“Radio frequency (RF) amplifiers are available in lead frame chip scale packages (LFCSPs) and flanged packages, and are mounted on printed circuit boards (PCBs) through proven reflow soldering processes. Not only does the PCB serve as an electrical interconnection between components, it is also the main way for the amplifier to dissipate heat (using the metal bumps on the bottom of the package).
“
By Eamon Nash Analog Devices
Introduction
Radio frequency (RF) amplifiers are available in lead frame chip scale packages (LFCSPs) and flanged packages, and are mounted on printed circuit boards (PCBs) through proven reflow soldering processes. Not only does the PCB serve as an electrical interconnection between components, it is also the main way for the amplifier to dissipate heat (using the metal bumps on the bottom of the package).
This application note introduces the concept of thermal resistance and provides a technique for modeling the flow of heat from the die to the heat sink of a typical RF amplifier in an LFCSP or flange package.
Hot Concept Review
heat flow
When there is a temperature difference between different areas of the material, heat flows from the high temperature area to the low temperature area. This process is similar to electric current, which flows through a circuit from a highpotential region to a lowpotential region.
Thermal resistance
All materials have some thermal conductivity. Thermal conductivity is a measure of a material’s ability to conduct heat. Thermal conductivity values are usually given in watts per meter kelvin (W/mK) or watts per inch kelvin (W/inK). If the thermal conductivity of the material is known, the following formula is used to calculate the thermal resistance (θ) per unit volume of the material in C/W or K/W:
in:
Length indicates the length or thickness of the material in meters.
k is the thermal conductivity of the material.
Area represents the crosssectional area, in m^{2}for the unit.
temperature
Using the analogy that heat flow is equivalent to the amount of electric current, the temperature difference of a material that inherently has thermal resistance and supports heat flow is as follows:
ΔT = Q × θ (2)
in:
ΔT represents the temperature difference (K or °C) between different regions of the material.
Q represents heat flow (W).
θ represents the thermal resistance (C/W or K/W) of the material.
device thermal resistance
The thermal resistance of a device is quite complex and tends to be nonlinear with temperature. Therefore, we use the finite element analysis method to establish the thermal model of the device. Infrared photography can determine the temperature at the device connections and the temperature of the package during operation. Based on these analyses and measurements, the equivalent thermal resistance can be determined. Equivalent thermal resistance is valid under the specific conditions under which the device is measured, typically at the maximum operating temperature.
Refer to Table 1 for a table of absolute maximum ratings for typical RF amplifiers.
Table 1. Absolute Maximum Ratings for Typical RF Amplifiers
parameter 
Rated value 
Drain Bias Voltage (V_{DD}) 
60Vdc 
Gate Bias Voltage (V_{GG1}) 
8 V to 0 V dc 
Radio Frequency (RF) Input Power (RFIN) 
35dBm 
Continuous power consumption (P_{DISS}) (T = 85°C) (636 mW/°C derated above 85°C) 
89.4W 
Thermal Resistance, Junction to Pad Backside (θ_{JC}) 
1.57°C/W 
temperature range 

storage 
55°C to +150°C 
Operating temperature 
40°C to +85°C 
Junction temperature range (T_{J}) 
225°C 
Nominal Junction Temperature (T_{CASE} = 85°C, V_{DD} = 50 V) 
187°C 
For LFCSP and flange packages, the package case is assumed to be the metal block on the bottom of the package.
Maximum junction temperature
In a given data sheet, the maximum junction temperature (based on the device’s semiconductor process) is given in the Absolute Maximum Ratings table for each product. In Table 1, the specified maximum junction temperature for the millionhour MTTF is 225°C. This temperature specified is generally applicable to Gallium Nitride (GaN) devices. Exceeding this limit can result in reduced device life and permanent device failure.
range of working temperature
device operating temperature (T_{CASE}) are given on the package base. T_{CASE}is the temperature of the metal bump on the bottom of the package. The operating temperature is not the temperature of the air surrounding the device.
