Datasheet

AN815
Vishay Siliconix
www.vishay.com
2
Document Number: 71334
12-Dec-03
FIGURE 3.
Front of Board SC70-6
Back of Board SC70-6
vishay.com
THERMAL PERFORMANCE
Junction-to-Foot Thermal Resistance
(Package Performance)
The junction to foot thermal resistance is a useful method of
comparing different packages thermal performance.
A helpful way of presenting the thermal performance of the
6-Pin SC-70 copper leadframe device is to compare it to the
traditional Alloy 42 version.
Thermal performance for the 6-pin SC-70 measured as
junction-to-foot thermal resistance, where the “foot” is the
drain lead of the device at the bottom where it meets the PCB.
The junction-to-foot thermal resistance is typically 40_C/W in
the copper leadframe and 163_C/W in the Alloy 42 leadframe
— a four-fold improvement. This improved performance is
obtained by the enhanced thermal conductivity of copper over
Alloy 42.
Power Dissipation
The typical Rq
JA
for the single 6-pin SC-70 with copper
leadframe is 103_C/W steady-state, compared with 212_C/W
for the Alloy 42 version. The figures are based on the 1-inch
2
FR4 test board. The following example shows how the thermal
resistance impacts power dissipation for the two different
leadframes at varying ambient temperatures.
ALLOY 42 LEADFRAME
Room Ambient 25 _C Elevated Ambient 60 _C
P
D
+
T
J(max)
* T
A
Rq
JA
P
D
+
150
o
C * 25
o
C
212
o
CńW
P
D
+ 590 mW
P
D
+
T
J(max)
* T
A
Rq
JA
P
D
+
150
o
C * 25
o
C
212
o
CńW
P
D
+ 425 mW
COOPER LEADFRAME
Room Ambient 25 _C Elevated Ambient 60 _C
P
D
+
T
J(max)
* T
A
Rq
JA
P
D
+
150
o
C * 25
o
C
124
o
CńW
P
D
+ 1.01 W
P
D
+
T
J(max)
* T
A
Rq
JA
P
D
+
150
o
C * 60
o
C
124
o
CńW
P
D
+ 726 mW
As can be seen from the calculations above, the compact 6-pin
SC-70 copper leadframe LITTLE FOOT power MOSFET can
handle up to 1 W under the stated conditions.
Testing
To further aid comparison of copper and Alloy 42 leadframes,
Figure 5 illustrates single-channel 6-pin SC-70 thermal
performance on two different board sizes and two different pad
patterns. The measured steady-state values of Rq
JA
for the
two leadframes are as follows:
LITTLE FOOT 6-PIN SC-70
Alloy 42 Copper
1) Minimum recommended pad pattern on
the EVB board V (see Figure 3.
329.7_C/W 208.5_C/W
2) Industry standard 1-inch
2
PCB with
maximum copper both sides.
211.8_C/W 103.5_C/W
The results indicate that designers can reduce thermal
resistance (Rq
JA
) by 36% simply by using the copper
leadframe device rather than the Alloy 42 version. In this
example, a 121_C/W reduction was achieved without an
increase in board area. If increasing in board size is feasible,
a further 105_C/W reduction could be obtained by utilizing a
1-inch
2
square PCB area.
The copper leadframe versions have the following suffix:
Single: Si14xxEDH
Dual: Si19xxEDH
Complementary: Si15xxEDH