RFCM2680 PCB Thermal Design Requirements

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 RTH calculations are related to GaAs pHEMT die junction temperature
Module Parameters
Parameter
Value
Unit
Description
RTHMOD
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.
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RFCM2680 PCB Thermal Design Requirements
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.
APCB=38.7mm2
12x 0.550
136 x 0.400mm VIA (0.035mm plating)
9x 0.600
3.700
Pin1
2x 2.000
X
0.650
X
X
X
X
0.000
0.650
1.000
X
X
A
1.000
A
2x 2.000
X
X
X= 0.600 x 0.600 mm
5x 4.100
3.400
0.000
3.400
4x 4.100
3.700
Figure 1. Proposed RFCM2680 Land Pattern (Dimensions in Millimeters)
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RFCM2680 PCB Thermal Design Requirements
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 conductive zone
solder joint
die on heatspreader
module mounting base
molding cap
thermal via
module substrate
thermal grease
application PCB
RthMOD
RthSN
RthPCB
RthTG
Rth
heat sink
RthHS
Figure 2. Cross Section of RFCM2680 on Application PCB and RTH 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
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RFCM2680 PCB Thermal Design Requirements
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 RTH calculations are related to GaAs pHEMT die junction temperature.
Thermal Calculations
Calculation Of Thermal Resistance
Overall Thermal Resistance - RTH
(1)
R TH = R THMOD + c   R THSN + R THPCB + R THTG  + R THHS
(2)
T j = R TH  P
RTH
= Overall thermal resistance in K/W
RTHMOD
= 3.3 K/W; Thermal resistance of RFCM2680 module in K/W (die junction to module backside)
RTHSN
= Thermal resistance of solder joint in K/W
RTHPCB
= Thermal resistance of application PCB in K/W
RTHTG
= Thermal resistance of thermal grease in K/W
RTHHS
= Thermal resistance of heat sink in K/W
c
= 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.
Tj
= Temperature difference between die junction and ambient air in K
P
= Module DC power in W (supply voltage · module current)
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RFCM2680 PCB Thermal Design Requirements
Thermal Resistance of Solder Joint - RTHSN
(3)
2

A SN = x GND  y GND – n VIA  ---  d i
4
(4)
z SN
R THSN = ---------------------A SN   SN
ASN
= Effective area of solder joint underneath of RFCM2680 ground plane in m2
xGND
= Width of RFCM2680 ground plane in m
yGND
= Length of RFCM2680 ground plane in m
nVIA
= Number of vias underneath of RFCM2680 ground plane
di
= Inner diameter of thermal via in m (final plated via hole diameter; di = do - 2 · plating)
zSN
= Thickness of solder joint in m
SN
= Thermal conductivity of solder joint in W/(m · K)
Thermal Resistance of Application PCB - RTHPCB
(5)
z PCB
R THSPCB = ----------------------------------------------------------2
 2
n VIA  ---  d o – d i    CU
4
zPCB
= Thickness of application PCB in m
do
= Outer diameter of thermal via in m (drilled via hole diameter ; do = 2 · plating + di)
CU
= Thermal conductivity of copper in W/(m · K)
Thermal Resistance of Thermal Grease - RTHTG
(6)
2

A TG = A SN = x GND  y GND – n VIA  ---  d i
4
(7)
z TG
R THTG = ----------------------A TG   TG
ATG
= Effective area of thermal grease in m2
zTG
= 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 - RTHHS
Thermal resistance of heat sink in K/W is given by customer application.
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RFCM2680 PCB Thermal Design Requirements
Example of RTH Calculation (Die Junction to Heat Sink)
Per equation (3)
Per equation (4)
Per equation (5)
Per equation (6)
Per equation (7)
Per equation (1)
Per equation (2)
2
–3
–3
–3
–6 2