If T is known_{CASE}and P_{DISS}, it is easy to calculate the junction temperature (T_{J}). For example, if T_{CASE}=75°C, P_{DISS}=70 W, then T can be calculated using the following formula_{J}:
T_{J} = T_{CASE} + (θ_{JC} × P_{DISS})
= 75°C + (1.57°C/W × 70W)
= 184.9°C
When considering device reliability, T_{J}is the most important specification parameter and should never be exceeded. Conversely, if it is possible to reduce P_{DISS}, so that T_{J}remains below the maximum allowable level, the T_{CASE}Specified Absolute Maximum Ratings may be exceeded. In this example, when the case temperature exceeds the specified maximum value of 85°C, a derating value of 636 mW/°C can be used to calculate the maximum allowable P_{DISS}. For example, using the data in Table 1, when P_{DISS}When the limit is 83 W, the maximum allowable T_{CASE}is 95°C. P_{DISS}It can be calculated using the following formula:
P_{DISS} = 89.4 W − (636 mW/°C × 10°C)
= 83W
use this P_{DISS} value, the junction temperature of 225°C can be calculated using the following formula:
T_{J}_{ }= T_{CASE} + (θ_{JC} × P_{DISS})
= 95°C + (1.57°C/W × 83 W) (3)
Thermal model of device and PCB environment
To fully understand the entire thermal environment around a device, the heat dissipation paths and materials of the device must be modeled. Figure 1 shows a crosssectional schematic of the LFCSP package mounted on the PCB and heat sink. In this example, the die generates heat, which is then transferred to the heat sink via the package and PCB. To determine the temperature at the device junction, thermal resistance must be calculated. Using thermal resistance and heat flow, the junction temperature can be calculated. The junction temperature is then compared to the maximum specified junction temperature to determine if the device is operating reliably.
In Figure 1, the thermal path from the device connection to the heat sink is defined as follows:
• θ_{JA}is the thermal resistance from the device connection to the air around the top of the package.
• θ_{JC}is the thermal resistance from the connection to the case (the metal block on the bottom of the package).
• θ_{SN63}is the thermal resistance of the solder.
• θ_{CU}is the thermal resistance of the copper plating on the PCB.
• θ_{VIACU}is the thermal resistance of the copper plating on the through hole.
• θ_{VIASN63}is the thermal resistance of the solder filled in the via.
• θ_{PCB}is the thermal resistance of the PCB laminate.
In a typical circuit board, there are multiple vias and multiple PCB layers. When calculating the thermal resistance of the cross section of the system, the thermal circuit is used to calculate the individual thermal resistances, and the series thermal resistance is combined with the parallel thermal resistance to determine the total thermal resistance of the device.
Figure 1. Thermal model of LFCSP package mounted on PCB and heat sink
System thermal resistance calculation
For each heat dissipation path, use Equation 1 to calculate its thermal resistance. To calculate the individual thermal resistance values, the thermal conductivity of the material must be known. See Table 2 for thermal conductivity of commonly used materials in PCB assemblies.
Table 2. Thermal Conductivity of Commonly Used PCB Materials
Material 
Thermal conductivity (W/inK) 
Copper (Cu) 
10.008 
Aluminum (Al) 
5.499 
Rogers 4350 (RO4350) 
0.016 
FR4 or G10 laminate 
0.008 
Alumina (Al2O3) 
0.701 
SN63 Solder 
1.270 
Thermally Conductive Epoxy 
0.020 
Gallium Arsenide (GaAs) 
1.501 
molding compound 
0.019 
Figure 2 shows the equivalent thermal circuit based on the thermal model shown in Figure 1. T_{PKG}Indicates the temperature at the bottom of the package, T_{SINK}Indicates the temperature of the radiator. In Figure 2, it is assumed that the package (T_{A}) the ambient air temperature is constant. For real assemblies with outer shells, T_{A}May rise as power consumption increases. The thermal path to ambient air temperature is ignored for this analysis because for LFCSP and flange packages with metal blocks, θ_{JA}much larger than θ_{JC}.
Figure 2. Equivalent thermal circuit
Thermal Resistance Example: HMC408LP3 Evaluation Board
The HMC408LP3 power amplifier uses a 0.01inch thick evaluation board constructed of Rogers RO4350 laminate. The ground pad shown in Figure 3 has an area of 0.065 × 0.065 inches with five 0.012inch diameter vias. The top and bottom of the board each have 1 oz copper plating (0.0014″ thick). The vias are plated with ½ oz copper (0.0007″ thick). During assembly, the through holes are filled with SN63 solder. Analysis showed that almost all of the heat flow would flow through the solderfilled vias. Therefore, the rest of the board layout can be ignored in this analysis.
Figure 3. Ground Pad Layout
Each thermal resistance is calculated using Equation 1. Calculate theta_{SN63}, the thermal conductivity of the SN63 solder used is 1.27 W/inK, the length (or thickness of the solder joint) is 0.002 inches, and the solder area is 0.004225 inches (0.065 inches × 0.065 inches).
Next, calculate the value of the copper plating on top of the PCB in a similar manner. The thermal conductivity of the copper coating is 10.008 W/inK, the length is 0.0014 inches (1 oz copper), and the coating area is 0.00366 square inches (in^{2}).
For the area of copper plating on the through hole, use the following formula to calculate
Area = π × (r_{O}^{2} C r_{I}^{2}) (6)
in:
r_{O}Indicates the outer diameter.
r_{I}Indicates the inner diameter.
With an outside diameter of 0.006 inches and an inside diameter of 0.0053 inches, the calculated area is 0.00002485 in^{2}. The length of the vias is the thickness of the board (0.01 inches), and the thermal conductivity of copper is 10.008 W/inK.
