath RFCM2680 ground plane. Reduce thickness (z PCB ) of application PCB. Increase copper plating thickness inside of thermal via. In best case thermal vias are completely filled. Use thermal grease with better thermal conductivity (λ TG ). Table 2. Temperature Distribution for Example of R TH Calculation (Die Junction to Heat Sink) with 90 C Heat Sink Temperature underneath Thermal Vias Mold cap temperature 107.4 C Die junction temperature 137.3 C Solder joint temperature (module mounting base) 103.3 C Application PCB temperature 103.0 C Thermal grease temperature 93.4 C Heat sink temperature 90.0 C 7 of 11
T j [ C] 160 155 150 145 140 135 130 125 50 75 100 125 150 Figure 5. Die Junction Temperature over Number of Thermal Vias [Example of R TH Calculation (Die Junction to Heat Sink) With 90 C Heat Sink Temperature underneath Thermal Vias] n VIA Die Junction Temperature Max. Die Junction Temperature Example of R TH Calculation (Die Junction to Heat Sink) T j [ C] 160 155 150 145 140 135 130 125 0.0 1.0 2.0 3.0 z PCB [mm] Die Junction Temperature Max. Die Junction Temperature Example of R TH Calculation (Die Junction to Heat Sink) Figure 6. Die Junction Temperature over Application PCB Thickness [Example of R TH Calculation (Die Junction to Heat Sink) With 90 C Heat Sink Temperature underneath Thermal Vias] T j [ C] 160 155 150 145 140 135 130 125 0 50 100 150 200 plating [ m] Die Junction Temperature Max. Die Junction Temperature Example of R TH Calculation (Die Junction to Heat Sink) Figure 7. Die Junction Temperature over Plating Thickness [Example of R TH Calculation (Die Junction to Heat Sink) With 90 C Heat Sink Temperature underneath Thermal Vias] 8 of 11
RFCM2680 Mold Cap Temperature Due to the fact that it is very complicated to measure the module backside temperature or the mounting base temperature directly underneath the thermal via, the die junction temperature can be extrapolated by measuring the module mold cap temperature. Mold cap temperature must not exceed 115 C in any case. T [K] 32 30 28 26 T = T die - T cap 24 22 7.5 8.0 8.5 9.0 9.5 10.0 10.5 P [W] Figure 8. Difference between Die Junction Temperature and Mold Cap Temperature over Module DC Power Mold cap Temperature Calculation (8) T = T die T cap = s P (9) T cap = T die s P T die = Temperature of die junction in C T cap = Temperature of mold cap in C s = 2.9 K/W; temperature slope 9 of 11
Mold Cap Temperature Example Per equation (9) T cap = 137.3 C 2.9---- K 10.32W = 107.4 C W If the die junction temperature was calculated to be 137.3 C (see Table 2) the measured mold cap temperature should be 107.4 C. Table 3. Values for Mold Cap Temperature Calculation Symbol Value Unit T die 137.3 C s 2.9 K/W P 10.32 W Measure RFCM2680 Mold Cap Temperature The following points must be considered for the measurement of the mold cap temperature: Module must be run under stable electrical conditions. The thermal sensor must be placed at the hottest point of the module surface (position shown in Figure 9). A small thermocouple type K is recommended as thermal sensor. Thermal grease should be used to ensure a proper thermal contact between sensor and module mold cap. The measurement must be performed in a steady thermal state. RFCM2680 YYWW Trace Code 4.000 1.800 Figure 9. Measuring Point for RFCM2680 Molding Cap Temperature 10 of 11
Figure 10. Example of RFCM2680 Molding Cap Temperature Measurement Test Setup 11 of 11