A SN = 7.4  10 m  6.8  10 m – 136  ---   0.33  10 m  = 38.7  10 m
4
–6
K
50  10 m
R THSN = --------------------------------------------------------- = 0.02 ----W
–6 2
W
38.7  10 m  60 -----------mK
–3
K
1.60  10 m
R THSPCB = ----------------------------------------------------------------------------------------------------------------------------------- = 0.75 ----2
2
W
–3
–3
W

136  ---   0.40  10 m  –  0.33  10 m    390 -----------mK
4
–6
A TG = A SN = 38.7  10 m
2
–6
K
50  10 m
R THTG = ------------------------------------------------------ = 0.26 ----W
–6 2
W
38.7  10 m  5 -----------mK
K
K
K
K
K
R TH = 3.30 ----- + 125   0.02 ----- + 0.75 ----- + 0.26 ----- + R THHS = 4.59 ----- + R THHS


W
W
W
W
W
K
T j =  4.59 ----- + R THHS  10.32W = 47.4K + R THHS  10.32W


W
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.
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RFCM2680 PCB Thermal Design Requirements
Table 1. Values for RTH Calculation
Symbol
Value
Unit
RTHMOD
3.30
K/W
c
1.25
P
10.32
W
xGND
7.4 · 10-3
m
yGND
-3
m
nVIA
di
6.8 · 10
136
0.33 · 10-3
m
zSN
50 · 10-6
m
λSN
60
W/(m · K)
zPCB
-3
1.60 · 10
m
do
0.40 · 10-3
m
λCU
390
W/(m · K)
zTG
λTG
50 ·
10-6
5
m
W/(m · K)
How to Improve Thermal Design of Application PCB
• Increase number of thermal vias (nVIA) underneath RFCM2680 ground plane.
• Reduce thickness (zPCB) 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 (λTG).
Table 2. Temperature Distribution for Example of RTH 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
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RFCM2680 PCB Thermal Design Requirements
Tj [°C]
160
155
Die Junction Temperature
150
145
Max. Die Junction Temperature
140
Example of RTH Calculation
(Die Junction to Heat Sink)
135
130
125
50
75
100
125
150 nVIA
Figure 5. Die Junction Temperature over Number of Thermal Vias
[Example of RTH Calculation (Die Junction to Heat Sink) With 90°C Heat Sink Temperature underneath Thermal Vias]
Tj [°C]
160
155
Die Junction Temperature
150
145
Max. Die Junction Temperature
140
Example of RTH Calculation
(Die Junction to Heat Sink)
135
130
125
0.0
1.0
2.0
3.0
zPCB [mm]
Figure 6. Die Junction Temperature over Application PCB Thickness
[Example of RTH Calculation (Die Junction to Heat Sink) With 90°C Heat Sink Temperature underneath Thermal Vias]
Tj [°C]
160
155
Die Junction Temperature
150
145
Max. Die Junction Temperature
140
Example of RTH Calculation
(Die Junction to Heat Sink)
135
130
125
0
50
100
150
200
plating [m]
Figure 7. Die Junction Temperature over Plating Thickness
[Example of RTH Calculation (Die Junction to Heat Sink) With 90°C Heat Sink Temperature underneath Thermal Vias]
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RFCM2680 PCB Thermal Design Requirements
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.
ȴT [K]
32
30
28
T = Tdie - Tcap
26
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
Tdie
= Temperature of die junction in °C
Tcap
= Temperature of mold cap in °C
s
= 2.9 K/W; temperature slope
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RFCM2680 PCB Thermal Design Requirements
Mold Cap Temperature Example
Per equation (9)
K
T cap = 137.3C – 2.9 -----  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
137.3
°C
s
2.9
K/W
P
10.32
W
Tdie
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.
4.000
RFCM2680
YYWW
Trace Code
1.800
Figure 9. Measuring Point for RFCM2680 Molding Cap Temperature
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RFCM2680 PCB Thermal Design Requirements
Figure 10. Example of RFCM2680 Molding Cap Temperature Measurement Test Setup
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