Since there are 5 vias side by side, the thermal resistance is divided by 5. So, θ_{VIACU} = 8.05°C/W.
In a similar manner, the value of the fill solder for the vias is calculated.
Since there are 5 filled vias, the equivalent thermal resistance is θ_{VIASN63} = 17.85°C/W.
Next, use a Rogers RO4350 thermal conductivity of 0.01 inch length, 0.016 W/inK, and 0.00366 in^{2}Area calculates the thermal resistance of the PCB.
In the equivalent thermal circuit shown in Figure 2, the three thermal resistances (θ_{PCB}, θ_{VIACU}and θ_{VIASN63}) is 5.37°C/W after the parallel combination. After filling the vias with solder, the thermal resistance decreased from 8.05°C/W to 5.37°C/W. Finally, adding the value of the thermal resistance in series gives the thermal resistance of the entire PCB assembly.
θ_{ASSY} = theta_{SN63} + theta_{CU} + theta_{EQUIV} + theta_{CU} = 0.372 + 0.038 + 5.37 + 0.038 = 5.81°C/W (10)
where θ_{ASSY}Indicates the thermal resistance of the assembly.
Determine power consumption
After the thermal resistance value is determined, the heat flow (Q) value must be determined. For RF devices, the value of Q represents the difference between the total power input to the device and the total power output from the device. Total power includes RF power and DC power.
Q = P_{INTOTAL} − P_{OUTTOTAL} = (P_{INRF} +P_{INDC}) − P_{OUTRF} (11)
in:
P_{INTOTAL}Indicates the sum of DC power and RF input power.
P_{OUTTOTAL}represents the power output by the device, and P_{OUTRF}same.
P_{INRF}Indicates RF input power.
P_{INDC}Indicates DC input power.
P_{OUTRF}Indicates the RF output power delivered to the load.
Figure 4. HMC408LP3 power dissipation vs input power
For the HMC408LP3 power amplifier, use Equation 11 to calculate P shown in Figure 4_{DISS}value of . Figure 4 shows the following characteristics of the amplifier:
• The device consumes approximately 4 W with no RF input signal.
• When using RF signals, P_{DISS}The value of is determined by the frequency.
• At a certain input power, the device consumes the lowest power dissipation.
According to the equivalent thermal resistance, θ_{TOTAL}and Q, the junction temperature can be calculated using
ΔT = Q × θ_{TOTAL} (12)
θ_{TOTAL} = theta_{ASSY} + theta_{JC} = 5.81 + 13.79 = 19.6°C/W (13)
For quiescent state with no RF input power, Q = 4 W, and
ΔT = 4.0 × 19.6 = 78.4°C (14)
Because the maximum junction temperature of the HMC408LP3 is specified at 150°C, at P_{DISS} = 4 W, the temperature of the heat sink must be ≤71.6°C (that is, 78.4°C + 71.6°C = 150°C).
The HMC408LP3 power amplifier consumes less than 4 W during normal operation (eg, input power ≤ 5 dBm), which means that the heat sink temperature can be slightly higher than 71.6°C. However, if the amplifier is operating in a deep compression environment and the input power is equivalent to 15 dBm, then P_{DISS}increases and requires the heat sink temperature to be below 71.6°C.
Table 3. Thermal Operating Data Sheet
describe 
value 
unit 
Notes 
Heatsink maximum temperature 
70 
°C 

θ_{ASSY} 
5.81 
°C/W 
Calculated from an equivalent thermal circuit 
θ_{JC} 
13.79 
°C/W 
from datasheet 
θ_{TOTAL} 
19.6 
°C/W 
add theta_{ASSY}and θ_{JC} 
Q 
4.0 
W 

The resulting junction temperature 
148.4 
°C 
Heatsink maximum temperature + (θ_{TOTAL} × Q); do not exceed the maximum channel temperature listed in the data sheet 
reliability
The life expectancy of a component is closely related to the operating temperature. Operating at temperatures below the maximum junction temperature can extend the life of the device. Exceeding the maximum junction temperature will shorten the lifetime. Therefore, performing a thermal analysis ensures that the specified maximum junction temperature will not be exceeded under expected operating conditions.
in conclusion
Using low junction temperature surface mount RF power amplifiers in LFCSP and flange packages to enclose thermal resistance forces the PCB not only to act as an RF interconnect between devices, but also as a thermal path to conduct heat away from the power amplifier.
Therefore, θ_{JC} replace theta_{JA}, which has become an important thermal resistance index to measure the LFCSP or flange package.
In these calculations, the most critical metric is the RF amplifier’s junction or channel temperature (T_{J}). As long as the maximum junction temperature is not exceeded, then other nominal limits such as T_{CASE}, it can be higher than the limit.
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