NXP Semiconductors AN10714, Application Note

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AN10714_1 © NXP B.V. 2010. All rights reserved.
Application note Rev. 01 — 26 January 2010 17 of 21
NXP Semiconductors
AN10714
Using the BLF574 in the 88 MHz to 108 MHz FM band
6.3 Output base plate
7. Reliability
Time-to-Failure (TTF) is defined as the expected time elapsed until 0.1 % of the devices of
a sample size fail. This is different from Mean-Time-to-Failure (MTBF), where half the
devices would have failed and is orders of magnitude are shorter. The predominant failure
mode for LDMOS devices is electromigration. The TTF for this mode is primarily
dependant on junction temperature (T
j
) added to the effect of current density. Once the
device junction temperature is measured and in-depth knowledge is obtained of the
average operating current for the application, the TTF can be calculated using
Figure 14
and the related procedure.
7.1 Calculating TTF
The first step uses the thermal resistance (R
th
) of the device to calculate the junction
temperature. The R
th
from the junction to the device flange for the BLF574 is 0.25
°
C/W. If
the device is soldered down to the heatsink, this same value can be used to determine T
j
.
If the device is greased down to the heatsink, the R
th(j-h)
value becomes 0.4
°
C/W.
Fig 13. Output base plate drawing
001aak568
Unit
mm
A
0
B
10.922
C
37.211
D
45.847
E
65.278
F
76.200
G
6.350
H
9.068
I
12.573
J
71.120
Unit
mm
K
3.505
L
M5
M
M2
N
8
O
21
A
B
C
D
E
F
A
N
O
M
(2
×
)
(4
×
)
L
G
I
A
engraved letter "M"
J
A
K
H